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Unicode bug restyled as DoS tool

Big Poop drops a big one on IIS

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Security for virtualized datacentres

The infamous Unicode IIS Web server exploit can also be used as a denial of service attack tool.

Gray hat hacker Big Poop has published a site on the Internet explaining how the Unicode bug, which permits the execution of commands on a Web server, can be used to tie up system resources so that legitimate users can't access a site - a classic DoS attack technique.

Mark Read, a security consultant at MIS Corporate Defence Solutions, who has reviewed Big Poop's site, said the exploit works by running up a number of processes on a server that do not terminate, such as comp.exe (which will wait indefinitely until the filenames of files to be compared are entered).

"Since the program doesn't terminate, IIS keeps the connection open assuming that something is going to be passed back to the browser," said Reid. "After so many connections though, IIS will stop any further connections in an attempt to stop the server from crying itself to sleep."

The exploit would give a s'kiddie a good chance of bringing a Web server exposed to the Unicode bug down to its knees using a web browser and a simple three part procedure (which we won't publish here). Restarting a machine will, at least, interrupt (if not curtail) the denial of service, but the issue still gives serious cause for concern, particular given how easy it is to exploit.

As Big Poop said: "If you still don't get it [the exploit] give up hackin' cause it ain't going to get any simpler, maybe relax and play the classic old school game chuckie egg, you know it makes sense.

"I don't think you are going to find an easier DOS attack for a while, well not until WindowsXP comes out anyway," he added.

Some security experts said the Unicode DoS attack, which can be automated through the use of scripts, is "more efficient" that more familiar network-based DDoS attacks, which commonly rely on installing Trojan horse programs on a range of compromised "zombie" clients. Under the control of a cracker, these zombies then fire off a batch of spurious commands against target servers with the intention of making sites unavailable.

Despite the possibility of mounting denial of service attacks on vulnerable servers using the Unicode bug, MIS's Read still reckons s'kiddies are still more likely to deface a site than mount a denial of service attack against it.

"This is a DOS attack, but to be perfectly honest if the sys admin hasn't applied the patch for the Unicode vulnerability then it goes without saying that the server is going to be wide open," he said. ®

External Links

Patch to fix Unicode exploit
CERT advisory

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