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Open Software Foundation V. Samar (SunSoft)
Request For Comments: 86.0 R. Schemers (SunSoft)
October 1995
UNIFIED LOGIN WITH
PLUGGABLE AUTHENTICATION MODULES (PAM)
1. INTRODUCTION
Since low-level authentication mechanisms constantly evolve, it is
important to shield the high-level consumers of these mechanisms
(system-entry services and users) from such low-level changes. With
the Pluggable Authentication Module (PAM) framework, we can provide
pluggability for a variety of system-entry services -- not just
system authentication _per se_, but also for account, session and
password management. PAM's ability to _stack_ authentication modules
can be used to integrate `login' with different authentication
mechanisms such as RSA, DCE, and Kerberos, and thus unify login
mechanisms. The PAM framework can also provide easy integration of
smart cards into the system.
Modular design and pluggability have become important for users who
want ease of use. In the PC hardware arena, no one wants to set the
interrupt vector numbers or resolve the addressing conflict between
various devices. In the software arena, people also want to be able
to replace components easily for easy customization, maintenance, and
upgrades.
Authentication software deserves special attention because
authentication forms a very critical component of any secure computer
system. The authentication infrastructure and its components may
have to be modified or replaced either because some deficiencies have
been found in the current algorithms, or because sites want to
enforce a different security policy than what was provided by the
system vendor. The replacement and modification should be done in
such a way that the user is not affected by these changes.
The solution has to address not just how the applications use the new
authentication mechanisms in a generic fashion, but also how the user
will be authenticated to these mechanisms in a generic way. The
former is addressed by GSS-API [Linn 93], while this RFC addresses
the later; these two efforts are complementary to each other.
Since most system-entry services (for example, `login', `dtlogin',
`rlogin', `ftp', `rsh') may want to be independent of the specific
authentication mechanisms used by the machine, it is important that
there be a framework for _plugging_ in various mechanisms. This
requires that the system applications use a standard API to interact
Samar, Schemers Page 1
OSF-RFC 86.0 PAM October 1995
with the authentication services. If these system-entry services
remain independent of the actual mechanism used on that machine, the
system administrator can install suitable authentication modules
without requiring changes to these applications.
For any security system to be successful, it has to be easy to use.
In the case of authentication, the single most important ease-of-use
characteristic is that the user should not be required to learn about
various ways of authentication and remember multiple passwords.
Ideally, there should be one all-encompassing authentication system
where there is only one password, but for heterogeneous sites,
multiple authentication mechanisms have to co-exist. The problem of
integrating multiple authentication mechanisms such as Kerberos
[Steiner 88], RSA [Rivest 78], and Diffie-Hellman [Diffie 76, Taylor
88], is also referred to as _integrated login_, or _unified login_
problem. Even if the user has to use multiple authentication
mechanisms, the user should not be forced to type multiple passwords.
Furthermore, the user should be able to use the new network identity
without taking any further actions. The key here is in modular
integration of the network authentication technologies with `login'
and other system-entry services.
In this RFC we discuss the architecture and design of pluggable
authentication modules. This design gives the capability to use
field-replaceable authentication modules along with unified login
capability. It thus provides for both _pluggability_ and _ease-of-
use_.
The RFC is organized as follows. We first motivate the need for a
generic way to authenticate the user by various system-entry services
within the operating system. We describe the goals and constraints
of the design. This leads to the architecture, description of the
interfaces, and _stacking_ of modules to get unified login
functionality. We then describe our experience with the design, and
end with a description of future work.
2. OVERVIEW OF IDENTIFICATION AND AUTHENTICATION MECHANISMS
An identification and authentication ("I&A") mechanism is used to
establish a user's identity the system (i.e., to a local machine's
operating system) and to other principals on the network. On a
typical UNIX system, there are various ports of entry into the
system, such as `login', `dtlogin', `rlogin', `ftp', `rsh', `su', and
`telnet'. In all cases, the user has to be identified and
authenticated before granting appropriate access rights to the user.
The user identification and authentication for all these entry points
needs to be coordinated to ensure a secure system.
In most of the current UNIX systems, the login mechanism is based
upon verification of the password using the modified DES algorithm.
Samar, Schemers Page 2
OSF-RFC 86.0 PAM October 1995
The security of the implementation assumes that the password cannot
be guessed, and that the password does not go over the wire in the
clear. These assumptions, however, are not universally valid.
Various programs are now available freely on the Internet that can
run dictionary attack against the encrypted password. Further, some
of the network services (for example, `rlogin', `ftp', `telnet') send
the password over in clear, and there are "sniffer" programs freely
available to steal these passwords. The classical assumptions may be
acceptable on a trusted network, but in an open environment there is
a need to use more restrictive and stronger authentication
mechanisms. Examples of such mechanisms include Kerberos, RSA,
Diffie-Hellman, one-time password [Skey 94], and challenge-response
based smart card authentication systems. Since this list will
continue to evolve, it is important that the system-entry services do
not have hard-coded dependencies on any of these authentication
mechanisms.
3. DESIGN GOALS
The goals of the PAM framework are as follows:
(a) The system administrator should be able to choose the default
authentication mechanism for the machine. This can range from
a simple password-based mechanism to a biometric or a smart
card based system.
(b) It should be possible to configure the user authentication
mechanism on a per application basis. For example, a site may
require S/Key password authentication for `telnet' access,
while allowing machine `login' sessions with just UNIX password
authentication.
(c) The framework should support the display requirements of the
applications. For example, for a graphical login session such
as `dtlogin', the user name and the password may have to be
entered in a new window. For networking system-entry
applications such as `ftp' and `telnet', the user name and
password has to be transmitted over the network to the client
machine.
(d) It should be possible to configure multiple authentication
protocols for each of those applications. For example, one may
want the users to get authenticated by both Kerberos and RSA
authentication systems.
(e) The system administrator should be able to _stack_ multiple
user authentication mechanisms such that the user is
authenticated with all authentication protocols without
retyping the password.
Samar, Schemers Page 3
OSF-RFC 86.0 PAM October 1995
(f) The architecture should allow for multiple passwords if
necessary to achieve higher security for users with specific
security requirements.
(g) The system-entry services should not be required to change when
the underlying mechanism changes. This can be very useful for
third-party developers because they often do not have the
source code for these services.
(h) The architecture should provide for a _pluggable_ model for
system authentication, as well as for other related tasks such
as password, account, and session management.
(i) For backward-compatibility reasons, the PAM API should support
the authentication requirements of the current system-entry
services.
There are certain issues that the PAM framework does not specifically
address:
(a) We focus only on providing a generic scheme through which users
use passwords to establish their identities to the machine.
Once the identity is established, how the identity is
communicated to other interested parties is outside the scope
of this design. There are efforts underway at IETF [Linn 93]
to develop a Generic Security Services Application Interface
(GSSAPI) that can be used by applications for secure and
authenticated communication without knowing the underlying
mechanism.
(b) The _single-signon_ problem of securely transferring the
identity of the caller to a remote site is not addressed. For
example, the problem of delegating credentials from the
`rlogin' client to the other machine without typing the
password is not addressed by our work. We also do not address
the problem of sending the passwords over the network in the
clear.
(c) We do not address the source of information obtained from the
"`getXbyY()'" family of calls (e.g., `getpwnam()'). Different
operating systems address this problem differently. For
example, Solaris uses the name service switch (NSS) to
determine the source of information for the "`getXbyY()'"
calls. It is expected that data which is stored in multiple
sources (such as passwd entries in NIS+ and the DCE registry)
is kept in sync using the appropriate commands (such as
`passwd_export').
Samar, Schemers Page 4
OSF-RFC 86.0 PAM October 1995
4. OVERVIEW OF THE PAM FRAMEWORK
We propose that the goals listed above can be met through a framework
in which authentication modules can be _plugged_ independently of the
application. We call this the _Pluggable Authentication Modules_
(PAM) framework.
The core components of the PAM framework are the authentication
library API (the front end) and the authentication mechanism-specific
modules (the back end), connected through the Service Provider
Interface (SPI). Applications write to the PAM API, while the
authentication-system providers write to the PAM SPI and supply the
back end modules that are independent of the application.
ftp telnet login (Applications)
| | |
| | |
+--------+--------+
|
+-----+-----+
| PAM API | <-- pam.conf file
+-----+-----+
|
+--------+--------+
UNIX Kerberos Smart Cards (Mechanisms)
Figure 1: The Basic PAM Architecture
Figure 1 illustrates the relationship between the application, the
PAM library, and the authentication modules. Three applications
(`login', `telnet' and `ftp') are shown which use the PAM
authentication interfaces. When an application makes a call to the
PAM API, it loads the appropriate authentication module as determined
by the configuration file, `pam.conf'. The request is forwarded to
the underlying authentication module (for example, UNIX password,
Kerberos, smart cards) to perform the specified operation. The PAM
layer then returns the response from the authentication module to the
application.
PAM unifies system authentication and access control for the system,
and allows plugging of associated authentication modules through well
defined interfaces. The plugging can be defined through various
means, one of which uses a configuration file, such as the one in
Table 1. For each of the system applications, the file specifies the
authentication module that should be loaded. In the example below,
`login' uses the UNIX password module, while `ftp' and `telnet' use
the S/Key module.
Samar, Schemers Page 5
OSF-RFC 86.0 PAM October 1995
Table 1: A Simplified View of a Sample PAM Configuration File.
service module_path
------- -----------
login pam_unix.so
ftp pam_skey.so
telnet pam_skey.so
Authentication configuration is only one aspect of this interface.
Other critical components include account management, session
management, and password management. For example, the `login'
program may want to verify not only the password but also whether the
account has aged or expired. Generic interfaces also need to be
provided so that the password can be changed according to the
requirements of the module. Furthermore, the application may want to
log information about the current session as determined by the
module.
Not all applications or services may need all of the above
components, and not each authentication module may need to provide
support for all of the interfaces. For example, while `login' may
need access to all four components, `su' may need access to just the
authentication component. Some applications may use some specific
authentication and password management modules but share the account
and session management modules with others.
This reasoning leads to a partitioning of the entire set of
interfaces into four areas of functionality: (1) authentication, (2)
account, (3) session, and (4) password. The concept of PAM was
extended to these functional areas by implementing each of them as a
separate pluggable module.
Breaking the functionality into four modules helps the module
providers because they can use the system-provided libraries for the
modules that they are not changing. For example, if a supplier wants
to provide a better version of Kerberos, they can just provide that
new authentication and password module, and reuse the existing ones
for account and session.
4.1. Module Description
More details on specific API's are described in Appendix A. A brief
description of four modules follows:
(a) Authentication management: This set includes the
`pam_authenticate()' function to authenticate the user, and the
`pam_setcred()' interface to set, refresh or destroy the user
credentials.
(b) Account management: This set includes the `pam_acct_mgmt()'
function to check whether the authenticated user should be
Samar, Schemers Page 6
OSF-RFC 86.0 PAM October 1995
given access to his/her account. This function can implement
account expiration and access hour restrictions.
(c) Session management: This set includes the `pam_open_session()'
and `pam_close_session()' functions for session management and
accounting. For example, the system may want to store the
total time for the session.
(d) Password management: This set includes a function,
`pam_chauthtok()', to change the password.
5. FRAMEWORK INTERFACES
The PAM framework further provides a set of administrative interfaces
to support the above modules and to provide for application-module
communication. There is no corresponding service provider interface
(SPI) for such functions.
5.1. Administrative Interfaces
Each set of PAM transactions starts with `pam_start()' and ends with
the `pam_end()' function. The interfaces `pam_get_item()' and
`pam_set_item()' are used to read and write the state information
associated with the PAM transaction.
If there is any error with any of the PAM interfaces, the error
message can be printed with `pam_strerror()'.
5.2. Application-Module Communication
During application initialization, certain data such as the user name
is saved in the PAM framework layer through `pam_start()' so that it
can be used by the underlying modules. The application can also pass
opaque data to the module which the modules will pass back while
communicating with the user.
5.3. User-Module Communication
The `pam_start()' function also passes conversation function that has
to be used by the underlying modules to read and write module
specific authentication information. For example, these functions
can be used to prompt the user for the password in a way determined
by the application. PAM can thus be used by graphical, non-
graphical, or networked applications.
Samar, Schemers Page 7
OSF-RFC 86.0 PAM October 1995
5.4. Inter-Module Communication
Though the modules are independent, they can share certain common
information about the authentication session such as user name,
service name, password, and conversation function through the
`pam_get_item()' and `pam_set_item()' interfaces. These API's can
also be used by the application to change the state information after
having called `pam_start()' once.
5.5. Module State Information
The PAM service modules may want to keep certain module-specific
state information about the session. The interfaces `pam_get_data()'
and `pam_set_data()' can be used by the service modules to access and
update module-specific information as needed from the PAM handle.
The modules can also attach a cleanup function with the data. The
cleanup function is executed when `pam_end()' is called to indicate
the end of the current authentication activity.
Since the PAM modules are loaded upon demand, there is no direct
module initialization support in the PAM framework. If there are
certain initialization tasks that the PAM service modules have to do,
they should be done upon the first invocation. However, if there are
certain clean-up tasks to be done when the authentication session
ends, the modules should use `pam_set_data()' to specify the clean-up
functions, which would be called when `pam_end()' is called by the
application.
6. MODULE CONFIGURATION MANAGEMENT
Table 2 shows an example of a configuration file `pam.conf' with
support for authentication, session, account, and password management
modules. `login' has three entries: one each for authentication
processing, session management and account management. Each entry
specifies the module name that should be loaded for the given module
type. In this example, the `ftp' service uses the authentication and
session modules. Note that all services here share the same session
management module, while having different authentication modules.
Samar, Schemers Page 8
OSF-RFC 86.0 PAM October 1995
Table 2: Configuration File (pam.conf) with Different Modules
and Control Flow
service module_type control_flag module_path options
------- ----------- ------------ ----------- -------
login auth required pam_unix_auth.so nowarn
login session required pam_unix_session.so
login account required pam_unix_account.so
ftp auth required pam_skey_auth.so debug
ftp session required pam_unix_session.so
telnet session required pam_unix_session.so
login password required pam_unix_passwd.so
passwd password required pam_unix_passwd.so
OTHER auth required pam_unix_auth.so
OTHER session required pam_unix_session.so
OTHER account required pam_unix_account.so
The first field, _service_, denotes the service (for example,
`login', `passwd', `rlogin'). The name `OTHER' indicates the module
used by all other applications that have not been specified in this
file. This name can also be used if all services have the same
requirements. In the example, since all the services use the same
session module, we could have replaced those lines with a single
`OTHER' line.
The second field, _module_type_, indicates the type of the PAM
functional module. It can be one of `auth', `account', `session', or
`password' modules.
The third field, _control_flag_ determines the behavior of stacking
multiple modules by specifying whether any particular module is
_required_, _sufficient_, or _optional_. The next section describes
stacking in more detail.
The fourth field, _module_path_, specifies the location of the
module. The PAM framework loads this module upon demand to invoke
the required function.
The fifth field, _options_, is used by the PAM framework layer to
pass module specific options to the modules. It is up to the module
to parse and interpret the options. This field can be used by the
modules to turn on debugging or to pass any module specific
parameters such as a timeout value. It is also used to support
unified login as described below. The options field can be used by
the system administrator to fine-tune the PAM modules.
If any of the fields are invalid, or if a module is not found, that
line is ignored and the error is logged as a critical error via
`syslog(3)'. If no entries are found for the given module type, then
the PAM framework returns an error to the application.
Samar, Schemers Page 9
OSF-RFC 86.0 PAM October 1995
7. INTEGRATING MULTIPLE AUTHENTICATION SERVICES WITH STACKING
In the world of heterogeneous systems, the system administrator often
has to deal with the problem of integrating multiple authentication
mechanisms. The user is often required to know about the
authentication command of the new authentication module (for example,
`kinit', `dce_login') after logging into the system. This is not
user-friendly because it forces people to remember to type the new
command and enter the new password. This functionality should be
invisible instead of burdening the user with it.
There are two problems to be addressed here:
(a) Supporting multiple authentication mechanisms.
(b) Providing unified login in the presence of multiple mechanisms.
In the previous section, we described how one could replace the
default authentication module with any other module of choice. Now
we demonstrate how the same model can be extended to provide support
for multiple modules.
7.1. Design for Stacked Modules
One possibility was to provide hard-coded rules in `login' or other
applications requiring authentication services [Adamson 95]. But
this becomes very specific to the particular combination of
authentication protocols, and also requires the source code of the
application. Digital's Security Integration Architecture [SIA 95]
addresses this problem by specifying the same list of authentication
modules for all applications. Since requirements for various
applications can vary, it is essential that the configuration be on a
per-application basis.
To support multiple authentication mechanisms, the PAM framework was
extended to support _stacking_. When any API is called, the back
ends for the stacked modules are invoked in the order listed, and the
result returned to the caller. In Figure 2, the authentication
service of `login' is stacked and the user is authenticated by UNIX,
Kerberos, and RSA authentication mechanisms. Note that in this
example, there is no stacking for session or account management
modules.
Samar, Schemers Page 10
OSF-RFC 86.0 PAM October 1995
login
|
+--------+--------+
| | |
session auth account
| | |
+--+--+ +--+--+ +--+--+
| PAM | | PAM | | PAM |
+--+--+ +--+--+ +--+--+
| | |
UNIX UNIX UNIX
session auth account
|
Kerberos
auth
|
RSA
auth
Figure 2: Stacking With the PAM Architecture
Stacking is specified through additional entries in the configuration
file shown earlier. As shown in Table 2, for each application (such
as `login') the configuration file can specify multiple mechanisms
that have to be invoked in the specified order. When mechanisms
fail, the _control_flag_ decides which error should be returned to
the application. Since the user should not know which authentication
module failed when a bad password was typed, the PAM framework
continues to call other authentication modules on the stack even on
failure. The semantics of the control flag are as follows:
(a) `required': With this flag, the module failure results in the
PAM framework returning the error to the caller _after_
executing all other modules on the stack. For the function to
be able to return success to the application all `required'
modules have to report success. This flag is normally set when
authentication by this module is a _must_.
(b) `optional': With this flag, the PAM framework ignores the
module failure and continues with the processing of the next
module in sequence. This flag is used when the user is allowed
to login even if that particular module has failed.
(c) `sufficient': With this flag, if the module succeeds the PAM
framework returns success to the application immediately
without trying any other modules. For failure cases, the
_sufficient_ modules are treated as `optional'.
Table 3 shows a sample configuration file that stacks the `login'
command. Here the user is authenticated by UNIX, Kerberos, and RSA
authentication services. The `required' key word for _control_flag_
Samar, Schemers Page 11
OSF-RFC 86.0 PAM October 1995
enforces that the user is allowed to login only if he/she is
authenticated by _both_ UNIX and Kerberos services. RSA
authentication is optional by virtue of the `optional' key word in
the _control_flag_ field. The user can still log in even if RSA
authentication fails.
Table 3: PAM Configuration File with Support for Stacking
service module_type control_flag module_path options
------- ----------- ------------ ----------- -------
login auth required pam_unix.so debug
login auth required pam_kerb.so use_mapped_pass
login auth optional pam_rsa.so use_first_pass
Table 4 illustrates the use of the sufficient flag for the `rlogin'
service. The Berkeley `rlogin' protocol specifies that if the remote
host is trusted (as specified in the `/etc/hosts.equiv' file or in
the `.rhosts' file in the home directory of the user), then the
`rlogin' daemon should not require the user to type the password. If
this is not the case, then the user is required to type the password.
Instead of hard coding this policy in the `rlogin' daemon, this can
be expressed with the `pam.conf' file in Table 4. The PAM module
`pam_rhosts_auth.so.1' implements the `.rhosts' policy described
above. If a site administrator wants to enable remote login with
only passwords, then the first line should be deleted.
Table 4: PAM Configuration File for the rlogin service
service module_type control_flag module_path options
------- ----------- ------------ ----------- -------
rlogin auth sufficient pam_rhosts_auth.so
rlogin auth required pam_unix.so
7.2. Password-Mapping
Multiple authentication mechanisms on a machine can lead to multiple
passwords that users have to remember. One attractive solution from
the ease-of-use viewpoint is to use the same password for all
mechanisms. This, however, can also weaken the security because if
that password were to be compromised in any of the multiple
mechanisms, all mechanisms would be compromised at the same time.
Furthermore, different authentication mechanisms may have their own
distinctive password requirements in regards to its length, allowed
characters, time interval between updates, aging, locking, and so
forth. These requirements make it problematic to use the same
password for multiple authentication mechanisms.
The solution we propose, while not precluding use of the same
password for every mechanism, allows for a different password for
each mechanism through what we call _password-mapping_. This
basically means using the user's _primary_ password to encrypt the
Samar, Schemers Page 12
OSF-RFC 86.0 PAM October 1995
user's other (_secondary_) passwords, and storing these encrypted
passwords in a place where they are available to the user. Once the
primary password is verified, the authentication modules would obtain
the other passwords for their own mechanisms by decrypting the
mechanism-specific encrypted password with the primary password, and
passing it to the authentication service. The security of this
design for password-mapping assumes that the primary password is the
user's strongest password, in terms of its unguessability (length,
type and mix of characters used, etc.).
If there is any error in password-mapping, or if the mapping does not
exist, the user will be prompted for the password by each
authentication module.
To support password-mapping, the PAM framework saves the primary
password and provides it to stacked authentication modules. The
password is cleared out before the `pam_authenticate' function
returns.
How the password is encrypted depends completely on the module
implementation. The encrypted secondary password (also called a
"mapped password") can be stored in a trusted or untrusted place,
such as a smart card, a local file, or a directory service. If the
encrypted passwords are stored in an untrusted publicly accessible
place, this does provide an intruder with opportunities for potential
dictionary attack.
Though password-mapping is voluntary, it is recommended that all
module providers add support for the following four mapping options:
(a) `use_first_pass': Use the same password used by the first
mechanism that asked for a password. The module should not ask
for the password if the user cannot be authenticated by the
first password. This option is normally used when the system
administrator wants to enforce the same password across
multiple modules.
(b) `try_first_pass': This is the same as `use_first_pass', except
that if the primary password is not valid, it should prompt the
user for the password.
(c) `use_mapped_pass': Use the password-mapping scheme to get the
actual password for this module. One possible implementation
is to get the mapped-password using the XFN API [XFN 94], and
decrypt it with the primary password to get the module-specific
password. The module should not ask for the password if the
user cannot be authenticated by the first password. The XFN
API allows user-defined attributes (such as _mapped-password_)
to be stored in the _user-context_. Using the XFN API is
particularly attractive because support for the XFN may be
found on many systems in the future.
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OSF-RFC 86.0 PAM October 1995
(d) `try_mapped_pass': This is the same as `use_mapped_pass',
except that if the primary password is not valid, it should
prompt the user for the password.
When passwords get updated, the PAM framework stores both the old as
well as the new password to be able to inform other dependent
authentication modules about the change. Other modules can use this
information to update the encrypted password without forcing the user
to type the sequence of passwords again. The PAM framework clears
out the passwords before returning to the application.
Table 3 illustrates how the same password can be used by `login' for
authenticating to the standard UNIX login, Kerberos and RSA services.
Once the user has been authenticated to the primary authentication
service (UNIX `login' in this example) with the primary password, the
option `use_mapped_pass' indicates to the Kerberos module that it
should use the primary password to decrypt the stored Kerberos
password and then use the Kerberos password to get the ticket for the
ticket-granting-service. After that succeeds, the option
`use_first_pass' indicates to the RSA module that instead of
prompting the user for a password, it should use the primary password
typed earlier for authenticating the user. Note that in this
scenario, the user has to enter the password just once.
Note that if a one-time password scheme (e.g., S/Key) is used,
password mapping cannot apply.
7.3. Implications of Stacking on the PAM Design
Because of the stacking capability of PAM, we have designed the PAM
API's to not return any data to the application, except status. If
this were not the case, it would be difficult for the PAM framework
to decide which module should return data to the application. When
there is any error, the application does not know which of the
modules failed. This behavior enables (even requires) the
application to be completely independent from the modules.
Another design decision we have made is that PAM gives only the user
name to all the underlying PAM modules, hence it is the
responsibility of the PAM modules to convert the name to their own
internal format. For example, the Kerberos module may have to
convert the UNIX user name to a Kerberos principal name.
Stacking also forces the modules to be designed such that they can
occur anywhere in the stack without any side-effects.
Since modules such as the authentication and the password module are
very closely related, it is important they be configured in the same
order and with compatible options.
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OSF-RFC 86.0 PAM October 1995
8. INTEGRATION WITH SMART CARDS
Many networking authentication protocols require possession of a long
key to establish the user identity. For ease-of-use reasons, that
long key is normally encrypted with the user's password so that the
user is not required to memorize it. However, weak passwords can be
compromised through a dictionary attack and thus undermine the
stronger network authentication mechanism. Furthermore, the
encrypted data is normally stored in a centrally accessible service
whose availability depends upon the reliability of the associated
service. Solutions have been proposed to use a pass-phrase or one-
time-password, but those are much longer than the regular eight
character passwords traditionally used with UNIX `login'. This makes
the solution user-unfriendly because it requires longer strings to be
remembered and typed.
For most authentication protocol implementations, the trust boundary
is the local machine. This assumption may not be valid in cases
where the user is mobile and has to use publicly available networked
computers. In such cases, it is required that the clear text of the
key or the password never be made available to the machine.
Smart cards solve the above problems by reducing password exposure by
supporting a _two factor_ authentication mechanism: the first with
the possession of the card, and the second with the knowledge of the
PIN associated with the card. Not only can the smart cards be a
secure repository of multiple passwords, they can also provide the
encryption and authentication functions such that the long (private)
key is never exposed outside the card.
The PAM framework allows for integrating smart cards to the system by
providing a smart card specific module for authentication.
Furthermore, the unified login problem is simplified because the
multiple passwords for various authentication mechanisms can be
stored on the smart card itself. This can be enabled by adding a
suitable key-word such as `use_smart_card' in the _options_ field.
9. SECURITY ISSUES
It is important to understand the impact of PAM on the security of
any system so that the site-administrator can make an informed
decision.
(a) Sharing of passwords with multiple authentication mechanisms.
If there are multiple authentication modules, one possibility
is to use the same password for all of them. If the password
for any of the multiple authentication system is compromised,
the user's password in all systems would be compromised. If
this is a concern, then multiple passwords might be considered
Samar, Schemers Page 15
OSF-RFC 86.0 PAM October 1995
at the cost of ease-of-use.
(b) Password-mapping.
This technique of encrypting all other passwords with the
primary password assumes that it is lot more difficult to crack
the primary password and that reasonable steps have been taken
to ensure limited availability of the encrypted primary
password. If this is not done, an intruder could target the
primary password as the first point of dictionary attack. If
one of the other modules provide stronger security than the
password based security, the site would be negating the strong
security by using password-mapping. If this is a concern, then
multiple passwords might be considered at the cost of ease-of-
use. If smart cards are used, they obviate the need for
password-mapping completely.
(c) Security of the configuration file.
Since the policy file dictates how the user is authenticated,
this file should be protected from unauthorized modifications.
(d) Stacking various PAM modules.
The system administrator should fully understand the
implications of stacking various modules that will be installed
on the system and their respective orders and interactions.
The composition of various authentication modules should be
carefully examined. The trusted computing base of the machine
now includes the PAM modules.
10. EXPERIENCE WITH PAM
The PAM framework was first added in Solaris 2.3 release as a private
internal interface. PAM is currently being used by several system
entry applications such as `login', `passwd', `su', `dtlogin',
`rlogind', `rshd', `telnetd', `ftpd', `in.rexecd', `uucpd', `init',
`sac', and `ttymon'. We have found that PAM provides an excellent
framework to encapsulate the authentication-related tasks for the
entire system. The Solaris 2.3 PAM API's were hence enhanced and
simplified to support stacking.
PAM modules have been developed for UNIX, DCE, Kerberos, S/Key,
remote user authentication, and dialpass authentication. Other PAM
modules are under development, and integration with smart cards is
being planned.
Some third parties have used the PAM interface to extend the security
mechanisms offered by the Solaris environment.
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OSF-RFC 86.0 PAM October 1995
The PAM API has been accepted by Common Desktop Environment (CDE)
vendors as the API to be used for integrating the graphical interface
for login, `dtlogin' with multiple authentication mechanisms.
11. FUTURE WORK
Amongst the various components of PAM, the password component needs
to be carefully examined to see whether the stacking semantics are
particularly applicable, and how PAM should deal with partial
failures when changing passwords.
The _control_flag_ of the configuration file can be extended to
include other semantics. For example, if the error is "name service
not available", one may want to retry. It is also possible to offer
semantics of "return success if any of the modules return success".
In an earlier section, we had mentioned integration of smart cards
with PAM. Though we feel that integration should be straight forward
from the PAM architecture point of view, there may be some issues
with implementation because the interfaces to the smart cards have
not yet been standardized.
One possible extension to PAM is to allow the passing of module-
specific data between applications and PAM modules. For example, the
`login' program likes to build its new environment from a select list
of variables, yet the DCE module needs the `KRB5CCNAME' variable to
be exported to the child process. For now we have modified the
`login' program to explicitly export the `KRB5CCNAME' variable.
Administrative tools are needed to help system administrators modify
`pam.conf', and perform sanity checks on it (i.e., a `pam_check'
utility).
12. CONCLUSION
The PAM framework and the module interfaces provide pluggability for
user authentication, as well as for account, session and password
management. The PAM architecture can be used by `login' and by all
other system-entry services, and thus ensure that all entry points
for the system have been secured. This architecture enables
replacement and modification of authentication modules in the field
to secure the system against the newly found weaknesses without
changing any of the system services.
The PAM framework can be used to integrate `login' and `dtlogin' with
different authentication mechanisms such as RSA and Kerberos.
Multiple authentication systems can be accessed with the same
password. The PAM framework also provides easy integration of smart
cards into the system.
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OSF-RFC 86.0 PAM October 1995
PAM provides complementary functionality to GSS-API, in that it
provides mechanisms through which the user gets authenticated to any
new system-level authentication service on the machine. GSS-API then
uses the credentials for authenticated and secure communications with
other application-level service entities on the network.
13. ACKNOWLEDGEMENTS
PAM development has spanned several release cycles at SunSoft.
Shau-Ping Lo, Chuck Hickey, and Alex Choy did the first design and
implementation. Bill Shannon and Don Stephenson helped with the PAM
architecture. Rocky Wu prototyped stacking of multiple modules.
Paul Fronberg, Charlie Lai, and Roland Schemers made very significant
enhancements to the PAM interfaces and took the project to completion
within a very short time. Kathy Slattery wrote the PAM
documentation. John Perry integrated PAM within the CDE framework.
APPENDIX A. PAM API'S
This appendix gives an informal description of the various interfaces
of PAM. Since the goal here is just for the reader to get a working
knowledge about the PAM interfaces, not all flags and options have
been fully defined and explained. The API's described here are
subject to change.
The PAM Service Provider Interface is very similar to the PAM API,
except for one extra parameter to pass module-specific options to the
underlying modules.
A.1. Framework Layer API's
int
pam_start(
char *service_name,
char *user,
struct pam_conv *pam_conversation,
pam_handle_t **pamh
);
`pam_start()' is called to initiate an authentication transaction.
`pam_start()' takes as arguments the name of the service, the name of
the user to be authenticated, the address of the conversation
structure. `pamh' is later used as a handle for subsequent calls to
the PAM library.
The PAM modules do not communicate directly with the user; instead
they rely on the application to perform all such interaction. The
application needs to provide the conversation functions, `conv()',
and associated application data pointers through a `pam_conv'
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OSF-RFC 86.0 PAM October 1995
structure when it initiates an authentication transaction. The
module uses the `conv()' function to prompt the user for data,
display error messages, or text information.
int
pam_end(
pam_handle_t *pamh,
int pam_status
);
`pam_end()' is called to terminate the PAM transaction as specified
by `pamh', and to free any storage area allocated by the PAM modules
with `pam_set_item()'.
int
pam_set_item(
pam_handle_t *pamh,
int item_type,
void *item
);
int
pam_get_item(
pam_handle_t *pamh,
int item_type,
void **item);
`pam_get_item()' and `pam_set_item()' allow the parameters specified
in the initial call to `pam_start()' to be read and updated. This is
useful when a particular parameter is not available when
`pam_start()' is called or must be modified after the initial call to
`pam_start()'. `pam_set_item()' is passed a pointer to the object,
`item', and its type, `item_type'. `pam_get_item()' is passed the
address of the pointer, `item', which is assigned the address of the
requested object.
The `item_type' is one of the following:
Table 5: Possible Values for Item_type
Item Name Description
--------- -----------
PAM_SERVICE The service name
PAM_USER The user name
PAM_TTY The tty name
PAM_RHOST The remote host name
PAM_CONV The pam_conv structure
PAM_AUTHTOK The authentication token (password)
PAM_OLDAUTHTOK The old authentication token
PAM_RUSER The remote user name
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OSF-RFC 86.0 PAM October 1995
Note that the values of `PAM_AUTHTOK' and `PAM_OLDAUTHTOK' are only
available to PAM modules and not to the applications. They are
explicitly cleared out by the framework before returning to the
application.
char *
pam_strerror(
int errnum
);
`pam_strerror()' maps the error number to a PAM error message string,
and returns a pointer to that string.
int
pam_set_data(
pam_handle_t *pamh,
char *module_data_name,
char *data,
(*cleanup)(pam_handle_t *pamh, char *data,
int error_status)
);
The `pam_set_data()' function stores module specific data within the
PAM handle. The `module_data_name' uniquely specifies the name to
which some data and cleanup callback function can be attached. The
cleanup function is called when `pam_end()' is invoked.
int
pam_get_data(
pam_handle_t *pamh,
char *module_data_name,
void **datap
);
The `pam_get_data()' function obtains module-specific data from the
PAM handle stored previously by the `pam_get_data()' function. The
`module_data_name' uniquely specifies the name for which data has to
be obtained. This function is normally used to retrieve module
specific state information.
A.2. Authentication API's
int
pam_authenticate(
pam_handle_t *pamh,
int flags
);
The `pam_authenticate()' function is called to verify the identity of
the current user. The user is usually required to enter a password
or similar authentication token, depending upon the authentication
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OSF-RFC 86.0 PAM October 1995
module configured with the system. The user in question is specified
by a prior call to `pam_start()', and is referenced by the
authentication handle, `pamh'.
int
pam_setcred(
pam_handle_t *pamh,
int flags
);
The `pam_setcred()' function is called to set the credentials of the
current process associated with the authentication handle, `pamh'.
The actions that can be denoted through `flags' include credential
initialization, refresh, reinitialization and deletion.
A.3. Account Management API
int
pam_acct_mgmt(
pam_handle_t *pamh,
int flags
);
The function `pam_acct_mgmt()' is called to determine whether the
current user's account and password are valid. This typically
includes checking for password and account expiration, valid login
times, etc. The user in question is specified by a prior call to
`pam_start()', and is referenced by the authentication handle,
`pamh'.
A.4. Session Management API's
int
pam_open_session(
pam_handle_t *pamh,
int flags
);
`pam_open_session()' is called to inform the session modules that a
new session has been initialized. All programs which use PAM should
invoke `pam_open_session()' when beginning a new session.
int
pam_close_session(
pam_handle_t *pamh,
int flags
);
Upon termination of this session, the `pam_close_session()' function
should be invoked to inform the underlying modules that the session
has terminated.
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OSF-RFC 86.0 PAM October 1995
A.5. Password Management API's
int
pam_chauthtok(
pam_handle_t *pamh,
int flags
);
`pam_chauthtok()' is called to change the authentication token
associated with the user referenced by the authentication handle
`pamh'. After the call, the authentication token of the user will be
changed in accordance with the authentication module configured on
the system.
APPENDIX B. SAMPLE PAM APPLICATION
This appendix shows a sample `login' application which uses the PAM
API's. It is not meant to be a fully functional login program, as
some functionality has been left out in order to emphasize the use of
PAM API's.
#include <security/pam_appl.h>
static int login_conv(int num_msg, struct pam_message **msg,
struct pam_response **response, void *appdata_ptr);
static struct pam_conv pam_conv = {login_conv, NULL};
static pam_handle_t *pamh; /* Authentication handle */
void
main(int argc, char *argv[], char **renvp)
{
/*
* Call pam_start to initiate a PAM authentication operation
*/
if ((pam_start("login", user_name, &pam_conv, &pamh))
!= PAM_SUCCESS)
login_exit(1);
pam_set_item(pamh, PAM_TTY, ttyn);
pam_set_item(pamh, PAM_RHOST, remote_host);
while (!authenticated && retry < MAX_RETRIES) {
status = pam_authenticate(pamh, 0);
authenticated = (status == PAM_SUCCESS);
}
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OSF-RFC 86.0 PAM October 1995
if (status != PAM_SUCCESS) {
fprintf(stderr,"error: %s\n", pam_strerror(status));
login_exit(1);
}
/* now check if the authenticated user is allowed to login. */
if ((status = pam_acct_mgmt(pamh, 0)) != PAM_SUCCESS) {
if (status == PAM_AUTHTOK_EXPIRED) {
status = pam_chauthtok(pamh, 0);
if (status != PAM_SUCCESS)
login_exit(1);
} else {
login_exit(1);
}
}
/*
* call pam_open_session to open the authenticated session
* pam_close_session gets called by the process that
* cleans up the utmp entry (i.e., init)
*/
if (status = pam_open_session(pamh, 0) != PAM_SUCCESS) {
login_exit(status);
}
/* set up the process credentials */
setgid(pwd->pw_gid);
/*
* Initialize the supplementary group access list.
* This should be done before pam_setcred because
* the PAM modules might add groups during the pam_setcred call
*/
initgroups(user_name, pwd->pw_gid);
status = pam_setcred(pamh, PAM_ESTABLISH_CRED);
if (status != PAM_SUCCESS) {
login_exit(status);
}
/* set the real (and effective) UID */
setuid(pwd->pw_uid);
pam_end(pamh, PAM_SUCCESS); /* Done using PAM */
/*
* Add DCE/Kerberos cred name, if any.
* XXX - The module specific stuff should be removed from login
* program eventually. This is better placed in DCE module and
* will be once PAM has routines for "exporting" environment
Samar, Schemers Page 23
OSF-RFC 86.0 PAM October 1995
* variables.
*/
krb5p = getenv("KRB5CCNAME");
if (krb5p != NULL) {
ENVSTRNCAT(krb5ccname, krb5p);
envinit[basicenv++] = krb5ccname;
}
environ = envinit; /* Switch to the new environment. */
exec_the_shell();
/* All done */
}
/*
* login_exit - Call exit() and terminate.
* This function is here for PAM so cleanup can
* be done before the process exits.
*/
static void
login_exit(int exit_code)
{
if (pamh)
pam_end(pamh, PAM_ABORT);
exit(exit_code);
/*NOTREACHED*/
}
/*
* login_conv():
* This is the conv (conversation) function called from
* a PAM authentication module to print error messages
* or garner information from the user.
*/
static int
login_conv(int num_msg, struct pam_message **msg,
struct pam_response **response, void *appdata_ptr)
{
while (num_msg--) {
switch (m->msg_style) {
case PAM_PROMPT_ECHO_OFF:
r->resp = strdup(getpass(m->msg));
break;
case PAM_PROMPT_ECHO_ON:
(void) fputs(m->msg, stdout);
r->resp = malloc(PAM_MAX_RESP_SIZE);
fgets(r->resp, PAM_MAX_RESP_SIZE, stdin);
/* add code here to remove \n from fputs */
Samar, Schemers Page 24
OSF-RFC 86.0 PAM October 1995
break;
case PAM_ERROR_MSG:
(void) fputs(m->msg, stderr);
break;
case PAM_TEXT_INFO:
(void) fputs(m->msg, stdout);
break;
default:
/* add code here to log error message, etc */
break;
}
}
return (PAM_SUCCESS);
}
APPENDIX C. DCE MODULE
This appendix describes a sample implementation of a DCE PAM module.
In order to simplify the description, we do not address the issues
raised by password-mapping or stacking. The intent is to show which
DCE calls are being made by the DCE module.
The `pam_sm_*()' functions implement the PAM SPI functions which are
called from the PAM API functions.
C.1. DCE Authentication Management
The algorithm for authenticating with DCE (not including error
checking, prompting for passwords, etc.) is as follows:
pam_sm_authenticate()
{
sec_login_setup_identity(...);
pam_set_data(...);
sec_login_valid_and_cert_ident(...);
}
pam_sm_setcred()
{
pam_get_data(...);
sec_login_set_context(...);
}
The `pam_sm_authenticate()' function for DCE uses the
`pam_set_data()' and `pam_get_data()' functions to keep state (like
the `sec_login_handle_t' context) between calls. The following
cleanup function is also registered and gets called when `pam_end()'
Samar, Schemers Page 25
OSF-RFC 86.0 PAM October 1995
is called:
dce_cleanup()
{
if (/* PAM_SUCCESS and
sec_login_valid_and_cert_ident success */) {
sec_login_release_context(...);
} else {
sec_login_purge_context(...);
}
}
If everything was successful we release the login context, but leave
the credentials file intact. If the status passed to `pam_end()' was
not `PAM_SUCCESS' (i.e., a required module failed) we purge the login
context which also removes the credentials file.
C.2. DCE Account Management
The algorithm for DCE account management is as follows:
pam_sm_acct_mgmt()
{
pam_get_data(...);
sec_login_inquire_net_info(...);
/* check for expired password and account */
sec_login_free_net_info(...);
}
The `sec_login_inquire_net_info()' function is called to obtain
information about when the user's account and/or password are going
to expire. A warning message is displayed (using the conversation
function) if the user's account or password is going to expire in the
near future, or has expired. These warning messages can be disabled
using the `nowarn' option in the `pam.conf' file.
C.3. DCE Session Management
The DCE session management functions are currently empty. They could
be modified to optionally remove the DCE credentials file upon
logout, etc.
C.4. DCE Password Management
The algorithm for DCE password management is as follows:
Samar, Schemers Page 26
OSF-RFC 86.0 PAM October 1995
pam_sm_chauthtok
{
sec_rgy_site_open(...);
sec_rgy_acct_lookup(...);
sec_rgy_acct_passwd(...);
sec_rgy_site_close(...);
}
The `sec_rgy_acct_passwd()' function is called to change the user's
password in the DCE registry.
REFERENCES
[Adamson 95] W. A. Adamson, J. Rees, and P. Honeyman, "Joining
Security Realms: A Single Login for Netware and
Kerberos", CITI Technical Report 95-1, Center for
Information Technology Integration, University of
Michigan, Ann Arbor, MI, February 1995.
[Diffie 76] W. Diffie and M. E. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information
Theory, November 1976.
[Linn 93] J. Linn, "Generic Security Service Application
Programming Interface", Internet RFC 1508, 1509, 1993.
[Rivest 78] R. L. Rivest, A. Shamir, and L. Adleman., "A Method
for Obtaining Digital Signatures and Pubic-key
Cryptosystems", Communications of the ACM, 21(2),
1978.
[SIA 95] "Digital UNIX Security", Digital Equipment
Corporation, Order Number AA-Q0R2C-TE, July 1995.
[Skey 94] N. M. Haller, "The S/Key One-Time Password System",
ISOC Symposium on Network and Distributed Security,
1994.
[Steiner 88] J.G. Steiner, B. C. Neuman, and J. I. Schiller,
"Kerberos, An Authentication Service for Open Network
Systems", in Proceedings of the Winter USENIX
Conference, Dallas, Jan 1988.
[Taylor 88] B. Taylor and D. Goldberg, "Secure Networking in the
Sun Environment", Sun Microsystems Technical Paper,
1988.
[XFN 94] "Federated Naming: the XFN Specifications", X/Open
Preliminary Specification, X/Open Document #P403,
ISBN:1-85912-045-8, X/Open Co. Ltd., July 1994.
Samar, Schemers Page 27
OSF-RFC 86.0 PAM October 1995
AUTHOR'S ADDRESS
Vipin Samar Internet email: vipin@eng.sun.com
SunSoft, Inc. Telephone: +1-415-336-1002
2550 Garcia Avenue
Mountain View, CA 94043
USA
Roland J. Schemers III Internet email: schemers@eng.sun.com
SunSoft, Inc. Telephone: +1-415-336-1035
2550 Garcia Avenue
Mountain View, CA 94043
USA
Samar, Schemers Page 28