Articles / SuSE: screen local root com…

SuSE: screen local root compromise

The screen package allows a local attacker to obtain root privileges if the /usr/bin/screen command is installed setuid root and if a directory below /tmp/screens/ exists. The screen program needs root permissions from the setuid-root bit for two reasons: multi-attached sessions are only possible with root privileges, and writing terminal allocation information to /var/run/utmp (the who(1) and finger(1) commands). If the screen command is not running with special privileges, all functionality except these two features will continue to work, but the local root compromise will not be possible. In order to provide the features mentioned, the screen package used to be installed setuid-root in SuSE Linux distributions. Packages which work around this problem are available from ftp.suse.com.
-----BEGIN PGP SIGNED MESSAGE-----

______________________________________________________________________________

                        SuSE Security Announcement

        Package:                screen
        Announcement-ID:        SuSE-SA:2001:030
        Date:                   Wednesday, September 5 2001 18:00 MEST
        Affected SuSE versions: (6.0, 6.1, 6.2) 6.3, 6.4, 7.0, 7.1, 7.2
        Vulnerability Type:     local root compromise
        Severity (1-10):        5
        SuSE default package:   yes
        Other affected systems: some systems with the screen program
                                installed setuid root

    Content of this advisory:
        1) security vulnerability resolved: screen
           problem description, discussion, solution and upgrade information
        2) pending vulnerabilities, solutions, workarounds
        3) standard appendix (further information)

______________________________________________________________________________

1)  problem description, brief discussion, solution, upgrade information

    screen is a terminal multiplexer program that allows reattaching to
    a detached session as well as multi-attached (shared) sessions.

    The screen package allows a local attacker to obtain root privileges if
    the /usr/bin/screen command is installed setuid root and if a directory
    below /tmp/screens/ exists.

    The screen program needs root permissions from the setuid-root bit for
    two reasons: multi-attached sessions are only possible with root
    privileges, and writing terminal allocation information to /var/run/utmp
    (the who(1) and finger(1) commands). If the screen command is not running
    with special privileges, all functionality except these two features will
    continue to work, but the local root compromise will not be possible.
    In order to provide the features mentioned, the screen package used to
    be installed setuid-root in SuSE Linux distributions.

    The update packages that we provide for the supported distributions 6.3,
    6.4, 7.0, 7.1 and 7.2 eliminate the error in the source code. In addition
    to that, the rpm package does not contain the setuid-bit on the screen
    program any more. If there is any more security-related bug in the screen
    package or libraries that it is linked against to be found in the future,
    these errors will not open local security holes any more. Users of the
    screen package who need the multi-attach feature must enable it again by
    adding the setuid-bit to the /usr/bin/screen file.
    Please note that you should reflect the changes to permissions in the
    files /etc/permissions*. If unsure, use the setting "secure local" for
    the variable PERMISSION_SECURITY in /etc/rc.config and execute "SuSEconfig"
    as root. Alternatively, change the settings for the screen program in
    /etc/permissions* and run "chkstat -set " for each file that you
    need. Use the command "rpm -qlv screen | grep /usr/bin" to find out which
    files to add to the permissions file (/usr/bin/screen is a symlink).

    The authors of screen have released the new version screen-3.9.10 that
    fixes the multi-attach-error, and some other uncritical bug. Our update
    packages contain the necessary patches applied to the version as shipped
    with the original distribution.
    We thank Christian Hürgemann  for reporting this
    error on Monday, 3 Sep 2001.

    Please download the update package for your distribution and verify its
    integrity by the methods listed in section 3) of this announcement.
    Then, install the package using the command "rpm -Uhv file.rpm" to apply
    the update.


    i386 Intel Platform:

    SuSE-7.2
    ftp://ftp.suse.com/pub/suse/i386/update/7.2/ap1/screen-3.9.8-115.i386.rpm
      e85453f50170ecdabe97dd2b33b51e4a
    source rpm:
    ftp://ftp.suse.com/pub/suse/i386/update/7.2/zq1/screen-3.9.8-115.src.rpm
      74ac0119ec42f7c5ad4405a7f1c00f6a

    SuSE-7.1
    ftp://ftp.suse.com/pub/suse/i386/update/7.1/ap1/screen-3.9.8-115.i386.rpm
      fb0a3ba99db246c1f08e3814557858ae
    source rpm:
    ftp://ftp.suse.com/pub/suse/i386/update/7.1/zq1/screen-3.9.8-115.src.rpm
      5edb0caa388be9f1ddc2cd4d0a4f3a54

    SuSE-7.0
    ftp://ftp.suse.com/pub/suse/i386/update/7.0/ap1/screen-3.9.8-115.i386.rpm
      b7bfa891b49db29093f6ab1830d84838
    source rpm:
    ftp://ftp.suse.com/pub/suse/i386/update/7.0/zq1/screen-3.9.8-115.src.rpm
      fb1e3bf070adec10296a59f2d1231c8c

    SuSE-6.4
    ftp://ftp.suse.com/pub/suse/i386/update/6.4/ap1/screen-3.9.8-115.i386.rpm
      a889c9090b0a64eb57a407ee00444491
    source rpm:
    ftp://ftp.suse.com/pub/suse/i386/update/6.4/zq1/screen-3.9.8-115.src.rpm
      29b6e24378153a4f9d2445414253759c

    SuSE-6.3
    ftp://ftp.suse.com/pub/suse/i386/update/6.3/ap1/screen-3.9.8-115.i386.rpm
      5a47bdc2407890d9a940c710d4806383
    source rpm:
    ftp://ftp.suse.com/pub/suse/i386/update/6.3/zq1/screen-3.9.8-115.src.rpm
      f3bace3114cfcb9b1b6e798ea8b93b34




    Sparc Platform:

    SuSE-7.1
    ftp://ftp.suse.com/pub/suse/sparc/update/7.1/ap1/screen-3.9.8-61.sparc.rpm
      3bfe4ac28d54ff2cab6582b2d09f3b75
    source rpm:
    ftp://ftp.suse.com/pub/suse/sparc/update/7.1/zq1/screen-3.9.8-61.src.rpm
      17da9f63ec2b0d48b3593dacfe341a7b

    SuSE-7.0
    ftp://ftp.suse.com/pub/suse/sparc/update/7.0/ap1/screen-3.9.8-62.sparc.rpm
      13037ae193ebd81b92ce1d94d2409421
    source rpm:
    ftp://ftp.suse.com/pub/suse/sparc/update/7.0/zq1/screen-3.9.8-62.src.rpm
      c542f1c51a701ad5dd06d9f9f3701cfb




    AXP Alpha Platform:

    SuSE-7.1
    ftp://ftp.suse.com/pub/suse/axp/update/7.1/ap1/screen-3.9.8-71.alpha.rpm
      c5239b5ca727a4f7d54589399b62a5c5
    source rpm:
    ftp://ftp.suse.com/pub/suse/axp/update/7.1/zq1/screen-3.9.8-71.src.rpm
      4c389bca8641e3c081e992e82bc5065d

    SuSE-7.0
    ftp://ftp.suse.com/pub/suse/axp/update/7.0/ap1/screen-3.9.8-71.alpha.rpm
      9f84734233284fc9f7be494c37a4f857
    source rpm:
    ftp://ftp.suse.com/pub/suse/axp/update/7.0/zq1/screen-3.9.8-71.src.rpm
      31920bb58b696739100e75ce363ade6d

    SuSE-6.4
    ftp://ftp.suse.com/pub/suse/axp/update/6.4/ap1/screen-3.9.8-71.alpha.rpm
      a7f64468ecc5be193795ea3a530d3ead
    source rpm:
    ftp://ftp.suse.com/pub/suse/axp/update/6.4/zq1/screen-3.9.8-71.src.rpm
      dae933854ae9a7efd3e80e09641b9963

    SuSE-6.3
    ftp://ftp.suse.com/pub/suse/axp/update/6.3/ap1/screen-3.9.8-71.alpha.rpm
      f66d9c18a27cd7915d5134528b47c342
    source rpm:
    ftp://ftp.suse.com/pub/suse/axp/update/6.3/zq1/screen-3.9.8-71.src.rpm
      13157d46457be72fd7684ca0cbe6c49b




    PPC Power PC Platform:

    SuSE-7.1
    ftp://ftp.suse.com/pub/suse/ppc/update/7.1/ap1/screen-3.9.8-52.ppc.rpm
      090e4781fb4949697fb08f755a0ae12d
    source rpm:
    ftp://ftp.suse.com/pub/suse/ppc/update/7.1/zq1/screen-3.9.8-52.src.rpm
      ce19f08412c6517798690fa7d99ed690

    SuSE-7.0
    ftp://ftp.suse.com/pub/suse/ppc/update/7.0/ap1/screen-3.9.8-52.ppc.rpm
      e37b478c4ce85febc13785b4a48c68c0
    source rpm:
    ftp://ftp.suse.com/pub/suse/ppc/update/7.0/zq1/screen-3.9.8-52.src.rpm
      0594837833e05e430c52421745f1abfa

    SuSE-6.4
    ftp://ftp.suse.com/pub/suse/ppc/update/6.4/ap1/screen-3.9.8-52.ppc.rpm
      53cb3d0fdf964a4013b6aed326d77c9e
    source rpm:
    ftp://ftp.suse.com/pub/suse/ppc/update/6.4/zq1/screen-3.9.8-52.src.rpm
      89300f304c709a10975f5906bedd1529

______________________________________________________________________________

2)  Pending vulnerabilities in SuSE Distributions and Workarounds:

  - sendmail
    SuSE Security announcement SuSE-SA:2001:028 (sendmail) offers a sendmail
    update package for the 7.0-i386 distribution that has a lower version
    number than the package in the tree for ftp installation on ftp.suse.com.
    If your update rpm command fails with a complaint that the version
    installed is newer than the one you wish to be installed, then please
    use the "--oldpackage" commandline option to rpm:
        "rpm --oldpackage -Uhv sendmail-*.rpm".
    We are correcting the problem as soon as possible. The new package
    will be named sendmail-8.11.0-10.i386.rpm and should appear on the ftp
    server within the next 11 hours. The *-10 package is technically identical
    to the *-5 package that is currently available on our ftp server.

______________________________________________________________________________

3)  standard appendix: authenticity verification, additional information

  - Package authenticity verification:

    SuSE update packages are available on many mirror ftp servers all over
    the world. While this service is being considered valuable and important
    to the free and open source software community, many users wish to be
    sure about the origin of the package and its content before installing
    the package. There are two verification methods that can be used
    independently from each other to prove the authenticity of a downloaded
    file or rpm package:
    1) md5sums as provided in the (cryptographically signed) announcement.
    2) using the internal gpg signatures of the rpm package.

    1) execute the command
        md5sum 
       after you downloaded the file from a SuSE ftp server or its mirrors.
       Then, compare the resulting md5sum with the one that is listed in the
       announcement. Since the announcement containing the checksums is
       cryptographically signed (usually using the key security@suse.de),
       the checksums show proof of the authenticity of the package.
       We disrecommend to subscribe to security lists which cause the
       email message containing the announcement to be modified so that
       the signature does not match after transport through the mailing
       list software.
       Downsides: You must be able to verify the authenticity of the
       announcement in the first place. If RPM packages are being rebuilt
       and a new version of a package is published on the ftp server, all
       md5 sums for the files are useless.

    2) rpm package signatures provide an easy way to verify the authenticity
       of an rpm package. Use the command
        rpm -v --checksig 
       to verify the signature of the package, where  is the
       filename of the rpm package that you have downloaded. Of course,
       package authenticity verification can only target an uninstalled rpm
       package file.
       Prerequisites:
        a) gpg is installed
        b) The package is signed using a certain key. The public part of this
           key must be installed by the gpg program in the directory
           ~/.gnupg/ under the user's home directory who performs the
           signature verification (usually root). You can import the key
           that is used by SuSE in rpm packages for SuSE Linux by saving
           this announcement to a file ("announcement.txt") and
           running the command (do "su -" to be root):
            gpg --batch; gpg < announcement.txt | gpg --import
           SuSE Linux distributions version 7.1 and thereafter install the
           key "build@suse.de" upon installation or upgrade, provided that
           the package gpg is installed. The file containing the public key
           is placed at the toplevel directory of the first CD (pubring.gpg)
           and at ftp://ftp.suse.com/pub/suse/pubring.gpg-build.suse.de .


  - SuSE runs two security mailing lists to which any interested party may
    subscribe:

    suse-security@suse.com
        -   general/linux/SuSE security discussion.
            All SuSE security announcements are sent to this list.
            To subscribe, send an email to
                .

    suse-security-announce@suse.com
        -   SuSE's announce-only mailing list.
            Only SuSE's security annoucements are sent to this list.
            To subscribe, send an email to
                .

    For general information or the frequently asked questions (faq)
    send mail to:
         or
         respectively.

    ===================================================
    SuSE's security contact is .
    The  public key is listed below.
    ===================================================
______________________________________________________________________________

    The information in this advisory may be distributed or reproduced,
    provided that the advisory is not modified in any way. In particular,
    it is desired that the cleartext signature shows proof of the
    authenticity of the text.
    SuSE GmbH makes no warranties of any kind whatsoever with respect
    to the information contained in this security advisory.

Type Bits/KeyID    Date       User ID
pub  2048R/3D25D3D9 1999-03-06 SuSE Security Team 
pub  1024D/9C800ACA 2000-10-19 SuSE Package Signing Key 

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