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A public claim by hackers that Mozilla’s Firefox browser is vulnerable to multiple code execution vulnerabilities may be an overblown hoax.

On the heels of a ToorCon presentation where two security researchers—Mischa Spiegelmock and Andrew Wbeelsoi—warned that Firefox’s implementation of JavaScript was badly flawed and could allow PC takeover attacks, Mozilla’s engineers say the risk is limited to a denial-of-service issue.

Spiegelmock, a developer at Six Apart, a blog software company in San Francisco, now says the ToorCon talk was meant “to be humorous” and insists the code presented at the conference cannot result in code execution.

Spiegelmock’s strange about-face comes as Mozilla’s security response team is racing to piece together information from the ToorCon talk to figure out how to fix the issue.

Mozilla security chief Window Snyder, who was an attendee at the conference, said the company is treating the claims as real until it can be verified otherwise but, as of Oct. 2, the open-source group could only reproduce a denial-of-service issue that caused a browser crash.

“In some cases this causes a crash based on an out-of-memory error. Based on the information we have at this time we have not been able to confirm whether an attacker can achieve code execution. We’re still investigating,” Snyder said.

A few hours later on Oct. 2, after discussions with Spiegelmock, Snyder said the researcher provided more code along with a note explaining the extent of the risk.

In Spiegelmock’s note, posted to the Mozilla developer blog, the researcher admitted the claims presented at ToorCon were a bit overblown.

“As part of our talk we mentioned that there was a previously known Firefox vulnerability that could result in a stack overflow ending up in remote code execution. However, the code we presented did not in fact do this, and I personally have not gotten it to result in code execution, nor do I know of anyone who has,” Spiegelmock said.

“I have not succeeded in making this code do anything more than cause a crash and eat up system resources, and I certainly haven’t used it to take over anyone else’s computer and execute arbitrary code,” he added.

On the claim that there are 30 undisclosed Firefox vulnerabilities, Spiegelmock pinned that entirely on co-presenter Wbeelsoi. “I have no undisclosed Firefox vulnerabilities. The person who was speaking with me made this claim, and I honestly have no idea if he has them or not. I apologize to everyone involved, and I hope I have made everything as clear as possible,” Spiegelmock added.

Wbeelsoi could not be reached for comment.

“Even though Mischa hasn’t been able to achieve code execution, we still take this issue seriously. We will continue to investigate,” Mozilla’s Snyder added.

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