OWASP Top10 2010

Editor's Notes

• #8: Input validation. Use a standard input validation mechanism to validate all input data for length, type, syntax, and business rules before accepting the data to be displayed or stored. Use an "accept known good" validation strategy. Reject invalid input rather than attempting to sanitize potentially hostile data. Do not forget that error messages might also include invalid data. Strong output encoding. Ensure that all user-supplied data is appropriately entity encoded (either HTML or XML depending on the output mechanism) before rendering, taking the approach to encode all characters other than a very limited subset. This is the approach of the Microsoft Anti-XSS library, and the forthcoming OWASP PHP Anti-XSS library. Also, set the character encodings for each page you output, which will reduce exposure to some variants. Specify the output encoding (such as ISO 8859-1 or UTF 8). Do not allow the attacker to choose this for your users. Do not use "blacklist" validation to detect XSS in input or to encode output. Searching for and replacing just a few characters ("<" ">" and other similar characters or phrases such as “script”) is weak and has been attacked successfully. Even an unchecked “<b>” tag is unsafe in some contexts. XSS has a surprising number of variants that make it easy to bypass blacklist validation. Watch out for canonicalization errors. Inputs must be decoded and canonicalized to the application’s current internal representation before being validated. Make sure that your application does not decode the same input twice. Such errors could be used to bypass whitelist schemes by introducing dangerous inputs after they have been checked.
• #11: Input validation. Use a standard input validation mechanism to validate all input data for length, type, syntax, and business rules before accepting the data to be displayed or stored. Use an "accept known good" validation strategy. Reject invalid input rather than attempting to sanitize potentially hostile data. Do not forget that error messages might also include invalid data Use strongly typed parameterized query APIs with placeholder substitution markers, even when calling stored procedures Enforce least privilege when connecting to databases and other backend systems Avoid detailed error messages that are useful to an attacker Show care when using stored procedures since they are generally safe from SQL Injection. However, be careful as they can be injectable (such as via the use of exec() or concatenating arguments within the stored procedure) Do not use dynamic query interfaces (such as mysql_query() or similar) Do not use simple escaping functions , such as PHP's addslashes() or character replacement functions like str_replace("'", " "). These are weak and have been successfully exploited by attackers. . For PHP, use mysql_real_escape_string() if using MySQL, or preferably use PDO which does not require escaping When using simple escape mechanisms, note that simple escaping functions cannot escape table names ! Table names must be legal SQL, and thus are completely unsuitable for user supplied input Watch out for canonicalization errors. Inputs must be decoded and canonicalized to the application's current internal representation before being validated. Make sure that your application does not decode the same input twice. Such errors could be used to bypass whitelist schemes by introducing dangerous inputs after they have been checked