A serious security flaw surfaced on Tuesday that turns conventional security assumptions on their head -- affecting Firefox and Linux, but leaving Microsoft's Internet Explorer and Windows unscathed.
The bug is in the Linux shell scripts that Firefox and the Mozilla browser suite use to parse Web addresses supplied via the command line or by external programs such as email clients. Researcher Peter Zelezny discovered that commands included in the URL and enclosed in backticks (') were executed by the Linux or Unix shell.
The flaw doesn't require Web interaction to be effective. If a user with affected versions of Firefox or Mozilla set as the default browser clicks on a maliciously crafted URL in an email program, for example, malicious commands would be executed before the browser was launched.
Security advisory aggregators Secunia and FrSIRT both gave the flaw their most severe ratings.
The Mozilla Foundation, which develops Firefox and other Mozilla-based software such as the Thunderbird email client, on Wednesday issued a Firefox update -- version 1.0.7 - fixing the flaw, as well as a week-old security bug in the handling of International Domain Names (IDN). The update can be found on the Firefox Web site.
The flaw arrives amid mounting challenges for Firefox, which has gained a significant user base in the past few months, mostly at the expense of Internet Explorer. A report from Symantec earlier this week revealed that nearly twice as many flaws had been discovered in Firefox as in Explorer over the first six months of this year. A few days earlier, developers were forced to rush out a patch for a critical hole in Firefox involving IDN parsing.
Linux is also generally seen as a lower-risk platform than Windows, partly because it is less widely used on the desktop and therefore isn't targeted as often. The security picture is changing, though, according to the Symantec report, with platforms like Linux and Mac OS X coming under increasing scrutiny by potential attackers.
Tristan Nitot, president of Mozilla Europe, has said Symantec's figures don't tell the whole story. For one thing, Firefox patches arrive faster than those for Explorer, he said, pointing out that Microsoft won't even issue its monthly patch in September. More flaws are being discovered in Firefox in the short term because of its newfound popularity, but overall, Explorer's flaws are more numerous and more severe, according to Nitot.