Incorrect Behavior Order: Validate Before Filter

The software validates data before it has been filtered, which prevents the software from detecting data that becomes invalid after the filtering step.


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

SFP Secondary Cluster: Faulty Input Transformation

This category identifies Software Fault Patterns (SFPs) within the Faulty Input Transformation cluster.

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Weaknesses without Software Fault Patterns

CWE identifiers in this view are weaknesses that do not have associated Software Fault Patterns (SFPs), as covered by the CWE-888 view. As such, they represent gaps in...

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.


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