Incorrect Behavior Order: Validate Before Filter

Description

This can be used by an attacker to bypass the validation and launch attacks that expose weaknesses that would otherwise be prevented, such as injection.

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

This script creates a subdirectory within a user directory and sets the user as the owner.

function createDir($userName,$dirName){
  $userDir = '/users/'. $userName;
  if(strpos($dirName,'..') !== false){
    echo 'Directory name contains invalid sequence';
    return;
  }
  //filter out '~' because other scripts identify user directories by this prefix
  $dirName = str_replace('~','',$dirName);
  $newDir = $userDir . $dirName;
  mkdir($newDir, 0700);
  chown($newDir,$userName);
}

While the script attempts to screen for '..' sequences, an attacker can submit a directory path including ".~.", which will then become ".." after the filtering step. This allows a Path Traversal (CWE-21) attack to occur.

See Also