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FreeBSD-SA-19:08.rack Security Advisory
The FreeBSD Project
Topic: Resource exhaustion in non-default RACK TCP stack
Credits: Jonathan Looney (Netflix)
Peter Lei (Netflix)
Affects: FreeBSD 12.0 and later
Corrected: 2019-06-19 16:25:39 UTC (stable/12, 12.0-STABLE)
2019-06-19 16:43:05 UTC (releng/12.0, 12.0-RELEASE-p6)
CVE Name: CVE-2019-5599
For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.
The Transmission Control Protocol (TCP) of the TCP/IP protocol suite provides
a connection-oriented, reliable, sequence-preserving data stream service.
A TCP loss detection algorithm called RACK ("Recent ACKnowledgment") uses the
notion of time, in addition to packet or sequence counts, to detect losses
for modern TCP implementations that support per-packet timestamps and the
selective acknowledgment (SACK) option.
FreeBSD ships an optional implementation of RACK. Please note this is not
included by default. If RACK was not specifically compiled, installed, and
loaded, the system is not vulnerable.
II. Problem Description
While processing acknowledgements, the RACK code uses several linked lists to
maintain state entries. A malicious attacker can cause the lists to grow
unbounded. This can cause an expensive list traversal on every packet being
processed, leading to resource exhaustion and a denial of service.
An attacker with the ability to send specially crafted TCP traffic to a
victim system can degrade network performance and/or consume excessive CPU by
exploiting the inefficiency of traversing the potentially very large RACK
linked lists with relatively small bandwidth cost.
By default RACK is not compiled or loaded into the TCP stack. To determine
if you are using RACK, check the net.inet.tcp.functions_available sysctl.
If it includes a line with "rack", the RACK stack is loaded.
To disable RACK, unload the kernel module with:
# kldunload tcp_rack
Note: it may be required to use the force flag (-f) with the kldunload.
Upgrade your vulnerable system to a supported FreeBSD stable or release /
security branch (releng) dated after the correction date.
Perform one of the following:
1) 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
Since the tcp_rack kernel module is not built by default, recompile,
reinstall, and reload the kernel module.
2) 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 https://security.FreeBSD.org/patches/SA-19:08/rack.patch
# fetch https://security.FreeBSD.org/patches/SA-19:08/rack.patch.asc
# gpg --verify rack.patch.asc
b) Apply the patch. Execute the following commands as root:
# cd /usr/src
# patch < /path/to/patch
c) Recompile, reinstall, and reload the tcp_rack kernel module.
VI. Correction details
The following list contains the correction revision numbers for each
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://svn.freebsd.org/base
Or visit the following URL, replacing NNNNNN with the revision number:
The latest revision of this advisory is available at
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