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Category:   Application (VPN)  >   OpenSSL Vendors:   OpenSSL.org
(FreeBSD Issues Fix) OpenSSL Alternative Certificate Chain Validation Flaw Lets Remote Users Forge Certificates
SecurityTracker Alert ID:  1032818
SecurityTracker URL:  http://securitytracker.com/id/1032818
CVE Reference:   CVE-2015-1793   (Links to External Site)
Date:  Jul 9 2015
Impact:   Modification of authentication information
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 1.0.1o, 1.0.1n, 1.0.2b, 1.0.2c
Description:   A vulnerability was reported in OpenSSL. A remote user can bypass certificate validation on the target system.

When the validation of a certificate chain fails, the system attempts to validate an alternate certificate chain but does not check the CA flag of untrusted certificates. As a result, a remote user can cause the target system to validate an invalid certificate using a valid leaf certificate.

Applications that verify certificates are affected.

SSL/TLS/DTLS clients and SSL/TLS/DTLS servers using client authentication are affected.

The vendor was notified on June 24, 2015.

Adam Langley/David Benjamin (Google/BoringSSL) reported this vulnerability.

Impact:   A remote user can bypass certificate validation on the target system.
Solution:   FreeBSD has issued a fix (10.2-PRERELEASE, 10.2-BETA1).

FreeBSD 10.1-RELEASE is not affected.

The FreeBSD advisory is available at:

https://security.FreeBSD.org/advisories/FreeBSD-SA-15:12.openssl.asc

Vendor URL:  security.FreeBSD.org/advisories/FreeBSD-SA-15:12.openssl.asc (Links to External Site)
Cause:   Authentication error
Underlying OS:  UNIX (FreeBSD)
Underlying OS Comments:  10.1-STABLE

Message History:   This archive entry is a follow-up to the message listed below.
Jul 9 2015 OpenSSL Alternative Certificate Chain Validation Flaw Lets Remote Users Forge Certificates



 Source Message Contents

Subject:  FreeBSD Security Advisory FreeBSD-SA-15:12.openssl

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

=============================================================================
FreeBSD-SA-15:12.openssl                                    Security Advisory
                                                          The FreeBSD Project

Topic:          OpenSSL alternate chains certificate forgery vulnerability

Category:       contrib
Module:         openssl
Announced:      2015-07-09
Credits:        Adam Langley/David Benjamin (Google/BoringSSL), OpenSSL
Affects:        FreeBSD 10.1-STABLE after 2015-06-11 and prior to the
                correction date.
Corrected:      2015-07-09 17:17:22 UTC (stable/10, 10.2-PRERELEASE,
                                         10.2-BETA1)
CVE Name:       CVE-2015-1793

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.

II.  Problem Description

During certificate verification, OpenSSL will attempt to find an alternative
certificate chain if the first attempt to build such a chain fails, unless
the application explicitly specifies X509_V_FLAG_NO_ALT_CHAINS.

An error in the implementation of this logic could erroneously mark
certificate as trusted when they should not.

III. Impact

An attacker could cause certain checks on untrusted certificates, such as the
CA (certificate authority) flag, to be bypassed, which would enable them to
use a valid leaf certificate to act as a CA and issue an invalid certificate.

IV.  Workaround

No workaround is available.

NOTE WELL: This issue does not affect earlier FreeBSD releases, including the
supported 8.4, 9.3 and 10.1-RELEASE because the alternative certificate chain
feature was not introduced in these releases.  Only 10.1-STABLE after
2015-06-11 and prior to the correction date is affected.

V.   Solution

Upgrade your vulnerable system to the latest supported FreeBSD stable/10
branch dated after the correction date.

Recompile the operating system using buildworld and installworld as
described in <URL:https://www.FreeBSD.org/handbook/makeworld.html>.

Restart all deamons using 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/10/                                                        r285330
- -------------------------------------------------------------------------

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:https://www.openssl.org/news/secadv_20150709.txt>

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-1793>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-15:12.openssl.asc>
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