Opera Software found and patched what it's calling a "highly severe" bug in its flagship browser, using a security tool released by its competitor, Mozilla.

During the recent Black Hat security conference, the Mozilla Foundation made the JavaScript fuzzer, an open-source application testing security testing tool, available to anyone who wants to use it. Mozilla has been using it to detect and fix dozens of security bugs in Firefox, according Window Snyder, who is head of Mozilla's product security.

Security fuzzers are software tools that test an application for problems like buffer overflows, format string vulnerabilities, and error handling. For example, Mozilla's JavaScript fuzzer recently found 280 bugs in Firefox, 27 of which were exploitable.

Now, Opera is putting the tool to a similar use.

Opera noted in an advisory that the flaw could allow a hacker to execute code on the victim machine. A virtual function call on an invalid pointer, which may reference data crafted by the attacker, can be used to execute arbitrary code.

Opera Software released Opera V9.23 to fix the problem. The company publicly thanked Mozilla for providing them the JavaScript fuzzer.

In a blog posted on Aug. 6, Snyder said she had communicated with Claudio Santambrogio, a test manager at Opera, and he told her that at that point they had found and fixed four flaws with the fuzzer.

"I am thrilled," wrote Snyder. "This is exactly what we hoped would happen. Hopefully, this will encourage other vendors to share their internal security tools with everyone so we call all make our software more secure."

Mozilla worked with Microsoft, Apple, and Opera before making the JavaScript fuzzer widely available in order to reduce the possibility that the tool might be used to expose vulnerabilities in those browsers. According to Snyder, all of these browser vendors reviewed the tool and told Mozilla know they were okay with the release.