US-CERT is aware of a class of vulnerabilities related to how some Windows applications may load external dynamic link libraries (DLLs). When an application loads a DLL without specifying a fully qualified path name, Windows will attempt to locate the DLL by searching a defined set of directories. If an application does not securely load DLL files, an attacker may be able to cause the affected application to load an arbitrary library.

By convincing a user to open a file from a location that is under an attacker's control, such as a USB drive or network share, a remote attacker may be able to exploit this vulnerability. Exploitation of this vulnerability may result in the execution of arbitrary code.

More: http://www.us-cert.gov/current/index.html#insecure_loading_of_dynamic_link