A security researcher has uncovered four bugs in web browsers. Two of the vulnerabilities affect Mozilla's open-source Firefox browser, and two flaws have been found in IE (Internet Explorer). Update: Microsoft fixes four critical Windows flaws

The flaws were discovered by security researcher Michael Zalewski. He then posted examples of each on the Full-disclosure mailing list for browser vulnerabilities.

More: Mozilla confused over Firefox flaws

One of the bugs regarded as critical is found in Internet Explorer 6.0 and IE7. Zawelski said the flaw gave hackers a window through which they could run malicious JavaScript to hijack a PC. Zawelski also compared the Internet Explorer vulnerability to a browser 'collapsing'.

Although Firefox is not at risk from the Internet Explorer bugs, Zawelski found two other vulnerabilities, one of which was considered major. Attackers could intercept keystrokes or insert malicious content into a legitimate website due to an IFrame vulnerability in Firefox 2.0. A patch Firefox manufacturer Mozilla issued had been unsuccessful in solving this Firefox problem when it was uncovered last year.

The other two bugs found are less critical. The less-important Firefox vulnerability could enable unauthorised downloads of execution files.

A bar-spoofing flaw found in Internet Explorer 6.0 could enable hackers to mimic a site by spoofing the URL bar, page information and SSL certificates.

The bugs have been brought to Mozilla’s attention and been posted on the Bugzilla website, Mozilla’s bug-tracking system.

Although Microsoft is unaware of any ongoing attacks due to the Internet Explorer flaws, the company is investigating Zalewski’s claims over the IE vulnerabilities. It may be necessary for Microsoft to issue a security advisor or provide a security update.

See also Firefox and Google combine to block malware

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