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A TOCTOU (time-of-check, time-of-use) race condition is possible when two or more concurrent processes are operating on a shared file system [Seacord 2013b].   Typically, the first access is a check to verify some attribute of the file, followed by a call to use the file.   An attacker can alter the file between the two accesses, or replace the file with a symbolic or hard link to a different file.   These TOCTOU conditions can be exploited when a program performs two or more file operations on the same file name or path name.

...

#include <stdio.h>

void open_some_file(const char *file) {
  FILE *f = fopen(file, "r");
  if (NULL != f) {
    /* File exists, handle error */
  } else {
    if (fclose(f) == EOF) {
      /* Handle error */
    }
    f = fopen(file, "w");
    if (NULL == f) {
      /* Handle error */
    }
 
    /* Write to file */
    if (fclose(f) == EOF) {
      /* Handle error */
    }
  }
}

...

This compliant solution invokes fopen() at a single location and uses the x mode of fopen(), which was added in C11. This mode causes fopen() to fail if the file exists. This check and subsequent open is performed without creating a race window. The x mode provides exclusive access to the file only if the host environment provides this support.

#include <stdio.h>

void open_some_file(const char *file) {
  FILE *f = fopen(file, "wx");
  if (NULL == f) {
    /* Handle error */
  }
  /* Write to file */
  if (fclose(f) == EOF) {
    /* Handle error */
  }
}

...

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

void open_some_file(const char *file) {
  int fd = open(file, O_CREAT | O_EXCL | O_WRONLY);
  if (-1 != fd) {
    FILE *f = fdopen(fd, "w");
    if (NULL != f) {
      /* Write to file */

      if (fclose(f) == EOF) {
        /* Handle error */
      }
    }
    else {
      if (close(fd) == -1) {
        /* Handle error */
      }
    }
  }
}

Exceptions

FIO45-EX1: TOCTOU race conditions require that the vulnerable process is more privileged than the attacker; otherwise there is nothing to be gained from a successful attack. An unprivileged process is not subject to this rule.FIO45C-EX2: Accessing a file name or path name multiple times is permitted if the file referenced resides in a secure directory. (for For more information, see FIO15-C. Ensure that file operations are performed in a secure directory.).

FIO45-C-EX3: Accessing a file name or path name multiple times is permitted if the program can verify that every operation operates on the same file.

This POSIX code example verifies that each subsequent file access operates on the same file. In POSIX, every file can be uniquely identified by using its device and i-node attributes. This code example checks that a file name refers to a regular file (and not a directory, symbolic link, or other special file) by invoking lstat(). This call also retrieves its device and i-node. The file is subsequently opened. Finally, the program verifies that the file that was opened is the same one (matching device and i-nodes) as the file that was confirmed as a regular file.

An attacker can still exploit this code if they have the ability to delete the benign file and create the malicious file within the race window between lstat() and open(). It is possible that the OS kernel will reuse the same device and i-node for both files. This can be mitigated by making sure that the attacker lacks the permissions to delete the benign file.

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

int open_regular_file(char *filename, int flags) {
  struct stat lstat_info;
  struct stat fstat_info;
  int f;
 
  if (lstat(filename, &lstat_info) == -1) {
    /* File does not exist, handle error */
  }
 
  if (!S_ISREG(lstat_info.st_mode)) {
    /* File is not a regular file, handle error */
  }
 
  if ((f = open(filename, flags)) == -1) {
    /* File has disappeared, handle error */
  }
 
  if (fstat(f, &fstat_info) == -1) {
    /* Handle error */
  }
 
  if (lstat_info.st_ino != fstat_info.st_ino  ||
      lstat_info.st_dev != fstat_info.st_dev) {
    /* Open file is not the expected regular file, handle error */
  }
 
  /* f is the expected regular open file */
  return f;
}

...

TOCTOU race conditions can result in unexpected behavior, including privilege escalation.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

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