Security Experts:

Flaws Found In ZRTPCPP Library Used to Encrypt Mobile Apps

Mark Dowd, co-founder of Azimuth Security, disclosed details of vulnerabilities he discovered in the ZRTPCPP library last week, which is a core component for many secure phone applications including Silent Circle, the private communications company co-founded by PGP creator Phil Zimmermann.

The ZRTP protocol was originally designed by Zimmermann.

“ZRTP seeks to negotiate and establish a cryptographically secure, authenticated channel over the pre-established RTP connection, rather than using out-of-band channels such as SIP. The ZRTPCPP library, now part of GNU's secure telephony stack, implements the ZRTP protocol and is utilized by several securephone solutions,” Dowd wrote, outlining the basics of the protocol.

ZRTPCPP Code VulnerabilitiesDowd examined the code library as a matter of interest, especially given the fallout over the revealing of the NSA’s PRISM program, and the subsequent market push by vendors and consumers for encrypted or otherwise secured mobile communications.

Dowd’s work with ZRTPCPP discovered three flaws, which if exploited could have allowed “un-authenticated, untrusted, remote parties” to target applications such as SilentCircle, CSipSimple, LinPhone, or Twinkle – but in reality they impacted any application that hinged on ZRTPCPP. 

“These vulnerabilities were recently reported by Azimuth to ZRTPCPP author/maintainer Werner Dittman who turned around fixes in a very short space of time,” a post by Dowd explains.

Dowd discovered three vulnerabilities, two of which are overflow-based, and the final one centers on information disclosure from data stored in memory. The overflow flaws would have enabled an attacker the chance to execute code, either by sending overly-large ZRTP packets or malformed ones; while the information disclosure would require a truncated ZRTP Ping packet.

“In working with Mark, our team moved quickly to engage with the author who published the affected library and it was updated to close the vulnerability and posted on GitHub,” a Silent Circle spokesperson told SecurityWeek in an emailed statement. “We have updated the app and submitted to Google Play and the Apple App Store. The app update is currently live in Google Play and in the Apple App Store.”

All of the aforementioned vendors have patched their products, and cleaned code has been submitted to the ZRTPCPP project, which can be obtained from github. For those who need a fast technical breakdown, a general overview was submitted to the OSS-Sec mailing list as part of a CVE request, which can be viewed here.  

Silent Circle’s CTO, Jon Callas, also published a blog post addressing the ZRTP issues, which provides context around Silent Circle's thoughts on the matter.

Additional reporting by Mike Lennon

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Steve Ragan is a security reporter and contributor for SecurityWeek. Prior to joining the journalism world in 2005, he spent 15 years as a freelance IT contractor focused on endpoint security and security training.