Jorge Luis Alvarez Medina, a security consultant with Core Security Technologies, says popular features in IE, such as URL Security Zones and the browser's file-sharing protocol, can together be abused to execute an attack that results in the attacker being able to read all files on the victim's machine. Medina plans to release proof-of-concept code for the attack next month after Black Hat DC, and after Microsoft issues a security update for the attack, which affects IE versions 6 and above, he says.
"These vulnerabilities are just features ... the implementation of the features allow you to obtain certain information, which by itself is harmless. But when combined together with other features, it renders an attack vector," Medina says. The attack requires the user to click on a malicious link.
Microsoft had previously patched two vulnerabilities in URL Security Zones -- initially discovered by Core -- that allow an attacker to cheat the Security Zones feature. But the patches don't prevent this new attack, Medina says.
"In its hot fixes, Microsoft is just slightly modifying some of the features so they cannot be strung, but as their base design is insecure, most of them are still there," he says.
Dave Forstrom, group manager of Microsoft's Trustworthy Computing, says Microsoft is investigating the vulnerability and hasn't yet seen any attacks in the wild. "Once we're done investigating, we will take appropriate action to help protect customers. This may include providing a security update through the monthly release process, an out-of-cycle update or additional guidance to help customers protect themselves," Forstrom said in a statement. He says users in the meantime should upgrade to IE 8 and enable Microsoft's automatic update feature.
The attack basically abuses the way features in IE are designed, Medina says, and it only works when a combination of features are abused in the attacks. A single feature can't be abused to wage the attack, he says. It does not, however, allow the attacker to execute code remotely or to control the victim's machine.
With IE's Security Zones, an Internet zone would not be allowed to read files from a local machine, for instance, Medina says. But if a local machine is considered part of the Internet zone, its files could be accessed by an attacker, he says.
"There are certain conditions when dealing with local equipment where it doesn't behave as expected," he says. "If it refers to a local machine by its local machine name, the computer will be treated as if it belongs to the local intranet zone. But if it's referred to by its IP address, it will be treated as part of the Internet zone, which brings complications" and opens up the possibility of this attack, he says.
Another step in the attack would trigger an SMB connection between the victim's browser and the attacker's server, which forces a handshake between the two that exposes the victim's Windows user name and other identifying information.
"The attack is not that complicated," Medina says. "And the attacker can get any kind of files -- text or binary."
The attack doesn't apply to other browsers, even though many of them use the same features that IE does, Medina says. "The problem is not just in the feature itself, but in the way it's implemented," he says. And some of that has to do with IE's relationship with Windows Explorer in the operating system, he says.
There are a few ways to mitigate these attacks, Medina says, including deploying IE's Protocol Lockdown feature to restrict the file protocol; setting the security level to "High"; disabling active scripting in the Intranet and Internet Zones; running IE in Protected Mode if available in the OS; using a different Web browser when visiting untrusted sites; and locking down and disabling the MHTML protocol handler.
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