Unrestricted Externally Accessible Lock

Description

This prevents the product from acting on associated resources or performing other behaviors that are controlled by the presence of the lock. Relevant locks might include an exclusive lock or mutex, or modifying a shared resource that is treated as a lock. If the lock can be held for an indefinite period of time, then the denial of service could be permanent.

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 tries to obtain a lock for a file, then writes to it.

function writeToLog($message){
  $logfile = fopen("logFile.log", "a");
  //attempt to get logfile lock
  if (flock($logfile, LOCK_EX)) {
    fwrite($logfile,$message);
    // unlock logfile
    flock($logfile, LOCK_UN);
  }
  else {
    print "Could not obtain lock on logFile.log, message not recorded\n";
  }
}
fclose($logFile);

PHP by default will wait indefinitely until a file lock is released. If an attacker is able to obtain the file lock, this code will pause execution, possibly leading to denial of service for other users. Note that in this case, if an attacker can perform an flock() on the file, they may already have privileges to destroy the log file. However, this still impacts the execution of other programs that depend on flock().

See Also