Security experts are warning Microsoft customers about silent internet attacks, which exploit a security flaw in Internet Explorer and, potentially, allow remote attackers to run malicious code on vulnerable machines.
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The vulnerability is similar in scope to those exploited by devastating worms such as Nimda, Badtrans and Klez, according to one security company. And, to make matters worse, the flaw is one Microsoft said it fixed weeks ago.
The security hole, known as the "Object Data vulnerability", affects Internet Explorer versions 5.01, 5.5 and 6.0. It concerns the way that Explorer processes HTML (Hypertext Markup Language) pages containing a special element called the Object Data tag.
If properly exploited, the vulnerability could enable an attacker to place a malicious computer program on a user's machine. No user actions would be required aside from opening an e-mail message or visiting a web page containing the attack.
On 20 August, Microsoft released patch MS03-032 for Explorer which, it claimed, closed the hole, in addition to patching other security holes in the browser.
However, a message posted to a prominent security discussion group on Sunday warned that the vulnerability still exists on machines using Explorer even after applying the patch.
A Microsoft spokesman confirmed that it is investigating the reports of new exploits for one of the vulnerabilities addressed in the MS03-032 security bulletin.
However, Microsoft still recommends that customers install that patch, he said.
Microsoft claimed to be unaware of any customers who have been attacked using the vulnerability.
However, security researchers know of at least one exploitation of the Object Data vulnerability already circulating on the internet. An alert by security company Secunia in Copenhagen said that an e-mail message containing HTML code that exploits the vulnerability is used to silently retrieve and run a file, "drg.exe", which installs a file called "surferbar.dll" onto the victim's computer.
That file adds a bar to the affected users' Explorer browser, which has links to pornographic websites.
The Object Data vulnerability is also similar to an earlier Explorer security hole dating to 2001, MS01-020, that was exploited by virulent e-mail worms such as Nimda and Klez, according to Secunia.
Security experts familiar with the issue say that Microsoft's failure to test their patch thoroughly against attack scenarios using the Object Data vulnerability is a black eye for the company.
"Microsoft should be ashamed. This is a major embarrassment," said Richard Smith, an independent security analyst based in Boston.
The problem with the Object Data vulnerability is similar to a hole found in an earlier Microsoft patch, according to Israeli security company GreyMagic Software, which issued a report on the problem in February 2002.
That fact points to problems with Microsoft's patch testing process, Smith said.
"They need to go back and look at how this slip-up occurred. They keep saying they can't prevent bugs, but when the same problems keep occurring over and over, that's a management issue," he said.
A Microsoft spokesman said the company is committed to keeping customers data safe and will take "appropriate action" to protect customers when its investigation into the new exploits is complete.
In the absence of a patch from Microsoft to fix the problem, security experts recommended disabling support for Active Scripting on affected versions of Explorer. Failing that, users should consider uninstalling Explorer to protect themselves from attack.
Paul Roberts writes for IDG News Service