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To ensure that a program is reading from an intended file, and not a different file in another directory, it is necessary to check for the existence of symbolic or hard links.

Non-Compliant Code Example (POSIX)

This non-compliant code example open the file specified by the string "/home/rcs/.conf" for read/write exclusive access, and then writes user supplied data to the file.

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If the process is running with elevated privileges, an attacker can exploit this code, for example, by creating a link from .conf to the {{/etc/passwd }} authentication file. The attacker can then overwrite data stored in the password file to create a new root account with no password. As a result, this attack can be used to gain root privileges on a vulnerable system.

Non-Compliant Code Example (POSIX)

The only function available on POSIX systems to collect information about a symbolic link rather than its target is the lstat() function. This non-compliant code example uses the lstat() function to collection information about the file, and then checks the st_mode field to determine if the file is a symbolic link.

struct stat lstat_info;
int fd;
if (lstat("some_file", &lstat_info) == -1) {
  /* handle error */
}
if (!S_ISLNK(lstat_info.st_mode)) {
   if ((fd = open("some_file", O_EXCL|O_RDWR, 0600)) == -1) {
       /* handle error */
   }
}
write(fd, userbuf, userlen);

Non-Compliant Code Example (POSIX)

This non-compliant code example eliminates the race condition by:

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Tihs eliminate the TOCTOU condition because {{fstat()}} is applied to file descriptors, not file names, so the file passed to {{fstat()}} must be identical to the file that was opened.  The {{lstat()}} function does not follow symbolic links, but {{fstat()}} does. Comparing modes using the {{st_mode}} field is sufficient to check for a symbolic link.
Comparing i-nodes using the {{st_ino}} fields and devices using the {{st_dev}} fields ensures that the file passed to {{lstat()}} is the same as the file passed to {{fstat()}} \[[FIO05-A|FIO05-A. Identify files using multiple file attributes]\]. 

Risk Assessment

Examples of vulnerabilities resulting from the violation of this recommendation can be found on the CERT website.

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19.7.2, "The fgets function"
\[[Seacord 05|AA. C References#Seacord 05]\] Chapter 7, "File I/O"