The Mozilla Foundation last week revealed that it's had to patch several serious security flaws in its Firefox browser. The Firefox bugs also affect the SeaMonkey browser and Thunderbird email application. According to security firm Secunia, Firefox and IE 7 have been affected by a similar number of advisories so far this year, although Microsoft's Internet Explorer has been hit by more serious bugs than Firefox.

The Firefox flaws could allow an attacker to take over a system, Mozilla said. The Firefox bugs also include less serious exploits such as spoofing or security bypass.

Firefox flaws show there's no safety in numbers

While browser bug patches, even for critical flaws, have become routine, these latest Firefox alerts highlight the fact that Firefox no longer has as clear a security advantage over Microsoft's Internet Explorer as it once did.

In an advisory, Mozilla said the Firefox and update releases had fixed bugs that appeared to allow remote system access.

"Some of these crashes showed evidence of memory corruption under certain circumstances and we presume that with enough effort at least some of these could be exploited to run arbitrary code," the company stated.

One bug involves Firefox's JavaScript engine and could cause memory corruption and allow malicious code execution, Mozilla said.

The Thunderbird email client uses Firefox's browser engine and is thus also vulnerable - however, since JavaScript is disabled by default in Thunderbird, users are unlikely to be affected, the company said.

"We strongly discourage users from running JavaScript in mail," the company said. The advisory added that it may be possible to exploit the memory corruption vulnerability through some other means than JavaScript, such as large images.

Also fixed were an error in the "addEventListener" method which could be exploited to inject script from one site into another, and a bug in the handling of XUL pop-ups that could allow spoofing of the location bar or other browser interface components.

Secunia called the bugs "highly critical".

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To date, Firefox has been affected by five vulnerabilities and IE 7 by six, according to Secunia's statistics.

Forty percent of the Firefox bugs remain unpatched, compared with 50 percent for IE 7. But only 13 percent of the Firefox bugs allowed system access, compared to 43 percent for IE 7, which is more closely integrated with Windows.

Correspondingly, Secunia ranked 17 percent of the IE bugs as "extremely critical," a status attained by none of the Firefox advisories.

Forty percent of the Firefox advisories were "highly critical", with the rest less serious. Along with IE 7's 17 percent "extremely critical" bugs, 33 percent were "highly critical" and the others were less serious.

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