OSEC

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Subject: Sendmail Workaround for Linux Capabilities Bug
From: Sendmail Security (sendmail-securitySendmail.ORG)
Date: Wed Jun 07 2000 - 20:38:01 CDT


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                SENDMAIL SECURITY TEAM ADVISORY

        Sendmail Workaround for Linux Capabilities Bug

The Sendmail Consortium and Sendmail, Inc. has been informed of a
serious problem in the Linux kernel that can be used to get root
access. This is not a sendmail security problem, although sendmail
is one of the vectors for this attack.

PROBLEM

        There is a bug in the Linux kernel capability model for versions
        through 2.2.15 that allows local users to get root. Sendmail is
        one of the programs that can be attacked this way. This problem
        may occur in other capabilities-based kernels.

SOLUTION

        The correct fix is to update your Linux kernel to version
        2.2.16. This is the only way to ensure that other programs
        running on Linux cannot be attacked by this bug.

WORKAROUND

        Sendmail 8.10.2 has added a check to see if the kernel has
        this bug, and if so will refuse to run. This version also
        does more detailed checks on certain system calls, notably
        setuid(2), to detect other possible attacks. Although there
        are no known attacks, this version is strongly recommended,
        whether or not you have a vulnerable kernel.

        Sendmail 8.10.2 can be obtained from:

        ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.10.2.tar.gz
        ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.10.2.tar.Z
        ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.10.2.tar.sig

        and has MD5 signatures:

        acb8b6f50869a058a9baaa4fb4692c4b sendmail.8.10.2.tar.Z
        00705e5ca3412604cebd052e0d7aefcd sendmail.8.10.2.tar.gz
        92dca37fb68a2a44f02c292656c123b6 sendmail.8.10.2.tar.sig

        You only need one of the first two files (either the gzip'ed
        version or the compressed version). The .sig file is a PGP
        signatures of the tar file (after uncompressing it). It is
        signed with the Sendmail Signing Key/2000, available on the web
        site (http://www.sendmail.org/) or on the public key servers.

        Note however that installing this sendmail patch does not
        fully protect you from attack. Other programs are probably
        vulnerable.

ACKNOWLEDGEMENTS

        Several people contributed to this advisory. Wojciech Purczynski
        of Elzab Soft first identified the problem. Alan Cox verified
        and patched the Linux kernel bug. Gregory Neil Shapiro and other
        members of the Sendmail Consortium helped identify the problem
        and produce the sendmail workaround.

DETAILS OF THE VULNERABILITY

        The problem lies in the setcap(2) call, which is not documented
        on most Linux-based systems (we think there might be a man page
        on Mandrake). This call, based on the unratified Posix 1e draft,
        attempts to break down root permissions into a series of
        capabilities. Normally root has all capabilities and normal
        users have none of the capabilities.

        One such capability is the ability of a process to do an
        arbitrary setuid(2) call. As documented in ISO/IEC 9945-1
        (ANSI/IEEE Std 1003.1) POSIX Part 1:

                4.2.2.2 Description
                ...
                   If {_POSIX_SAVED_IDS} is defined:

                   (1) If the process has appropriate privileges, the
                       setuid() function sets the real user ID, effective
                       user ID, and the saved set-user-ID to uid.

                   (2) If the process does not have the appropriate
                       privileges, but uid is equal to the real user ID
                       or the saved set-user-ID, the setuid() function
                       sets the effective user ID to uid; the real user
                       ID and saved set-user-ID remain unchanged by this
                       function call.

        The CAP_SETUID capability represents the "appropriate privileges".

        Normally this would not be an issue, since a setuid root program
        would simply have capability reinstated. However, Linux has
        an added capability CAP_SETPCAP that controls the ability of a
        process to inherit capabilities; this capability does affect
        setuid programs. It is possible to set the capabilities such
        that a setuid program does not have "appropriate privileges."
        The effect of this is that a root program cannot fully give up
        its root privileges (since the saved set-user-ID cannot be
        reset).

        Note that checking the return value from setuid() is insufficient;
        the setuid(getuid()) succeeds even when the process does not have
        "appropriate privileges."

        The sendmail patch attempts a setuid(0) after a setuid(getuid());
        under normal circumstances this should fail (unless of course
        the real uid is root). If this setuid(0) succeeds, then the
        kernel has failed to properly give up permissions and sendmail
        will refuse to continue running.

        This problem can, of course, appear in any setuid root program
        that attempts to cede special permissions.

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