Incomplete List of Disallowed Inputs

Description

Developers often try to protect their products against malicious input by performing tests against inputs that are known to be bad, such as special characters that can invoke new commands. However, such lists often only account for the most well-known bad inputs. Attackers may be able to find other malicious inputs that were not expected by the developer, allowing them to bypass the intended protection mechanism.

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

The following code attempts to stop XSS attacks by removing all occurences of "script" in an input string.

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

Because the code only checks for the lower-case "script" string, it can be easily defeated with upper-case script tags.

See Also