Cisco has released a proper fix for the critical remote code execution vulnerability affecting the WebEx browser extension, but the patch is currently only available for the Chrome version.

Google Project Zero researcher Tavis Ormandy reported a few days ago that Cisco’s WebEx extension for Chrome, which has roughly 20 million active installs, was affected by a serious flaw that could have been exploited to execute arbitrary code simply by getting a user to access a specially crafted website.

The expert disclosed the details of the flaw after Cisco claimed to have patched it, but it later turned out that the initial fix was incomplete. Ormandy warned that the security hole could still be exploited without any user interaction if an attacker could find a cross-site scripting (XSS) vulnerability on *.webex.com domains – which he did find.

Cisco on Thursday released version 1.0.7 of the WebEx extension for Chrome and Ormandy said he had not found a way to defeat the new patch.

According to Cisco, the vulnerability also affects Firefox and Internet Explorer on Windows, but patches have yet to be released for these web browsers.

Google has restored the WebEx extension in its Chrome Web Store, but Mozilla is still blocking it in Firefox. The networking giant said Mac OS X and Linux systems are not impacted.

The vulnerability, tracked as CVE-2017-3823, can be exploited through a “magic string” used to activate the WebEx extension inside the browser. In an advisory describing the security hole, Cisco said the flaw is caused by a design defect in an API response parser.

In addition to the patch for the Chrome extension, Cisco released Snort rules to help organizations detect potential attacks. Other security vendors have also provided information on how their products can block exploitation attempts. For the time being, there is no evidence that the vulnerability has been exploited in the wild.

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Eduard Kovacs is an international correspondent for 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.

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