pam_cookie PAM module

Description

pam_cookie is intended to allow a one-time-password (OTP) token to remain valid over a period of time instead of just once. For example this allows the use of OTPs for authenticating against a web or imap server. An optional 'cookie' mode may extend the validity period every time the correct OTP is entered. It basically 'wraps' around an existing pam auth module, caches correct passwords, and skips the wrapped auth module for a certain time period. It was conceived with pam_mobile_otp in mind but it should work with any pam authentication module.

Pwauth may be used in conjunction with mod-authn-external to connect the apache web server to PAM.

Send comments and bug reports to John Whitbeck <john@whitbeck.fr>

Platforms

This module has been developed and tested on the following platforms:

• Ubuntu Linux 11.04 and 10.04

Please let me know if you were able to get it to work on different platforms.

Requirements

• Openssl development headers
• LibPAM development headers

Installation

The only supported pam module is "auth"

For installation you must:

1. Run 'sudo make install'. This will move pam_cookie.so to /lib/security and create the directory /var/cache/pam_cookie

2. Set the permissions on the /var/cache/pam_cookie directory so that the process using the module will have write access to it. For example:

    $ sudo chmod 6660 /var/cache/pam_cookie
    $ sudo chown www-data:www-data /var/cache/pam_cookie
   

3. Configure the relevant pam config file. This part is a little tricky so having a good knowledge of how PAM modules are configured and interact work is essential. Make sure you understand the example config below. It is very easy to either completely brick you system or open it up to the world.

4. Try to log in!

Configuration

The auth module has two modes: 'auth' and 'touch'. One or the either must be specified. Here are the pam config options:

auth

Set auth mode. Auth mode read the current password and compares it to its database to check it is valid and if has not expired.

touch

Set touch mode. Touch mode creates new entries in the database for passwords that were validated by another module, or updates the timestamps of existing passwords.

use_first_pass

Only use with auth mode. If set, the module does not ask for the passcode, but uses the password given earlier to an other module.

try_first_pass

Only use with auth mode. If set, the module only asks for a passcode, if password that was set earlier is not the correct passcode.

interval=<value>

Only use with auth mode. Value is set in minutes and defaults to 10. Let last_t be the password's last timestamp and cur_t be the system's current time. If (cur_t > last_t+value), then the password has expired and the module rejects it.

lifetime=<value>

Only use with touch mode. Value is set in minutes and defaults to 0 (ie infinity). Let crea_t be the time when the password was entered into the database. Let cur_t be the system's current time. If (cur_t > crea_t + value), then the password has expired and the module rejects it.

cookie

Only use with touch mode. Every time a saved password is entered correctly, update it's timestamp to the system's current time. If lifetime is unset, then that password could conceivably remain valid as long as it keeps getting updated every 'interval' minutes.

debug

Log debugging information to /var/log/auth.log. Warning: this will print passwords in cleartext into your log files.

strict

User gets authenticated only when he uses the same IP address. The IP address is determind by the PAM_RHOST item. The option should be set in both auth and touch mode.

strip_last_n_pw_chars=<value>

Only use with auth mode. Strip last n characters from the password when successful authentication has been made. Note that the password hash used to compare with the one store in the cookie is also made with characters stripped. Useful when using some kind of OTP system in the pam chain. See example below.

digest_name=<value>

Use with both auth and touch mode. Name of the digest to use when storing the password in the cookie. The value should be accepted by the EVP_get_digestbyname function. The default value is 'SHA256'.

Example #1

To enable 10-minute renewable OTPs with a maximum lifetime of 30 minutes, you could configure one of your pam services (e.g. /etc/pam.d/pwauth) to look like this:

auth    [success=1 default=ignore]      pam_cookie.so auth interval=10
auth    [success=ok default=1]          pam_mobile_otp.so use_first_pass
auth    [default=1]                     pam_cookie.so touch cookie lifetime=30
auth    requisite                       pam_deny.so
auth    required                        pam_permit.so

A quick explanation of the various lines:

• Line 1: This is the 'auth' mode. Here pam_cookie asks for the user password an compares it to the ones in its database. If it is identical and is was last used less than 10minutes ago (interval=10) and first used less 30mins ago (lifetime=30 on line 3) the module succeeds. It then skips pam_mobile_otp.so and goes straight to line 3. If it fails, the auth process goes to line 3.

• Line 2: standard pam_mobile_otp.so authentication. If this succeeds, proceed to line 3, if it fails, it skips line 3 and goes straight to line 4 (default=1 param).

• Line 3: Here pam_cookie.so is in the 'touch' mode. Two options: (i) if the password does not exist in the database, then create it and set a 30min lifetime; or (ii) if the password is already in the databse, since the 'cookie' option was passed, update the password's timestamp with the current time. This line always succeeds and then proceeds the line 5.

• Line 4: fail and return

• Line 5: return success

Example #2

Imagine you want to have HTTP Basic Auth enabled to authenticate users, along with pam_google_authenticator which adds support of OTP codes. In this case the pam configuration could look like this:

auth    [success=3 default=ignore]  pam_cookie.so auth strip_last_n_pw_chars=6
auth    [success=ok default=1]      pam_google_authenticator.so forward_pass
auth    [success=1 default=ignore]  pam_pgsql.so use_first_pass
auth    requisite           pam_deny.so
auth    required            pam_permit.so
auth    optional            pam_cookie.so cookie touch

The password send to the browser consists of two parts <password>+<otp>.

Explanation:

• Line 1: Here pam_cookie gets password and compares it to the ones in its database. The strip_last_n_pw_chars tells module to remove the <otp> part so the <password> is always the same. When module authenticates the user it removes <otp> part and forwords only <password> to the next module. When authentication is negative the whole phrase <password>+<otp> is forwarded to the next module.

• Line 2: pam_google_authenticator always gets full passphrase: <password>+<otp> and forwards <password> part to the next module on successful authentication.

• Line 3: PAM module perform actual authentication. It could be any module. The point is that the authentication can last several seconds. That is why we need pam_cookie.

• Line 4: Fail when previous module fails to authenticate the user.

• Line 5: Permit user.

• Line 6: Touch the cookie. This module always gets only <password> part.

Security considerations

• Extending the validity period of a one-time password degrades its security. Only use where striclty needed (e.g. web server) and not where it is useless (e.g. ssh). Make sure you understand the 'interval' and 'lifetime' options and set them accordingly. Furthermore, you should protect your web server against brute-force attacks using for example fail2ban.

• Functions are not thread safe.

License

This program is free software; you can redistribute it and/or modify it under the terms of the GNU Library General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.

This software is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public License for more details.