Internet Explorer Zero-Day Exploited in Malware Attack

A malware attack was observed exploiting a new zero-day vulnerability in Internet Explorer, prompting Microsoft to issue an advisory.

Security researchers have uncovered a new Internet Explorer zero-day being used in malware attacks.

The situation was uncovered by Symantec after researchers observed a limited spam campaign seeking to trick users into clicking on a link. The spam had the subject line "re: hotel reservations," and posed as a message about a hotel room. Once inside the e-mail, users were greeted with a link leading to a page on a legitimate, but compromised, site.

"The link pointed to a page which contained a script looking to see what version of the browser and Operating System the visitor was using," explained Vikram Thakur, threat researcher at Symantec Security Response. "Since the specific exploit page only worked when someone was using Internet Explorer 6 and 7, the script only transferred the visitor to the page hosting the exploit when this condition was met. In other cases the users didn't see anything but a blank website."

"Visitors who were served the exploit page didn't realize it, but went on to download and run a piece of malware on their computer without any interaction at all," Thakur blogged. "The vulnerability allowed for any remote program to be executed without the end user's notice."

According to Microsoft, the vulnerability in question exists due to an invalid flag reference within Internet Explorer. Under certain conditions, the invalid flag reference can be accessed after an object is deleted. In attempting to access a freed object, the browser can open users up to remote code execution, according to Microsoft's advisory.

In the attack discovered by Symantec, the malware--a backdoor Trojan dubbed Pirpi--set itself to start up with the computer along with a service named "NetWare Workstation." The Trojan, which targets Windows computers, installs a backdoor and reaches out to remote servers.

"It tries to contact a specific server hosted in Poland for small files named with a .gif extension," Thakur blogged. "These small files are actually encrypted files with commands telling the Trojan what to do next."

"The files being downloaded by the attacker were hosted on yet another hacked Website," the researcher continued. "The owners of this server were also unaware of their computer being involved in hosting of malicious programs...(and) immediately took down the malicious content."

The IE vulnerability impacts IE 6, 7 and 8, Microsoft noted, but there are mitigations. Data Execution Prevention (DEP) protects against code execution and is enabled by default in IE 8 on Windows XP Service Pack 3, Windows Vista Service Pack 1, Windows Vista Service Pack 2, and Windows 7. Additionally, Protected Mode in Internet Explorer on Windows Vista and later Windows versions limits the impact of the attack.

"On completion of this investigation, Microsoft will take the appropriate action to protect our customers, which may include providing a solution through our monthly security update release process, or an out-of-cycle security update, depending on customer needs," Microsoft said in its advisory.