Twitter is vulnerable to a serious cross-site scripting (XSS) vulnerability that could allow an attacker to hijack users' accounts or, in conjunction with other exploit code, compromise their computers.

Proof-of-concept exploit code has been posted by Secure Science researchers Lance James and Eric Wastl. They say that Twitter has been notified but has not yet responded to them.

The proof-of-concept code page offers those clicking on the link a choice of whether they want to be exploited or not. Those who accept will trigger the exploit, causing the message "I just got owned!" to be posted to the Twitter XSSExploits account.

Twitter did not immediately respond to a request for comment.

"The vulnerability is still active," said Wastl. "Basically, we produce a link and if a Twitter user clicks on it, it allows us to hijack their accounts."

XSS vulnerabilities allow attackers to inject malicious code into Web pages, including HTML and client-side scripts. They can be used to bypass access controls, steal information, and conduct phishing attacks.

James cautions that XSS vulnerabilities should be taken seriously because they can reach beyond Web pages. "A lot of people think XSS is limited to the Web," he said. If there's another vulnerability in the victim's browser, the Twitter flaw could be used to launch additional malicious code, he explained.

This is particularly germane to Twitter users because so many of them rely on specialized third-party Twitter browsing applications, which aren't subjected to the security scrutiny given to major Web browsers.