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Category:   Application (VPN)  >   OpenSSL Vendors:   OpenSSL.org
(FreeBSD Issues Fix) OpenSSL SSL3_AL_WARNING Alert Processing Flaw Lets Remote Users Consume Excessive CPU Resources on the Target System
SecurityTracker Alert ID:  1037172
SecurityTracker URL:  http://securitytracker.com/id/1037172
CVE Reference:   CVE-2016-8610   (Links to External Site)
Date:  Nov 2 2016
Impact:   Denial of service via network
Fix Available:  Yes  Vendor Confirmed:  Yes  Exploit Included:  Yes  
Version(s): 0.9.8, 1.0.1, 1.0.2 - 1.0.2h, 1.1.0
Description:   A vulnerability was reported in OpenSSL. A remote user can consume excessive CPU resources on the target system.

The ssl3_read_bytes() function in 'ssl/s3_pkt.c' does not properly handle certain packets. A remote user can send a flood of SSL3_AL_WARNING alerts during the SSL handshake to consume excessive CPU resources on the target system. As a result, services that use the OpenSSL library may become unavailable for the duration of the packet flood.

Shi Lei from Gear Team, Qihoo 360 Inc., reported this vulnerability.

Impact:   A remote user can consume excessive CPU resources on the target system.
Solution:   FreeBSD has issued a fix.

The FreeBSD advisory is available at:

https://security.FreeBSD.org/advisories/FreeBSD-SA-16:35.openssl.asc

Vendor URL:  security.FreeBSD.org/advisories/FreeBSD-SA-16:35.openssl.asc (Links to External Site)
Cause:   State error
Underlying OS:  UNIX (FreeBSD)
Underlying OS Comments:  9.3, 10.1, 10.2, 10.3

Message History:   This archive entry is a follow-up to the message listed below.
Oct 24 2016 OpenSSL SSL3_AL_WARNING Alert Processing Flaw Lets Remote Users Consume Excessive CPU Resources on the Target System



 Source Message Contents

Subject:  FreeBSD Security Advisory FreeBSD-SA-16:35.openssl

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

=============================================================================
FreeBSD-SA-16:35.openssl                                    Security Advisory
                                                          The FreeBSD Project

Topic:          OpenSSL Remote DoS vulnerability

Category:       contrib
Module:         openssl
Announced:      2016-11-02
Affects:        FreeBSD 9.x and FreeBSD 10.x.
Corrected:      2016-11-02 07:09:31 UTC (stable/10, 10.3-STABLE)
                2016-11-02 07:23:36 UTC (releng/10.3, 10.3-RELEASE-p12)
                2016-11-02 07:24:14 UTC (releng/10.2, 10.2-RELEASE-p25)
                2016-11-02 07:24:14 UTC (releng/10.1, 10.1-RELEASE-p42)
                2016-11-02 07:09:31 UTC (stable/9, 9.3-STABLE)
                2016-11-02 07:24:34 UTC (releng/9.3, 9.3-RELEASE-p50)
CVE Name:       CVE-2016-8610

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/>.

I.   Background

FreeBSD includes software from the OpenSSL Project.  The OpenSSL Project is
a collaborative effort to develop a robust, commercial-grade, full-featured
Open Source toolkit implementing the Secure Sockets Layer (SSL v2/v3)
and Transport Layer Security (TLS v1) protocols as well as a full-strength
general purpose cryptography library.

The SSL alert protocol is a way to communicate problems within a SSL/TLS session.

II.  Problem Description

Due to improper handling of alert packets, OpenSSL would consume an excessive
amount of CPU time processing undefined alert messages.

III. Impact

A remote attacker who can initiate handshakes with an OpenSSL based server
can cause the server to consume a lot of computation power with very little
bandwidth usage, and may be able to use this technique in a leveraged Denial
of Service attack.

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.

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

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.

[FreeBSD 10.x]
# fetch https://security.FreeBSD.org/patches/SA-16:35/openssl-10.patch
# fetch https://security.FreeBSD.org/patches/SA-16:35/openssl-10.patch.asc
# gpg --verify openssl-10.patch.asc

[FreeBSD 9.3]
# fetch https://security.FreeBSD.org/patches/SA-16:35/openssl-9.patch
# fetch https://security.FreeBSD.org/patches/SA-16:35/openssl-9.patch.asc
# gpg --verify openssl-9.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:https://www.FreeBSD.org/handbook/makeworld.html>.

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/                                                         r308200
releng/9.3/                                                       r308205
stable/10/                                                        r308200
releng/10.1/                                                      r308204
releng/10.2/                                                      r308204
releng/10.3/                                                      r308203
- -------------------------------------------------------------------------

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:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:http://seclists.org/oss-sec/2016/q4/224>

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-8610>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-16:35.openssl.asc>
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