FIO10-A. Take care when using the rename() function

The rename() function has the following prototype.

int rename(char const *old, char const *new);

If the file referenced by new exists prior to calling rename(), the behavior is implementation-defined. For portability, you must ensure that the file referenced by new does not exist when rename() is invoked.

Non-Compliant Code Example

In this non-compliant code example, a file is moved using rename().

char const *old = /* ... */;
char const *new = /* ... */;
if (rename(old, new) != 0) {
  /* Handle Error */
}

If the file named by new exists at the time of the call to rename(), the result is implementation-defined.

Compliant Solution

This compliant solution checks for the existence of the new file before callling rename(). This code contains an unavoidable race condition between the call to fopen() and the call to rename(). Consequently, this code can only be safely executed within a secure directory.

char const *old = /* ... */;
char const *new = /* ... */;
FILE *file = fopen(new, "r");

if (!file) {
  if (rename(old, new) != 0) {
    /* Handle Error */
  }
} else {
  fclose(file);
  /* handle Error */
}

Unfortunately, fopen() may fail on many file systems when the file exists but the program does not have sufficient permissions to read it.

Compliant Solution (POSIX)

A somewhat better solution involves using the POSIX access() function which can check for the existence of a file explicitly [[Open Group 04]]. As with the fopen() solution, this code contains an unavoidable race condition and consequently can only be safely executed within a secure directory. In fact the existence check is not really needed on POSIX systems, because POSIX specifies the behavior of rename() when the file referenced by new exists.

char const *old = /* ... */;
char const *new = /* ... */;

if (access(new,F_OK) != 0) {
  if (rename(old, new) != 0) {
    /* Handle Error */
  }
} else {
  /* Handle Error */
}

While the likelihood of access() returning a false negative is lower than that of fopen(), on file systems where the program does not have sufficient permissions in the directory to view the file, access() may return -1 even when the file exists. In such cases, rename() will also fail since the program does not have adequate permissions inside the directory.

Compliant Solution (Windows)

On Windows, _access_s() allows one to check for the existence of a file [[MSDN]]. As with the fopen() solution, this code contains an unavoidable race condition and consequently can only be safely executed within a secure directory.

char const *old = /* ... */;
char const *new = /* ... */;

if (_access_s(new,0) != 0) {
  if (rename(old, new) != 0) {
    /* Handle Error */
  }
} else {
  /* Handle Error */
}

This _access_s() example shares many of the same problems as the POSIX access() example above.

Risk Assessment

Calling rename() has implementation-defined behavior when the new file name refers to an existing file. Incorrect use of rename() can result in a file being unexpectedly overwritten or other unexpected behavior.

Related Vulnerabilities

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

References

[[ISO/IEC 9899:1999]] Section 7.9.4.2, "The rename function"
[[MSDN]] _access_s, _waccess_s
[[Open Group 04]] access()

FIO09-A. Be careful with binary data when transferring data across systems      09. Input Output (FIO)       FIO11-A. Take care when specifying the mode parameter of fopen()