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

The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. This can cause the product to perform invalid actions when the resource is in an unexpected state.


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.


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");

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+");
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
    $filename = readlink($filename);

  if(fileowner($filename) == $user){
    echo file_get_contents($realFile);
    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;
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

Comprehensive Categorization: Concurrency

Weaknesses in this category are related to concurrency.

SFP Secondary Cluster: Race Condition Window

This category identifies Software Fault Patterns (SFPs) within the Race Condition Window cluster (SFP20).

CERT C++ Secure Coding Section 09 - Input Output (FIO)

Weaknesses in this category are related to rules in the Input Output (FIO) section of the CERT C++ Secure Coding Standard. Since not all rules map to specific weakness...

Comprehensive CWE Dictionary

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

CWE Cross-section

This view contains a selection of weaknesses that represent the variety of weaknesses that are captured in CWE, at a level of abstraction that is likely to be useful t...

Weaknesses Introduced During Implementation

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

Common Weakness Enumeration content on this website is copyright of The MITRE Corporation unless otherwise specified. Use of the Common Weakness Enumeration and the associated references on this website are subject to the Terms of Use as specified by The MITRE Corporation.