An application might use the file name or extension of a user-supplied file to determine the proper course of action, such as selecting the correct process to which control should be passed, deciding what data should be made available, or what resources should be allocated. If the attacker can cause the code to misclassify the supplied file, then the wrong action could occur. For example, an attacker could supply a file that ends in a ".php.gif" extension that appears to be a GIF image, but would be processed as PHP code.

In extreme cases, code execution is possible, but the attacker could also cause exhaustion of resources, denial of service, exposure of debug or system data (including application source code), or being bound to a particular server side process. This weakness may be due to a vulnerability in any of the technologies used by the web and application servers, due to misconfiguration, or resultant from another flaw in the application itself.