Obscured Security-relevant Information by Alternate Name

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 code prints the contents of a file if a user has permission.

function readFile($filename){

  $user = getCurrentUser();
  $realFile = $filename;

  //resolve file if its a symbolic link
  if(is_link($filename)){
    $realFile = readlink($filename);
  }

  if(fileowner($realFile) == $user){
    echo file_get_contents($realFile);
    return;
  }
  else{
    echo 'Access denied';
    writeLog($user . ' attempted to access the file '. $filename . ' on '. date('r'));
  }

}

While the code logs a bad access attempt, it logs the user supplied name for the file, not the canonicalized file name. An attacker can obscure their target by giving the script the name of a link to the file they are attempting to access. Also note this code contains a race condition between the is_link() and readlink() functions (CWE-363).

See Also