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Cisco PIX security flaw possibility comes to light

Details of an alleged flaw related to SIP and PIX appliances, briefly mentioned in a Wednesday Black Hat presentation, are being kept under wraps as Cisco and US-CERT investigate.

LAS VEGAS -- Controversy looms for Cisco once again at Black Hat, as information revealed Wednesday could lead to another significant zero-day vulnerability and exploit.

Hendrik Scholz, lead VoIP developer and systems engineer with Freenet Cityline of Germany, saved the best for last during his Black Hat USA 2006 presentation Wednesday on SIP stack fingerprinting and attacks. His final slide appeared to featured limited details on an undisclosed flaw related to Session Initiation Protocol (SIP) in Cisco Systems Inc. PIX series of firewalls and security appliances.

According to Mike Caudill and Jeffrey Lanza, incident managers with Cisco's Product Security Incident Response Team (PSIRT), the networking giant is unsure whether the details describe a vulnerability or a misconfiguration. has learned that the information Scholz shared during his presentation involved the use of a proxy server to ring multiple phones simultaneously in conjunction with SIP "fixup" command. Essentially it pokes a hole through a PIX firewall to allow SIP data to pass through and potentially allows for the spoofing of a source device, in this case a telephony handset.

A news source said Scholz is working with San Jose, Calif.-based Cisco and United States Computer Emergency Readiness Team (US-CERT) on the matter, and is giving the networking giant time to address any outstanding vulnerabilities before disclosing more details.

Cisco is investigating the discovery, but said it may need several days to vet the issue because it must be tested on myriad PIX devices. The vendor has emphasized that since the issue involves the exposure of a service that shouldn't be exposed, it may be caused by a problem specific to Scholz's implementation and not a true vulnerability.

If proven to be a flaw, a source said, there is a potential for telephony denial-of-service or malicious call redirection, which could lead to voice phishing.

"There weren't enough details in the slide for anyone to be able to do anything with it," said a source with knowledge of Scholz's presentation. "He wanted to let people know it was there and to protect themselves."

Scholz reportedly stumbled upon the issue within the last month, recently returned from a vacation prior to Black Hat. "He didn't think it was a big deal," the source said.

"The [flaw] Michael Lynn revealed last year had the ability to essentially bring down routing," said another source. "So on a severity scale of one to 100, if Mike Lynn's was a 95, this might be a two."

Few Cisco products support SIP; for instance, its SIP Proxy Server call-control software uses it, and its SIP IP Phone software enables certain handsets to work in SIP-based VoIP environments. Hence the reaction from Cisco's lawyers pales in comparison to the furor caused last year when researcher Michael Lynn disclosed a serious vulnerability in IOS, Cisco's router operating system. Lynn subsequently lost his job, was sued and had a run-in with the FBI over the matter. Lynn, who now works for Cisco rival Juniper Networks Inc., is at this year's Black Hat.

This news comes just hours after a pair of presenters revealed a zero-day exploit for Cisco CallManager Express.

David Endler, director of security research for the TippingPoint division of Marlborough, Mass.-based 3Com Corp., and Mark Collier, CTO of San Antonio-based telephony management vendor SecureLogix Corp., authors of the book Hacking Exposed VoIP, told Black Hat attendees that the networking giant's CallManager Express VoIP management software is vulnerable to a flaw in which a remote user can supply specially crafted SIP requests to gain information from the SIP user directory, including the names of the users stored in the SIP user database.

A patch for that issue is not yet available, but Cisco said it is investigating the problem and will provide further information when it becomes available. Cisco was notified of the issue prior to Black Hat.

Victor R. Garza and News Editor Eric B. Parizo contributed to this article.

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