Page History

Many common operating systems, such as Windows and UNIX, support symbolic (soft) links. Symbolic links can be created in UNIX using the ln -s command or in Windows by using directory junctions in NTFS or the Linkd.exe (Win 2K resource kit) or "junction" freeware.

If not properly performed, checking for the existence of symbolic links can lead to time of creation to time of use (TOCTOU) race conditions.

...

race conditions.

This rule is a specific instance of rule FIO45-C. Avoid TOCTOU race conditions while accessing files.

Noncompliant Code Example

The only function available on POSIX systems to collect POSIX lstat() function collects information about a symbolic link rather than its target is the lstat() function. This non-compliant noncompliant code example uses the lstat() function to collect information about the file, checks the st_mode field to determine if the file is a symbolic link, and then opens the file if it is not a symbolic link.:

char *filename = /* file name */;
char *filename;
 *userbuf = /* user data */;
unsigned int userlen = /* length of userbuf string */;

struct stat lstat_info;
int fd;
/* ... */
if (lstat(filename, &lstat_info) == -1) {
  /* handleHandle error */
}

if (!S_ISLNK(lstat_info.st_mode)) {
   if ((fd = open(filename, O_RDWR, 0600)));
   if (fd == -1) {
       /* handleHandle error */
   }
}
if (write(fd, userbuf, userlen) < userlen);

This code contains a time of creation to time of use (TOCTOU) race condition betwen the call to {{lstat()}} and the subsequent call to {{open()}} because both functions operate on a file name \[[FIO01-A. Prefer functions that do not rely on file names for identification]\] that can be manipulated asynchronously to the execution of the program.

 {
  /* Handle error */
}

This code contains a time-of-check, time-of-use (TOCTOU) race condition between the call to lstat() and the subsequent call to open() because both functions operate on a file name that can be manipulated asynchronously to the execution of the program. (See FIO01-C. Be careful using functions that use file names for identification.)

Compliant Solution (POSIX.1-2008 or newer)

This compliant solution eliminates the race condition by using O_NOFOLLOW to cause open() to fail if passed a symbolic link, avoiding the TOCTOU by not having a separate "check" and "use":

char *filename = /* file name */;
char *userbuf = /* user data */;
unsigned int userlen = /* length of userbuf string */;

int fd = open(filename, O_RDWR|O_NOFOLLOW);
if (fd == -1) {
  /* Handle error */
}
if (write(fd, userbuf, userlen) < userlen) {
  /* Handle error */
}

Compliant Solution (POSIX.1-2001 or older)

...

This compliant solution eliminates the race condition by:

1. calling the Calling lstat() on the filenamefile name.
2. calling Calling open() to open the file.
3. calling Calling fstat() on the file descriptor returned by open().
4. comparing Comparing the fine file information returned by the calls to lstat() and fstat() to ensure that the files are the same.

Code Block
bgColor #ccccff lang c
char *filename = /* file name */; char *userbuf = /* user data */; unsigned int userlen = /* length of userbuf string *filename/; struct stat lstat_info,; struct stat fstat_info; int fd; /* ... */ if (lstat(filename, &lstat_info) == -1) { /* handle error */ } if (( fd = open(filename, O_RDWR, 0600))); if (fd == -1) { /* handle error */ } if (fstat(fd, &fstat_info) == -1) { /* handle error */ } if (lstat_info.st_mode == fstat_info.st_mode && lstat_info.st_ino == fstat_info.st_ino && lstat_info.st_dev == fstat_info.st_dev) { if (write(fd, userbuf, userlen); < userlen) { /* Handle Error */ } }

...

This code eliminates 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 {{fstatopen()}} 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()}} \[[. (See FIO05-AC. Identify files using multiple file attributes]\].)

Risk Assessment

Time of creation to time of use (TOCTOU ) race condition vulnerabilities can be exploited to gain elevated privileges.

Automated Detection

Related Vulnerabilities

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

References

\[[Dowd 06|AA. C References#Dowd 06]\] Chapter 9, "UNIX 1: Privileges and Files"
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19, "Input/output <stdio.h>"
\[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 365|http://cwe.mitre.org/data/definitions/365.html], "Race Condition in Switch"
\[[Open Group 04|AA. C References#Open Group 04]\] [lstat()|http://www.opengroup.org/onlinepubs/000095399/functions/lstat.html], [fstat()|http://www.opengroup.org/onlinepubs/009695399/functions/fstat.html], [open()|http://www.opengroup.org/onlinepubs/009695399/functions/open.html]
\[[Seacord 05|AA. C References#Seacord 05]\] Chapter 7, "File I/O"

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-764 and POS51-C/POS35-C

Independent( CWE-764, POS51-C, POS35-C)

CWE-764 is about semaphores, or objects capable of being locked multiple times. Deadlock arises from multiple locks being acquired in a cyclic order, and generally does not arise from semaphores or recursive mutexes.

Bibliography

...

Image Added Image Added Image Added Image Removed      50. POSIX (POS)       99. The Void