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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().

/* program code */
char const *old = "oldfile.ext";
char const *new = "newfile.ext";
if (rename(old, new) != 0) {
  /* Handle rename failure */
}
/* program code */

If newfile.ext 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.

/* program code */
char const *old = "oldfile.ext";
char const *new = "newfile.ext";
FILE *file = fopen(new, "r");

if (!file) {
  if (rename(old, new) != 0) {
    /* Handle rename failure */
  }
} else {
  fclose(file);
  /* handle error condition */
}
/* program code */

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.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

FIO10-A

medium

probable

medium

P8

L2

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"


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()

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