Zero-Day Hits IE-Firefox Combo

By Lisa Vaas  |  Posted 2007-07-10 Print this article Print

Fingers are being pointed regarding whether it's Firefox or IE that's responsible.

Security researcher Thor Larholm has discovered a zero-day vulnerability that could lead to remote attackers hijacking systems running both Internet Explorer and Firefox. Larholm is calling this an IE zero day, blaming the vulnerability on an input validation flaw in Internet Explorer that allows users to specify arbitrary arguments to the process responsible for handling URL protocols. Its the same type of input validation vulnerability that Larholm discovered in the Safari 3 beta, he said. Microsoft says no dice, it aint IE this time, although a spokesman neglected to say where the vulnerability lies if not in the browser. "Microsoft has thoroughly investigated the claim of a vulnerability in Internet Explorer and found that this is not a vulnerability in a Microsoft product," the spokesman said.
At any rate, Larholm says that the vulnerability consists of a flaw in IEs protocol handler that allows for command injection. A successful attack would entail passing and executing arbitrary commands and arguments through the browsers "firefox.exe" process.
The problem comes out of an input validation error when a vulnerable instance of IE is installed on the same system as Mozillas Firefox browser. Firefox registers a URL protocol handler named "FirefoxURL" when its installed. The handler lets other applications launch Firefox if theyre capable of handling HTML. The flaw arises when IE fails to validate the handler and passes any parameters in the request directly to the firefox.exe process as arguments or options. According to Symantec, attackers can also exploit the issue to execute cross-browser scripting attacks with the "-chrome" argument. This would allow attackers to run JavaScript with the privileges of trusted Chrome context that has full access to Firefoxs resources. Read more here about a critical .Net flaw. "Chrome" refers to an interface written in XUL and JavaScript thats used by the Mozilla applications Firefox, Thunderbird and the Gecko rendering engine. Gecko renders both Web pages and user interface, enabling a user to enter chrome URLs into the Firefox address bar and render inside the browser. Exploiting the issue allows a remote attacker to influence command options that can be called through the FirefoxURL handler and therefore execute commands and script code with the privileges of a user running the applications, according to Symantecs Deep Site threat analysis service. Successful attacks can lead to remote unauthorized access, among other consequences. Attack scenarios include an attacker constructing malicious HTML code to influence command-line parameters that will run when a URI is loaded. The malicious code can be embedded in a Web page or sent through HTML e-mail. The malicious code may be automatically loaded when the page or HTML e-mail is rendered. This scenario entails a user clicking on a link to get to the malicious site or opening the malicious e-mail. Even feedback on Larholms site concerned trying to figure out which browser is vulnerable and which is a vector. "It may be worth noticing that this is NOT an Internet Explorer flaw, but a Firefox flaw," said a feedback post from Michael Mattson on Larholms site. "Why the author would title this article Internet Explorer 0day Exploit is really misleading, and shows the authors lack of understanding of how programs register URIs. This is CLEARLY an issue with Firefox and its flawed URI registration." "Firefox is the current attack vector but Internet Explorer is to blame for not escaping … characters when passing on the input to the command line," Larholm responded. "I agree that Firefox could have registered its URL handler with pure DDE [Dynamic Data Exchange] instead and thereby have avoided the possibility of a command line argument injection, but IE should still be able to safely launch external applications safely." To mitigate risk, Symantec is advising customers to avoid following links from unknown or untrusted sources and for users to be wary of visiting untrusted or unfamilar sites or following links provided by unknown or suspicious sources. In addition, Symantec says, dont accept communications that originate from unknown or untrusted sources, and never open or accept unsolicited HTML e-mail, because it may provide an attack vector for numerous vulnerabilities. Also, filter all HTML e-mail or disable client support for HTML e-mail. Run all software as a nonprivileged user with minimal access rights, Symantec also advises. To limit the impact of client vulnerabilities, perform all nonadministrative tasks, such as reading e-mail and browsing, as an unprivileged user with minimal access rights. Check out eWEEK.coms Security Center for the latest security news, reviews and analysis. And for insights on security coverage around the Web, take a look at eWEEKs Security Watch blog.
Lisa Vaas is News Editor/Operations for and also serves as editor of the Database topic center. Since 1995, she has also been a Webcast news show anchorperson and a reporter covering the IT industry. She has focused on customer relationship management technology, IT salaries and careers, effects of the H1-B visa on the technology workforce, wireless technology, security, and, most recently, databases and the technologies that touch upon them. Her articles have appeared in eWEEK's print edition, on, and in the startup IT magazine PC Connection. Prior to becoming a journalist, Vaas experienced an array of eye-opening careers, including driving a cab in Boston, photographing cranky babies in shopping malls, selling cameras, typography and computer training. She stopped a hair short of finishing an M.A. in English at the University of Massachusetts in Boston. She earned a B.S. in Communications from Emerson College. She runs two open-mic reading series in Boston and currently keeps bees in her home in Mashpee, Mass.

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