Improper Neutralization of Alternate XSS Syntax

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

In the following example, an XSS neutralization method intends to replace script tags in user-supplied input with a safe equivalent:

public String preventXSS(String input, String mask) {
  return input.replaceAll("script", mask);
}

The code only works when the "script" tag is in all lower-case, forming an incomplete denylist (CWE-184). Equivalent tags such as "SCRIPT" or "ScRiPt" will not be neutralized by this method, allowing an XSS attack.

See Also