Format string attacks are a new class of [[Exploit_(computer_science)|vulnerbilitiesvulnerabilities]] discovered in June of 2000 previously thought harmless. The problem exists in the %n format token for [[C_programming_language|C]]'s <code>[[Printf|printf()]]</code> format strings that commands <code>printf()</code> to write back the number of bytes formatted to the same argument to <code>printf()</code>, assuming that the corresponding [[Argument|argument]] exists, and is of [[Datatype|type]] int * . [[Software]] that allows unfiltered user input as the first argument to <code>printf()</code> is vulnerable to format string attacks.
This is a common vulnerability due to the fact that format bugs were previously thought harmless and resulted in vulnerabilites in many common tools. [http://www.cve.mitre.org/cgi-bin/cvekey.cgi?keyword=format+string MITRE's CVE project] list roughly 150 vulnerable programs.
