Mozilla Partially Vulnerable to Internet Explorer URL Spoofing Security Flaw

Thursday December 11th, 2003

koody wrote in to tell us that Mozilla is partially vulnerable to the recently announced URL spoofing security hole in Internet Explorer. The latest IE flaw allows an attacker to disguise the true domain of a URL in the browser's Address Bar, allowing a page located at to appear to be from This exploit can be used to increase the effectiveness of the so-called 'phishing' scams that have recently been used to target customers of PayPal, eBay and several online banks.

The Address Bar URL spoofing flaw was originally reported by Sam "Zap The Dingbat" Greenhalgh, who provided details of the exploit and a demonstration. Security company Secunia issued an advisory about the vulnerability, with an update from Chris Hall reporting that the URL shown in the Status Bar while mousing over a link to a spoofed page is also affected. While Mozilla-based browsers such as the Mozilla Application Suite and Mozilla Firebird are immune to the more serious Address Bar spoofing, they appear to be vulnerable to the Status Bar variant.

The Secunia Internet Explorer Address Bar Spoofing Test page demonstrates both the full flaw in IE and the Status Bar aspect of it that affects Mozilla. The relevant Bugzilla report is bug 228176, which was filed today and already has a preliminary patch attached (please do not add unnecessary comments to the bug; the developers are already aware of its seriousness). Mozilla users are advised to not rely on the URL displayed in the Status Bar and to check the complete address of the destination page in the Location Bar upon arrival.

#14 Re: However

by jgraham

Friday December 12th, 2003 1:03 AM

You are replying to this message

That character only appears because they've set up the test incorrectly (from the point of view of testing the Mozilla bug) - all that's needed is a null character %00 in the URL - that has %01%00 and Mozilla tries to display %01 which leads (at least on Linux) to a 'character with no assosiated glyph' symbol. See the test page I set up: <…/~jg307/test/exploit.html> As far as I know, the first case causes problems in both Mozilla and IE, the second only in IE.

For the majority of people, this bug only represents something that can't otherwise be acheived via javascript in the case where javascript is not enabled by default (i.e. in mailnews) or in the case where there is a reasonable expectation that the author did not have access to javascript e.g. in weblog comments or other forums. However, in Mozilla, there is an option to prevent javascript from changing the statusbar text - if this is set, this exploit is (as far as I know), the only way to provide a fake address in the status bar.