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Category:   Application (VPN)  >   OpenSSL Vendors:
(FreeBSD Issues Fix) OpenSSL TLS Diffie-Hellman Export Cipher Downgrade Attack Lets Remote Users Decrypt Connections
SecurityTracker Alert ID:  1032575
SecurityTracker URL:
CVE Reference:   CVE-2015-4000   (Links to External Site)
Date:  Jun 12 2015
Impact:   Modification of authentication information
Fix Available:  Yes  Vendor Confirmed:  Yes  Exploit Included:  Yes  

Description:   A vulnerability was reported in OpenSSL. A remote user may be able to decrypt TLS connections in certain situations.

A remote user that can conduct a man-in-the-middle attack can cause the target system to downgrade the Diffie-Hellman algorithm to 512-bit export-grade cryptography. The remote user may then be able to decrypt the connection.

This vulnerability resides in the TLS protocol and not in the specific TLS implementation, but the vulnerability is exposed because the target system supports export-grade ciphers.

This attack is known as the "Logjam" attack.

The original advisory is available at:

David Adrian, Karthikeyan Bhargavan, Zakir Durumeric, Pierrick Gaudry, Matthew Green, J. Alex Halderman, Nadia Heninger, Drew Springall, Emmanuel Thome, Luke Valenta, Benjamin VanderSloot, Eric Wustrow, Santiago Zanella-Beguelin, and Paul Zimmermann reported this vulnerability.

Impact:   A remote user that can conduct a man-in-the-middle attack can cause the target system to use weak cryptography that can be decrypted.
Solution:   FreeBSD has issued a fix.

The FreeBSD advisory is available at:

Vendor URL: (Links to External Site)
Cause:   Authentication error
Underlying OS:  UNIX (FreeBSD)
Underlying OS Comments:  8.4, 9.3, 10.1

Message History:   This archive entry is a follow-up to the message listed below.
Jun 3 2015 OpenSSL TLS Diffie-Hellman Export Cipher Downgrade Attack Lets Remote Users Decrypt Connections

 Source Message Contents

Subject:  FreeBSD Security Advisory FreeBSD-SA-15:10.openssl

Hash: SHA512

FreeBSD-SA-15:10.openssl                                    Security Advisory
                                                          The FreeBSD Project

Topic:          Multiple OpenSSL vulnerabilities

Category:       contrib
Module:         openssl
Announced:      2015-06-12
Affects:        All supported versions of FreeBSD.
Corrected:      2015-06-11 19:07:45 UTC (stable/10, 10.1-STABLE)
                2015-06-12 07:23:55 UTC (releng/10.1, 10.1-RELEASE-p12)
                2015-06-11 19:39:27 UTC (stable/9, 9.3-STABLE)
                2015-06-12 07:23:55 UTC (releng/9.3, 9.3-RELEASE-p16)
                2015-06-11 19:39:27 UTC (stable/8, 8.4-STABLE)
                2015-06-12 07:23:55 UTC (releng/8.4, 8.4-RELEASE-p30)
CVE Name:       CVE-2015-1788, CVE-2015-1789, CVE-2015-1790, CVE-2015-1791
                CVE-2015-1792, CVE-2015-4000

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

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

A vulnerability in the TLS protocol would allow a man-in-the-middle
attacker to downgrade vulnerable TLS connections using ephemeral
Diffie-Hellman key exchange to 512-bit export-grade cryptography.
This vulnerability is also known as Logjam [CVE-2015-4000].

When processing an ECParameters structure OpenSSL enters an infinite
loop if the curve specified is over a specially malformed binary
polynomial field. [CVE-2015-1788]

X509_cmp_time does not properly check the length of the ASN1_TIME
string and can read a few bytes out of bounds. In addition,
X509_cmp_time accepts an arbitrary number of fractional seconds in
the time string. [CVE-2015-1789]

The PKCS#7 parsing code does not handle missing inner EncryptedContent
correctly.  [CVE-2015-1790]

When verifying a signedData message the CMS code can enter an infinite
loop if presented with an unknown hash function OID. [CVE-2015-1792]

If a NewSessionTicket is received by a multi-threaded client when
attempting to reuse a previous ticket then a race condition can occur,
potentially leading to a double free of the ticket data. [CVE-2015-1791]

The OpenSSL advisory also describes a problem that is identified as
CVE-2014-8176, which is already fixed by an earlier FreeBSD Errata
Notice, FreeBSD-EN-15:02.openssl.

III. Impact

A man-in-the-middle attacker may be able to downgrade vulnerable TLS
connections using ephemeral Diffie-Hellman key exchange to 512-bit
export-grade cryptography. [CVE-2015-4000].  On FreeBSD 10.1, the
patch contains a countermeasure for clients by rejecting handshakes
with DH parameters shorter than 768 bits.

An attacker who is able to use a certificate to authenticate with
a remote system perform denial of service against any system which
processes public keys, certificate requests or certificates.
[CVE-2015-1788].  This affects FreeBSD 10.1 only, as the problem
was no longer exist in OpenSSL 0.9.8 series since July 2012.

An attacker can use the CVE-2015-1789 issue by using specifically
crafted certificates and CRLs of various sizes and potentially
cause a segmentation fault, resulting in a DoS on applications that
verify certificates or CRLs.

An attacker who can create specifically crafted malformed ASN.1-encoded
PKCS#7 blobs with missing content and trigger a NULL pointer dereference
on parsing. [CVE-2015-1790].  Applications that decrypt PKCS#7 data
or otherwise parse PKCS#7 structures from untrusted sources are
affected. OpenSSL clients and servers are not affected.

An attacker can perform denial of service against any system which
verifies signedData messages using the CMS code. [CVE-2015-1792]

An attacker may be able to crash multi-thread applications that
supports resumed TLS handshakes. [CVE-2015-1791]

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

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 10.1]
# fetch
# fetch
# gpg --verify openssl-10.1.patch.asc

[FreeBSD 9.3 and 8.4]
# fetch
# fetch
# gpg --verify openssl-8.4.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 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/8/                                                         r284286
releng/8.4/                                                       r284295
stable/9/                                                         r284286
releng/9.3/                                                       r284295
stable/10/                                                        r284285
releng/10.1/                                                      r284295
- -------------------------------------------------------------------------

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
Version: GnuPG v2.1.4 (FreeBSD)


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