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 race conditions.
Noncompliant Code Example
The POSIX lstat()
function collects information about a symbolic link rather than its target. This 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 *userbuf = /* user data */; unsigned int userlen = /* length of userbuf string */; struct stat lstat_info; int fd; /* ... */ if (lstat(filename, &lstat_info) == -1) { /* Handle error */ } if (!S_ISLNK(lstat_info.st_mode)) { fd = open(filename, O_RDWR); if (fd == -1) { /* Handle error */ } } if (write(fd, userbuf, userlen) < userlen) { /* Handle error */ }
This code contains a time-of-creation-to-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
This compliant solution eliminates the race condition by
- calling
lstat()
on the file name - calling
open()
to open the file - calling
fstat()
on the file descriptor returned byopen()
- comparing the file information returned by the calls to
lstat()
andfstat()
to ensure that the files are the same
char *filename = /* file name */; char *userbuf = /* user data */; unsigned int userlen = /* length of userbuf string */; struct stat lstat_info; struct stat fstat_info; int fd; /* ... */ if (lstat(filename, &lstat_info) == -1) { /* handle error */ } fd = open(filename, O_RDWR); 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 open()
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-C. Identify files using multiple file attributes).
Risk Assessment
TOCTOU race condition vulnerabilities can be exploited to gain elevated privileges.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
---|---|---|---|---|---|
POS35-C |
high |
likely |
medium |
P18 |
L1 |
Automated Detection
Compass/ROSE can detect some violations of this rule. In particular, it ensures that calls to open()
that are preceded by a call to lstat()
are also followed by a call to fstat()
.
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
[[Dowd 06]] Chapter 9, "UNIX 1: Privileges and Files"
[[ISO/IEC 9899:1999]] Section 7.19, "Input/output <stdio.h>"
[[MITRE 07]] CWE ID 363, "Race Condition Enabling Link Following", and CWE ID 365 "Race Condition in Switch"
[[Open Group 04]] lstat(), fstat(), open()
[[Seacord 05a]] Chapter 7, "File I/O"