Cisco starts patching critical flaw in WebEx browser extension

The vulnerability could allow attackers to remotely execute malicious code on computers

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Cisco Systems has started to patch a critical vulnerability in its WebEx collaboration and conferencing browser extension that could allow attackers to remotely execute malicious code on computers.

The company released a patched version of the extension -- 1.0.7 -- for Google Chrome on Thursday and is working on similar patches for the Internet Explorer and Mozilla Firefox versions.

The vulnerability was found by Google security researcher Tavis Ormandy and stemmed from the fact that the WebEx extension exposed functionality to any website that had "cwcsf-nativemsg-iframe-43c85c0d-d633-af5e-c056-32dc7efc570b.html" in its URL or inside an iframe. Some of that WebEx functionality allowed for the execution of arbitrary code on computers.

Cisco tried to fix the problem in version 1.0.5 by restricting the sensitive features only to the *.webex.com or *.webex.com.cn domains. That didn't solve the problem completely because any cross-site scripting (XSS) vulnerability on those domains could be used to bypass the restrictions.

XSS is one of the most common types of vulnerabilities on the web, and webex.com has more than 500 defined subdomains. Chances are high that multiple XSS flaws exist on those websites and, in fact, Ormandy found one and proved a bypass of the initial patch.

Cisco added further restrictions in version 1.0.7 of the WebEx extension that appears to block all known bypass methods.

"It looks like they correctly handle Mac and Windows, and have also added some verification on GpcInitCall/GpcExitCall/etc so that functions have to match a RegEx," Ormandy said. "This looks like a huge improvement."

Ormandy added that he doesn't currently know of any way to defeat the new patch, so users should upgrade to the latest version as soon as possible.

The Chrome WebEx extension alone has around 20 million active users, so the risk of attacks is high, especially since details of this vulnerability have been public for days. IE and Firefox users who have the extension installed should disable it until a fixed version is released for those browsers.

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