TimThumb (timthumb.php) before 2.0 does not validate the entire source with the domain white list, which allows remote attackers to upload and execute arbitrary code via a URL containing a white-listed domain in the src parameter, then accessing it via a direct request to the file in the cache directory, as exploited in the wild in August 2011.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.