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Webmail bug puts 40m accounts in jeopardy

One attack pwns all

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A web-borne vulnerability lurking in a popular email application seriously compromised the security of 40 million accounts until it was fixed early last month, independent researchers said.

The flaw, in the Memova messaging application sold by a company known as Critical Path, is yet another testament to the awesome power of XSS, or cross site scripting, vulnerabilities. Combined with another bug, it allowed attackers to surreptitiously forward the email of millions of end-users from some of Europe's biggest internet service providers.

"The attacker only needs to send a specially crafted email to his victim," independent researchers Rosario Valotta and Matteo Carli wrote in an advisory. "As soon as the victim opens the mail (no further interaction required) the forwarding settings of his webmail account of silently modified."

The researchers tested a proof-of-concept attack on Italian ISPs Tiscali, Libero (also known as Wind) and Virgilio (aka Telecom) and found all three to be vulnerable. Using Critical Path press releases announcing customer deployments, they say about a dozen other large ISPs also used Memova, including Vodafone, Virgin, T-Mobile, and Telefonica. All told, that's 40 million combined users, they say.

A video of the PoC is here.

Critical Path representatives hadn't responded to requests for comment by time of publication, but Valotta told The Register the company issued an update patching the vulnerability shortly after it was brought to their attention. "They answered immediately to our advisory," he said. By last week, all of Critical Path's customers had installed it, he added.

What's notable here is that two of the three sites Valotta and Carli tested had implemented protections designed to mitigate the exploitation of XSS vulnerabilities. Specifically, the providers designated one domain for webmail and a separate domain for iframes that display the mail content. Even still, the researchers found a way to bypass the protection using a technique known as reflected XSS.

Yes, the vulnerability has been fixed, and no, there are no reports it was exploited in the wild. Still, the discovery that a single web bug could compromise the privacy of so many accounts had to be more than just a little stunning. More than two-thirds of websites suffer from XSS flaws, Jeremiah Grossman, CTO of WhiteHat Security and an expert in website security has estimated. Given the proliferation, odds are that plenty of other accounts are similarly susceptible. ®

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