Microsoft is alerting the public about a new vector that can be used to infect PCs when an application is tricked into loading a malicious library.
The software giant issued the advisory Monday after recent research revealed that a class of vulnerabilities known as "DLL (dynamic-link Library) preloading" can be exploited remotely by an attacker who places a malicious library on a network share, Christopher Budd, senior security response communications manager at Microsoft, said in a blog post. In the past, the attacker needed access to the local client system to perpetrate the exploit.
"In this [new] scenario, the attacker would create a data file that the vulnerable application would open, create a malicious library that the vulnerable application would use, post both of them on a network share that the user could access, and convince the user to open the data file," Budd wrote. "At that point, the application would load the malicious library and the attacker's code would execute on the user's system."
In addition to users disabling WebDAV and blocking outbound SMB (Server Message Block) traffic at the perimeter to prevent the threat, Microsoft released a tool Monday that prevents the loading of libraries from network shares.
Andrew Storms, director of security operations at nCircle, a vulnerability management firm, called the threat a "new take on an old attack vector."
"Programs that are trying to load libraries may try to do so from an arbitrary directory, not a specific one," he told SCMagazineUS.com on Tuesday. "It's going to load the malware library as opposed to the library form Windows."
HD Moore, chief security officer of Rapid7, was among the first researchers to discover the issue, which he called DLL hijacking. On Sunday, he posted exploit code as part of his open-source Metasploit hacking toolkit.
He said last week that 40 different Windows applications are susceptible to the attack. Now that the exploit is out, more and more researchers have claimed to discover vulnerable applications, including Windows Live Mail and PowerPoint.
"This vulnerability is triggered when a vulnerable file type is opened from within a directory controlled by the attacker," Moore wrote Sunday in a blog post. "This directory can be a USB drive, an extracted archive, or a remote network share. In most cases, the user will have to browse to the directory and then open the target file type for this exploit to work. The file opened by the user can be completely harmless. [T]he flaw is that the application launched to handle the file type will inadvertently load a DLL from the working directory."
Microsoft did not say whether any of its programs are vulnerable.
"We are currently conducting a thorough investigation into how this new vector may affect Microsoft products," Budd wrote. "As always, if we find this issue affects any of our products, we will address them appropriately."
Storms said organizations should play close attention to the issue — particularly whether any Microsoft products are vulnerable — and take the necessary mitigation steps, including educating users. However, they also should understand that a successful exploit is difficult to accomplish.
"It's not as easy as a click-a-link-and-be-owned kind of thing," he said. "Two steps of user interaction [are required]."