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Category:   Application (VPN)  >   OpenVPN Vendors:   openvpn.net
OpenVPN Blowfish Cipher Block Collision Weakness Lets Remote Users Decrypt Data in Certain Cases
SecurityTracker Alert ID:  1036695
SecurityTracker URL:  http://securitytracker.com/id/1036695
CVE Reference:   CVE-2016-6329   (Links to External Site)
Date:  Aug 26 2016
Impact:   Disclosure of system information, Disclosure of user information
Vendor Confirmed:  Yes  Exploit Included:  Yes  

Description:   A vulnerability was reported in OpenVPN. A remote user can decrypt transmitted data in certain cases.

A remote user that can monitor the network and can capture a long duration Blowfish CBC mode encrypted TLS session over which some amount of known plaintext is communicated can recover some plaintext in certain cases.

Over the duration of a long-lived connection, a cipher block collision may occur, allowing the remote user to recover the exclusive OR between the two plaintext blocks. If the communications protocol sends a fixed plaintext portion repeatedly and also sends some amount of known plaintext, the user can recover the secret plaintext.

[Editor's note: The report confirms a successful attack to recover secure HTTP cookies by capturing 785 GB of network traffic.]

The attack method is known as a SWEET32 attack.

64-bit block ciphers, such as 3DES and Blowfish, are affected by this type of attack.

The original advisory will be presented at the 23rd ACM Conference on Computer and Communications Security and is available at:

https://sweet32.info/SWEET32_CCS16.pdf

Karthikeyan Bhargavan and Gaetan Leurent from INRIA reported this vulnerability.

Impact:   A remote user that can monitor the network can decrypt transmitted data in certain cases.
Solution:   [Editor's note: The vulnerability is a known limitation of the cryptographic protocol.]

The vendor plans to issue a version (2.3.12) that will warning when 64-bit ciphers are selected.

The vendor plans to issue a future version that will use a default renegotiation data limit of 64MB for TLS connections.

Vendor URL:  openvpn.net/ (Links to External Site)
Cause:   Access control error
Underlying OS:  Linux (Any), UNIX (Any)

Message History:   This archive entry has one or more follow-up message(s) listed below.
Jan 10 2017 (IBM Issues Fix for IBM License Metric Tool) OpenVPN Blowfish Cipher Block Collision Weakness Lets Remote Users Decrypt Data in Certain Cases
IBM has issued a fix for IBM License Metric Tool
Jan 13 2017 (HPE Issues Fix for HPE SiteScope) OpenVPN Blowfish Cipher Block Collision Weakness Lets Remote Users Decrypt Data in Certain Cases
HPE has issued a fix for HPE SiteScope.
Jun 22 2017 (Ubuntu Issues Fix) OpenVPN Blowfish Cipher Block Collision Weakness Lets Remote Users Decrypt Data in Certain Cases
Ubuntu has issued a fix for Ubuntu Linux 14.04 LTS, 16.04 LTS, 16.10, and 17.04.



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