The vulnerability affects IE versions 6, 7 and 8. The latest versions of the browser, 9 and 10, are not affected. The company occasionally issues quick fixes as a temporary protective measure while a permanent security update is developed if a vulnerability is considered particularly dangerous.
Microsoft issued an advisory on Saturday warning of the problem, which involves how IE accesses "an object in memory that has been deleted or has not been properly allocated." The problem corrupts the browser's memory, allowing attackers to execute their own code.
The vulnerability can be exploited by manipulating a website in order to attack vulnerable browsers, one of the most dangerous types of attacks known as a drive-by download. Victims merely need to visit the tampered site in order for their computer to become infected. To be successful, the hacker would have to lure the person to the harmful website, which is usually done by sending a malicious link via email.
Security vendor Symantec described such a scenario as a "watering hole" attack, where victims are profiled and then lured to the malicious site. Last week, one of the websites discovered to have been rigged to delivered an attack was that of the Council on Foreign Relations, a reknowned foreign policy think tank.
The attack delivers a piece of malware nicknamed Bifrose, a malware family first detected around 2004. Bifrose is a "backdoor" that allows an attacker to steal files from a computer. Symantec wrote that the attacks using the IE vulnerability appear to be limited and concentrated in North America, indicating a targeted attack campaign.
Since the attacks already under way before the vulnerability was discovered, Symantec said it "suggests a high level of sophistication requiring access to resources and skills which would normally be outside most hackers capabilities."