8. A system for repairing Uniform Resource Locator (URL) request messages, the system comprising: computer memory that stores a mapping of escape sequences to special characters; and one more processors configured to: obtain a URL request from a user computing device, the URL request specifying a URL that includes a name of a domain followed by a character string, the character string including a first product identifier; in response to determining that the character string references an invalid location, generate a modified URL at least partly by converting one or more non-URL escape sequences in the character string into URL format based in part on the mapping stored in the computer memory, wherein generating the modified URL comprises at least one of the following: (a) performing URL decoding of the URL two times to repair a doubly URL encoded portion of the URL, and (b) removing malicious content embedded in the URL; determine whether the modified URL resolves to valid content; when the modified URL is determined to resolve to valid content, provide the valid content associated with the modified URL; and when the modified URL is determined to not resolve to valid content, identify, using the first product identifier from the URL, substitute content to return in response to the URL request.
8. A system for repairing Uniform Resource Locator (URL) request messages, the system comprising: computer memory that stores a mapping of escape sequences to special characters; and one more processors configured to: obtain a URL request from a user computing device, the URL request specifying a URL that includes a name of a domain followed by a character string, the character string including a first product identifier; in response to determining that the character string references an invalid location, generate a modified URL at least partly by converting one or more non-URL escape sequences in the character string into URL format based in part on the mapping stored in the computer memory, wherein generating the modified URL comprises at least one of the following: (a) performing URL decoding of the URL two times to repair a doubly URL encoded portion of the URL, and (b) removing malicious content embedded in the URL; determine whether the modified URL resolves to valid content; when the modified URL is determined to resolve to valid content, provide the valid content associated with the modified URL; and when the modified URL is determined to not resolve to valid content, identify, using the first product identifier from the URL, substitute content to return in response to the URL request. 10. The system of claim 8 , wherein the one or more processors are configured to generate the modified URL at least partly by performing URL decoding of the URL at least two times to repair a doubly URL encoded portion of the URL.