Parameter tampering is a form of Web-based attack in which certain parameters in the Uniform Resource Locator (URL) or Web page form field data entered by a user are changed without that user's authorization. This points the browser to a link, page or site other than the one the user intends (although it may look exactly the same to the casual observer).
Parameter tampering can be employed by criminals and identity thieves to surreptitiously obtain personal or business information about the user. Countermeasures specific to the prevention of parameter tampering involve the validation of all parameters to ensure that they conform to standards concerning minimum and maximum allowable length, allowable numeric range, allowable character sequences and patterns, whether or not the parameter is actually required to conduct the transaction in question, and whether or not null is allowed.
Whitelisting (accepting only allowable input) is more effective than blacklisting (refusing to accept forbidden input). A Web application firewall can provide some protection against parameter tampering, provided that it is configured properly for the site in use. Overall, the vulnerability of a computer or network to parameter tampering can be minimized by implementing a strict application security routine and making sure that it is kept up to date.