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Bug 598334 (CVE-2016-6321) - <app-arch/tar-1.29-r1: extract pathname bypass
Summary: <app-arch/tar-1.29-r1: extract pathname bypass
Alias: CVE-2016-6321
Product: Gentoo Security
Classification: Unclassified
Component: Vulnerabilities (show other bugs)
Hardware: All Linux
: Normal normal (vote)
Assignee: Gentoo Security
Whiteboard: A3 [glsa cve]
: 620012 (view as bug list)
Depends on:
Reported: 2016-10-28 10:09 UTC by Kristian Fiskerstrand
Modified: 2017-05-29 08:55 UTC (History)
2 users (show)

See Also:
Package list:
Runtime testing required: ---


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Description Kristian Fiskerstrand gentoo-dev Security 2016-10-28 10:09:47 UTC
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!

               `;+####',       .;####':`       .;####':`
            `#@@@@@@@@@@@,   ;@@@@@@@@@@#    ;@@@@@@@@@@#
           +@@@@@@@@@@@@@,  +@@@@@@@@@@@#   +@@@@@@@@@@@#
          #@@@@@#.    .+@,  @@@@@`    ,##   @@@@@`    ,##
         :@@@@@,         `  @@@@@:          @@@@@:
         #@@@@#             #@@@@@@@@'.     #@@@@@@@@'.
         @@@@@:              #@@@@@@@@@@;    #@@@@@@@@@@;
         #@@@@+                ;#@@@@@@@@'     ;#@@@@@@@@'
         '@@@@@`                   `#@@@@@         `#@@@@@
          @@@@@@;      `'#  #;      `@@@@@  #;      `@@@@@
          `@@@@@@@@@@@@@@#  #@@@@##@@@@@@+  #@@@@##@@@@@@+
            ;@@@@@@@@@@@@#  #@@@@@@@@@@@+   #@@@@@@@@@@@+
              .+@@@@@@@#:    :#@@@@@@#:      :#@@@@@@#:

           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 

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 | tar xv etc/motd
$ cat etc/shadow




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 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 regarding the status of the
11.08.2016  CVE-2016-6321 was assigned to the vulnerability
15.09.2016  polled 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
Comment 1 Patrick McLean gentoo-dev 2016-10-28 17:57:24 UTC
Added patch in app-arch/tar-1.29-r1

We should be good to stabilize this.
Comment 2 Patrick McLean gentoo-dev 2016-10-28 18:17:05 UTC
@arches: please stabilize =app-arch/tar-1.29-r1
Comment 3 Jeroen Roovers gentoo-dev 2016-11-01 11:05:26 UTC
Stable for HPPA.
Comment 4 Agostino Sarubbo gentoo-dev 2016-11-04 08:22:16 UTC
amd64 stable
Comment 5 Agostino Sarubbo gentoo-dev 2016-11-04 08:25:34 UTC
x86 stable
Comment 6 Jeroen Roovers gentoo-dev 2016-11-04 13:27:24 UTC
Stable for PPC64.
Comment 7 Markus Meier gentoo-dev 2016-11-10 17:50:36 UTC
arm stable
Comment 8 SpanKY gentoo-dev 2016-11-10 20:26:12 UTC
done the rest now
Comment 9 Aaron Bauman Gentoo Infrastructure gentoo-dev Security 2016-11-11 06:02:00 UTC
@maintainer(s), please clean the vulnerable versions from the tree.
Comment 10 GLSAMaker/CVETool Bot gentoo-dev 2016-11-22 11:42:29 UTC
This issue was resolved and addressed in
 GLSA 201611-19 at
by GLSA coordinator Aaron Bauman (b-man).
Comment 11 Lars Wendler (Polynomial-C) gentoo-dev 2016-12-22 23:01:53 UTC
commit 204a2aaac4657909edb11d2ebd3837707e9b9485
Author: Lars Wendler <>
Date:   Thu Dec 22 23:56:53 2016

    app-arch/tar: Removed old.

    Package-Manager: Portage-2.3.3, Repoman-2.3.1
Comment 12 Agostino Sarubbo gentoo-dev 2017-05-29 08:55:04 UTC
*** Bug 620012 has been marked as a duplicate of this bug. ***