Security Experts:

Critical DoS Flaws Patched in Asterisk Framework

Updates released on Friday for the Asterisk communications framework address three critical denial-of-service (DoS) vulnerabilities discovered by Sandro Gauci, a penetration tester and researcher who specializes in VoIP and communications systems.

Asterisk, considered the world’s most popular open source communications framework, is used by government agencies, carriers and other businesses, including most Fortune 1000 companies. According to its developers, more than one million IP PBX systems, VoIP gateways, conference servers and other solutions rely on Asterisk.

In collaboration with Alfred Farrugia, Gauci discovered in April that the project is affected by three potentially serious vulnerabilities that can be exploited to cause the system to crash. Separate advisories have been published by Asterisk developers for each of the flaws, and Gauci has also released technical details

The vulnerabilities affect all versions of Asterisk 13, 14 and Certified Asterisk 13.13. The issues have been addressed with the release of versions 13.15.1, 14.4.1 and 13.13-cert4.

One of the security holes can be exploited by a remote attacker to cause Asterisk to exhaust all available memory by sending a specially crafted Signalling Connection Control Part (SCCP) packet. Removing or disabling support for the SCCP protocol prevents potential attacks.

“A remote memory exhaustion can be triggered by sending an SCCP packet to Asterisk system with ‘chan_skinny’ enabled that is larger than the length of the SCCP header but smaller than the packet length specified in the header. The loop that reads the rest of the packet doesn’t detect that the call to read() returned end-of-file before the expected number of bytes and continues infinitely. The ‘partial data’ message logging in that tight loop causes Asterisk to exhaust all available memory,” Asterisk developers wrote in their advisory.

The other two vulnerabilities found by Gauci affect PJSIP, an open source multimedia communication library that implements SIP (Session Initiation Protocol) and other protocols. The flaws can be exploited remotely to cause a crash by sending specially crafted SIP packets.

The latest Asterisk releases include a version of PJSIP that addresses these vulnerabilities. However, other projects using the PJSIP library are vulnerable as well, and they will need to obtain upstream patches to protect their users against attacks.

*Updated with link to Gauci's advisory 

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.