A vulnerability found in Cisco’s 3000-series VPN concentrators running WebVPN appears to extend to all versions of the product, according to a security researcher who has been following the situation. Cisco has acknowledged the problem and confirmed that an advisory update is in the offing.

eSentire'sEldon Sprickerhoff, who described the vulnerability at last month’s Shmoocon security gathering, has been working with Cisco to pinpoint whether a recently released patch addressed the problem. Last Friday, the company set up a test installation of the concentrator running the problematic patch. Sprickerhoff successfully launched a single-machine DoS attack.

Cisco confirmed the results and the vulnerability today, though the company says that administrators on the scene retained console access to the box. Previous reports had indicated that the exploit could cause a total machine freeze.

The vulnerability is described by both Sprickerhoff and Cisco as obscure, but the exploit is trivial: A relatively small stream directed to TCP/80 can cause a concentrator running the webVPN service to drop all its connections. Interestingly, according to Sprickerhoff, PSIRT responders at Cisco suspect the problem might in fact be triggered by either of two separate vulnerabilities, either causing the dropped connections.

According to a Cisco representative, "Cisco has been contacted by Eldon Sprickerhoff with additional information about our recent Cisco VPN 3000 Series Concentrator security advisory. As a result, the Cisco product security incident response team (PSIRT) has been working directly with Mr. Sprickerhoff on this issue. Cisco PSIRT is still investigating this issue with additional research and testing and expect to update the security advisory accordingly and as necessary. Customers should take the recommended steps as outlined in the current security advisory to protect themselves from the potential impact of this vulnerability."

Cisco has attempted to patch the vulnerability in the past, most recently in their 4.7.2B release. Since the company believed until this week that the problem was solved, the hole is also likely to exist in subsequent releases 4.7.2C and 4.7.2D. It is also unclear whether the problematic code has been reused in the WebVPN service module in Cisco’s current Self-Defending Network security strategy.