Mozilla Flaw Lets Links Run Arbitrary Programs

By Larry Seltzer  |  Posted 2004-07-08 Print this article Print

Updated: The Mozilla Foundation has confirmed findings that its Mozilla and Firefox browsers are vulnerable to attacks using the "shell:" scheme, which execute arbitrary code under Windows without th

Security researchers are reporting another security issue in Web browsing under Windows, but this time Internet Explorer is not the culprit. The Mozilla Foundations Mozilla and Firefox are reported as vulnerable. The Mozilla Foundation has confirmed the problem and issued a fix, which is available here. The reports indicate that links in a Web page using the "shell:" scheme can execute arbitrary programs on the users system. The attacker would have to know the location in the file system of the program, but there are known programs in Windows with buffer overflows.
This means the attacker could create a link in a Web page that could execute arbitrary code under Windows. Through the use of an appropriate META tag, the attack could load without the user having to click a link explicitly.

In the definition of a URI (Uniform Resource Identifier), the technical name for a Web address, "shell:" is not a protocol like http but a scheme. Some schemes map directly to protocol handlers in the browser itself or externally, such as those that handle audio and video media. Current versions of Mozilla and Firefox pass unknown protocol handlers to the operating system shell to handle. In this case, the location passed to the shell is a program name that the shell executes. A new report by security firm Secunia finds that Internet Explorer may share a similar shell hole. Click here to read more. Other researchers reported that certain links in Mozilla could cause a denial of service in the system by causing Mozilla to open large numbers of windows and consume 100 percent of CPU capacity.

An old discussion in the Mozilla bug report database considers the possibility of addressing this problem, but the developers decided against it since the program has a facility for letting the user disallow specific external protocols and schemes, including shell:. It is not disabled by default, though.

The developers considered changing from scheme blacklisting to whitelisting, in which case all schemes and protocols would be disallowed unless explicitly allowed. Mozilla Foundation spokesmen said a future version of the browsers will change to whitelisting, but the interim fix just disables the shell protocol. Several other schemes, such as vbscript, are already disabled by default. Click here to read about the latest Internet Explorer exploit. Internet Explorer is reported as being less vulnerable. When the user clicks on the link, it opens an "open/save" dialog box in which the user is allowed either to run the program, save it to disk or cancel. Mozilla and Firefox simply run the program without any further user action.

For insights on security coverage around the Web, check out Security Center Editor Larry Seltzers Weblog. The shell: syntax works only on Windows XP systems. According to one report, similar functionality is available on Windows 2000 but with different syntax. tested the reported vulnerability on Mozilla Firefox and confirmed the reported behavior. We also confirmed the appearance of the open/save dialog on Windows XP SP1. In our tests on Windows XP SP2, links with the shell: protocol failed to operate at all.

Editors Note: This story was updated to include a response from the Mozilla Foundation and further explanation of the bugs nature. Check out eWEEK.coms Security Center at for security news, views and analysis.

Be sure to add our security news feed to your RSS newsreader or My Yahoo page:  

Larry Seltzer has been writing software for and English about computers ever since—,much to his own amazement—,he graduated from the University of Pennsylvania in 1983.

He was one of the authors of NPL and NPL-R, fourth-generation languages for microcomputers by the now-defunct DeskTop Software Corporation. (Larry is sad to find absolutely no hits on any of these +products on Google.) His work at Desktop Software included programming the UCSD p-System, a virtual machine-based operating system with portable binaries that pre-dated Java by more than 10 years.

For several years, he wrote corporate software for Mathematica Policy Research (they're still in business!) and Chase Econometrics (not so lucky) before being forcibly thrown into the consulting market. He bummed around the Philadelphia consulting and contract-programming scenes for a year or two before taking a job at NSTL (National Software Testing Labs) developing product tests and managing contract testing for the computer industry, governments and publication.

In 1991 Larry moved to Massachusetts to become Technical Director of PC Week Labs (now eWeek Labs). He moved within Ziff Davis to New York in 1994 to run testing at Windows Sources. In 1995, he became Technical Director for Internet product testing at PC Magazine and stayed there till 1998.

Since then, he has been writing for numerous other publications, including Fortune Small Business, Windows 2000 Magazine (now Windows and .NET Magazine), ZDNet and Sam Whitmore's Media Survey.

Submit a Comment

Loading Comments...
Manage your Newsletters: Login   Register My Newsletters

Rocket Fuel