15. A non-transitory computer readable storage system having stored thereon instructions that, when executed, direct a computing system to perform operations comprising: obtaining 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; determining whether the character string references an invalid location; in response to determining that the character string references an invalid location, generating a modified character string at least partly by replacing a non-URL escape sequence in the character string with a corresponding special character associated with the non-URL escape sequence, wherein generating the modified character string comprises at least one of the following: (a) performing URL decoding on the character string two times to repair a doubly URL encoded portion of the character string, and (b) removing malicious content embedded in the URL; determining whether the modified character string resolves to valid content; when the modified character string is determined to resolve to valid content, providing a modified URL including the name of the domain and the modified character string; and when the modified character string is determined to not resolve to valid content, identifying a substitute URL and providing the substitute URL.
20. The non-transitory computer readable storage system of claim 15 , wherein generating the modified character string comprises performing URL decoding on the character string two times to repair a doubly URL encoded portion of the character string.