New tls/ssl vulnerability Ticketbleed (CVE-2016-9244) Exposes F5 Appliances to Remote Attacks

Ticketbleed is a software vulnerability in the TLS/SSL stack of F5 BIG-IP appliances allowing a remote attacker to extract up to 31 bytes of uninitialized memory at a time.

This vulnerability was discovered by Filippo Valsorda, cryptography engineer at CloudFlare, and other employees of the content delivery network (CDN). The expert identified the weakness while investigating a bug report from a CloudFlare customer, and notified F5 in late October.




According to F5, the vulnerability affects BIG-IP SSL virtual servers that have the non-default Session Tickets option enabled.This memory can potentially contain key material or sensitive data from other connections.

Ticketbleed reminds use the dangerous Heartbleed flaw in the OpenSSL library, however, unlike Heartbleed. It is different in that it exposes 31 bytes at a time instead of 64k, requiring more rounds to carry out an attack, and in that it affects the proprietary F5 TLS stack, not OpenSSL.

Technical Details:

The vulnerability lies in the implementation of Session Tickets, a resumption technique used to speed up repeated connections.

When a client supplies a Session ID together with a Session Ticket, the server is supposed to echo back the Session ID to signal acceptance of the ticket. Session IDs can be anywhere between 1 and 31 bytes in length,” Valsorda explained.

The F5 stack always echoes back 32 bytes of memory, even if the Session ID was shorter. An attacker providing a 1-byte Session ID would then receive 31 bytes of uninitialized memory, the expert added.

The expert Filippo Valsorda has developed a free online tool that could be used to check if a product is affected by the Ticketbleed issue.

The company suggests as a workaround to disable the Session Tickets option on the vulnerable Client SSL profile, this is possible accessing to the menu item “Local Traffic > Profiles > SSL > Client ” of the Configuration utility.

Internet scans conducted by the researcher showed that 949 of the Alexa top one million websites were vulnerable, including 15 in the top 10,000 sites. Of the top one million hosts on Cisco’s Umbrella cloud security platform, over 1,600 were found to be affected.

Valsorda has provided detailed technical information on the vulnerability and made some recommendations for security vendors that might consider trying to detect potential Ticketbleed attacks.

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