Time-of-check Time-of-use (TOCTOU) Race Condition

Description

This weakness can be security-relevant when an attacker can influence the state of the resource between check and use. This can happen with shared resources such as files, memory, or even variables in multithreaded programs.

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 checks a file, then updates its contents.

struct stat *sb;
...
lstat("...",sb); // it has not been updated since the last time it was read
printf("stated file\n");
if (sb->st_mtimespec==...){
  print("Now updating things\n");
  updateThings();
}

Potentially the file could have been updated between the time of the check and the lstat, especially since the printf has latency.

Example Two

The following code is from a program installed setuid root. The program performs certain file operations on behalf of non-privileged users, and uses access checks to ensure that it does not use its root privileges to perform operations that should otherwise be unavailable the current user. The program uses the access() system call to check if the person running the program has permission to access the specified file before it opens the file and performs the necessary operations.

if(!access(file,W_OK)) {
  f = fopen(file,"w+");
  operate(f);
  ...
}
else {


  fprintf(stderr,"Unable to open file %s.\n",file);

}

The call to access() behaves as expected, and returns 0 if the user running the program has the necessary permissions to write to the file, and -1 otherwise. However, because both access() and fopen() operate on filenames rather than on file handles, there is no guarantee that the file variable still refers to the same file on disk when it is passed to fopen() that it did when it was passed to access(). If an attacker replaces file after the call to access() with a symbolic link to a different file, the program will use its root privileges to operate on the file even if it is a file that the attacker would otherwise be unable to modify. By tricking the program into performing an operation that would otherwise be impermissible, the attacker has gained elevated privileges. This type of vulnerability is not limited to programs with root privileges. If the application is capable of performing any operation that the attacker would not otherwise be allowed perform, then it is a possible target.

Example Three

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

function readFile($filename){

  $user = getCurrentUser();

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

  if(fileowner($filename) == $user){
    echo file_get_contents($realFile);
    return;
  }
  else{
    echo 'Access denied';
    return false;
  }

}

This code attempts to resolve symbolic links before checking the file and printing its contents. However, an attacker may be able to change the file from a real file to a symbolic link between the calls to is_link() and file_get_contents(), allowing the reading of arbitrary files. Note that this code fails to log the attempted access (CWE-778).

Example Four

This example is adapted from [REF-18]. Assume that this code block is invoked from multiple threads. The switch statement will execute different code depending on the time when MYFILE.txt was last changed.

#include <sys/types.h>
#include <sys/stat.h>

...

struct stat sb;
stat("MYFILE.txt",&sb);
printf("file change time: %d\n",sb->st_ctime);
switch(sb->st_ctime % 2){
  case 0: printf("Option 1\n"); break;
  case 1: printf("Option 2\n"); break;
  default: printf("this should be unreachable?\n"); break;

}

If this code block were executed within multiple threads, and MYFILE.txt changed between the operation of one thread and another, then the switch could produce different, possibly unexpected results.

See Also