CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition

The following code checks a file, then updates its contents.

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

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

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.

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.

if(!access(file,W_OK)) {f = fopen(file,"w+");operate(f);...}else {                                                     fprintf(stderr,"Unable to open file %s.\n",file);                      }

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

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).

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 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.

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.

#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; 		      }