FIO01-C. Be careful using functions that use file names for identification

Many file-related security vulnerabilities result from a program accessing an unintended file object because file names are only loosely bound to underlying file objects. File names provide no information regarding the nature of the file object itself. Furthermore, the binding of a file name to a file object is reasserted every time the file name is used in an operation. File descriptors and FILE pointers are bound to underlying file objects by the operating system (see FIO03-C. Do not make assumptions about fopen() and file creation).

Accessing files via file descriptors or FILE pointers rather than file names provides a greater degree of certainty as to which object is actually acted upon. It is recommended that files be accessed through file descriptors or FILE pointers where possible.

The following C99 functions rely solely on file names for file identification:

• remove()
• rename()
• fopen()
• freopen()

Use these functions with caution. See FIO10-C. Take care when using the rename() function, and FIO08-C. Take care when calling remove() on an open file.

Noncompliant Code Example

In this noncompliant code example, the file identified by file_name is opened, processed, closed, and removed. However, it is possible that the file object identified by file_name in the call to remove() is not the same file object identified by file_name in the call to fopen().

char *file_name;
FILE *f_ptr;

/* initialize file_name */

f_ptr = fopen(file_name, "w");
if (f_ptr == NULL) {
  /* Handle error */
}

/*... Process file ...*/

if (fclose(f_ptr) != 0) {
  /* Handle error */
}

if (remove(file_name) != 0) {
  /* Handle error */
}

Compliant Solution

There is not much that can be programmatically done to ensure the file removed is the same file that was opened, processed, and closed except to make sure that the file is opened in a secure directory with privileges that would prevent the file from being manipulated by an untrusted user (see FIO15-C. Ensure that file operations are performed in a secure directory).

Noncompliant Code Example (POSIX)

In this noncompliant code example, the function chmod() is called to set the permissions of a file. However, it is not clear whether the file object referred to by file_name refers to the same object in the call to fopen() and in the call to chmod().

char *file_name;
FILE *f_ptr;

/* initialize file_name */

f_ptr = fopen(file_name, "w");
if (f_ptr == NULL)  {
  /* Handle error */
}

/* ... */

if (chmod(file_name, S_IRUSR) == -1) {
  /* Handle error */
}

Compliant Solution (POSIX)

This compliant solution uses the POSIX fchmod() and open() functions [[Open Group 04]]. This guarantees that the file opened is the same file that is operated on.

char *file_name;
int fd;

/* initialize file_name */

fd = open(
  file_name,
  O_WRONLY | O_CREAT | O_EXCL,
  S_IRWXU
);
if (fd == -1) {
  /* Handle error */
}

/* ... */

if (fchmod(fd, S_IRUSR) == -1) {
  /* Handle error */
}

Mitigation Strategies (POSIX)

Many file-related race conditions can be eliminated by using

• fchown() rather than chown()
• fstat() rather than stat()
• fchmod() rather than chmod()

or simply by ensuring the security of the working directory as per FIO15-C. Ensure that file operations are performed in a secure directory.

POSIX functions that have no file descriptor counterpart should be used with caution:

• link() and unlink()
• mkdir() and rmdir()
• mount() and unmount()
• lstat()
• mknod()
• symlink()
• utime()

Risk Assessment

Many file-related vulnerabilities, such as time of check, time of use race conditions, can be exploited to cause a program to access an unintended file. Using FILE pointers or file descriptors to identify files instead of file names reduces the chance of accessing an unintended file. Remediation costs are medium because while insecure functions can be easily identified, simple drop-in replacements are not always available.

Automated Detection

The LDRA tool suite Version 7.6.0 can detect violations of this recommendation.

Compass/ROSE can detect some violations of this recommendation. In particular, it warns when chown(), stat(), or chmod() are called on an open file.

Klocwork Version 8.0.4.16 can detect violations of this rule with the SV.TOCTOU.FILE_ACCESS checker.

Related Vulnerabilities

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

References

[Apple Secure Coding Guide] "Avoiding Race Conditions and Insecure File Operations"
[[Drepper 06]] Section 2.2.1 "Identification When Opening"
[[ISO/IEC 9899:1999]] Section 7.19.3, "Files," and Section 7.19.4, "Operations on Files"
[[MITRE 07]] CWE ID 367, "Time-of-check Time-of-use Race Condition," and CWE ID 676, "Use of Potentially Dangerous Function"
[[Open Group 04]] "The open function"
[[Seacord 05a]] Chapter 7, "File I/O"

FIO00-C. Take care when creating format strings      09. Input Output (FIO)       FIO02-C. Canonicalize path names originating from untrusted sources