Over the weekend, Microsoft issued a new security advisory which covered a vulnerability in how Windows handles DLL files. The attack scenario would go this way: a vulnerable application would be used to open a file.
The opened file can be a perfectly legitimate file; however the malicious file must be located in the same directory and given the same file name as a legitimate DLL file. When the vulnerable application loads, instead of calling the legitimate DLL file the malicious file is loaded instead.
This is because of errors in how Windows selects which DLL files to load, giving preference to libraries located in the same directory as the opened file instead of those in the correct system directories. Any code in the malicious file would be executed, causing a full-fledged problem for users.
These kinds of attacks–known as binary planting or DLL preloading–have been known for years. However, they were not much of a threat because the malicious file had to already be on the user’s system. Recently, however, independent researchers have found a way to exploit this attack remotely, via network shares. This resulted in Microsoft issuing the said advisory.
Popular applications like Firefox and Powerpoint are among those initially reported as affected by the vulnerability. However, more exploits for many other applications have been found, and reports on attacks actively exploiting the bug have been posted.
The existence of malware attacks actively leveraging on the said vulnerability may drive Microsoft to take more drastic action. Until a clear solution is given, users are strongly advised to be careful about files opened from network shares.
Enterprise users with certain Trend Micro products such as Deep Security and OfficeScan with Intrusion Defense Firewall (IDF) plug-in may download the latest rules to help protect themselves against this threat; these rules prevent DLLs from being loaded from remote shares.