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Category:   Application (Generic)  >   libc Vendors:   [Multiple Authors/Vendors]
libc link_ntoa() Buffer Overflow Lets Users Execute Arbitrary Code
SecurityTracker Alert ID:  1037398
SecurityTracker URL:
CVE Reference:   CVE-2016-6559   (Links to External Site)
Updated:  Dec 8 2016
Original Entry Date:  Dec 6 2016
Impact:   Execution of arbitrary code via local system, Execution of arbitrary code via network, User access via network
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 9.3, 10.1, 10.2, 10.3, 11.0
Description:   A vulnerability was reported in libc. A user can execute arbitrary code on the target system.

A user may be able to trigger a buffer overflow in the link_ntoa() function and execute arbitrary code on the target system. The code will run with the privileges of the target application or service.

The specific impact depends on the application or service that uses the vulnerable link_ntoa() function.

FreeBSD is affected.

[Editor's note: US-CERT indicates that macOS is affected. The US-CERT advisory is available at:].

Impact:   A remote or local user can execute arbitrary code on the target system. The specific impact depends on the application or service that uses the vulnerable libc function.
Solution:   FreeBSD has issued a fix.

[Editor's note: On December 8, 2016, FreeBSD issued a revised fix.]

The FreeBSD advisory is available at:

Vendor URL: (Links to External Site)
Cause:   Boundary error
Underlying OS:  UNIX (Any)

Message History:   None.

 Source Message Contents

Subject:  FreeBSD Security Advisory FreeBSD-SA-16:37.libc

Hash: SHA512

FreeBSD-SA-16:37.libc                                       Security Advisory
                                                          The FreeBSD Project

Topic:          link_ntoa(3) buffer overflow 

Category:       core
Module:         libc
Announced:      2016-12-06
Affects:        All supported versions of FreeBSD.
Corrected:      2016-12-06 18:53:21 UTC (stable/11, 11.0-STABLE)
                2016-12-06 18:49:38 UTC (releng/11.0, 11.0-RELEASE-p4)
                2016-12-06 18:53:46 UTC (stable/10, 10.3-STABLE)
                2016-12-06 18:49:48 UTC (releng/10.3, 10.3-RELEASE-p13)
                2016-12-06 18:49:54 UTC (releng/10.2, 10.2-RELEASE-p26)
                2016-12-06 18:49:59 UTC (releng/10.1, 10.1-RELEASE-p43)
                2016-12-06 18:54:04 UTC (stable/9, 9.3-STABLE)
                2016-12-06 18:50:06 UTC (releng/9.3, 9.3-RELEASE-p51)
CVE Name:       CVE-2016-6559

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:>.

I.   Background

The link_ntoa(3) function generates ASCII representation of a link-level
address and is avaliable as part of standard C library (libc).

II.  Problem Description

A specially crafted argument can trigger a static buffer overflow in the
library, with possibility to rewrite following static buffers that belong to
other library functions.

III. Impact

Due to very limited use of the function in the existing applications,
and limited length of the overflow, exploitation of the vulnerability
does not seem feasible.  None of the utilities and daemons in the base
system are known to be vulnerable.  However, careful review of third
party software that may use the function was not performed.

IV.  Workaround

No workaround is available.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

2) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

Restart all daemons that use the library, or reboot the system.

3) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch
# fetch
# gpg --verify libc.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile the operating system using buildworld and installworld as
described in <URL:>.

Restart all daemons that use the library, or reboot the system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/9/                                                         r309646
releng/9.3/                                                       r309637
stable/10/                                                        r309645
releng/10.1/                                                      r309636
releng/10.2/                                                      r309635
releng/10.3/                                                      r309634
stable/11/                                                        r309644
releng/11.0/                                                      r309633
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://

Or visit the following URL, replacing NNNNNN with the revision number:


VII. References


The latest revision of this advisory is available at

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