The software checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. This can cause the software to perform invalid actions when the resource is in an unexpected state.
Extended Description
This weakness can be security-relevant when an attacker can influence the state of the resource between check and use. This can happen with shared resources such as files, memory, or even variables in multithreaded programs.
Alternate Terms
TOCTTOU:
The TOCTTOU acronym expands to "Time Of Check To Time Of Use".
TOCCTOU:
The TOCCTOU acronym is most likely a typo of TOCTTOU, but it has been
used in some influential documents, so the typo is repeated fairly
frequently.
Time of Introduction
Implementation
Applicable Platforms
Languages
All
Common Consequences
Scope
Effect
Integrity
Other
Technical Impact: Alter execution
logic; Unexpected state
The attacker can gain access to otherwise unauthorized
resources.
Integrity
Other
Technical Impact: Modify application
data; Modify files or
directories; Modify memory; Other
Race conditions such as this kind may be employed to gain read or
write access to resources which are not normally readable or writable by
the user in question.
Integrity
Other
Technical Impact: Other
The resource in question, or other resources (through the corrupted
one), may be changed in undesirable ways by a malicious user.
Non-Repudiation
Technical Impact: Hide activities
If a file or other resource is written in this method, as opposed to
in a valid way, logging of the activity may not occur.
Non-Repudiation
Other
Technical Impact: Other
In some cases it may be possible to delete files a malicious user
might not otherwise have access to, such as log files.
Likelihood of Exploit
Low to Medium
Demonstrative Examples
Example 1
(Bad Code)
Example Languages: C and C++
struct stat *sb;
...
lstat("...",sb); // it has not been updated since the last time it
was read
printf("stated file\n");
if (sb->st_mtimespec==...){
print("Now updating things\n");
updateThings();
}
Potentially the file could have been updated between the time of the
check and the lstat, especially since the printf has latency.
Example 2
The following code is from a program installed setuid root. The
program performs certain file operations on behalf of non-privileged users,
and uses access checks to ensure that it does not use its root privileges to
perform operations that should otherwise be unavailable the current user.
The program uses the access() system call to check if the person running the
program has permission to access the specified file before it opens the file
and performs the necessary operations.
(Bad Code)
Example
Language: C
if(!access(file,W_OK)) {
f = fopen(file,"w+");
operate(f);
...
}
else {
fprintf(stderr,"Unable to open file %s.\n",file);
}
The call to access() behaves as expected, and returns 0 if the user
running the program has the necessary permissions to write to the file,
and -1 otherwise. However, because both access() and fopen() operate on
filenames rather than on file handles, there is no guarantee that the
file variable still refers to the same file on disk when it is passed to
fopen() that it did when it was passed to access(). If an attacker
replaces file after the call to access() with a symbolic link to a
different file, the program will use its root privileges to operate on
the file even if it is a file that the attacker would otherwise be
unable to modify. By tricking the program into performing an operation
that would otherwise be impermissible, the attacker has gained elevated
privileges. This type of vulnerability is not limited to programs with
root privileges. If the application is capable of performing any
operation that the attacker would not otherwise be allowed perform, then
it is a possible target.
Example 3
This code prints the contents of a file if a user has permission.
(Bad Code)
Example
Language: PHP
function readFile($filename){
$user = getCurrentUser();
//resolve file if its a symbolic link
if(is_link($filename)){
$filename = readlink($filename);
}
if(fileowner($filename) == $user){
echo file_get_contents($realFile);
return;
}
else{
echo 'Access denied';
return false;
}
}
This code attempts to resolve symbolic links before checking the file and printing its contents. However, an attacker may be able to change the file from a real file to a symbolic link between the calls to is_link() and file_get_contents(), allowing the reading of arbitrary files. Note that this code fails to log the attempted access (CWE-778).
A multi-threaded race condition allows remote
attackers to cause a denial of service (crash or reboot) by causing two
threads to process the same RPC request, which causes one thread to use
memory after it has been freed.
chain: time-of-check time-of-use (TOCTOU) race
condition in program allows bypass of protection mechanism that was designed
to prevent symlink attacks.
chain: time-of-check time-of-use (TOCTOU) race
condition in program allows bypass of protection mechanism that was designed
to prevent symlink attacks.
Potential Mitigations
Phase: Implementation
The most basic advice for TOCTOU vulnerabilities is to not perform a
check before the use. This does not resolve the underlying issue of the
execution of a function on a resource whose state and identity cannot be
assured, but it does help to limit the false sense of security given by
the check.
Phase: Implementation
When the file being altered is owned by the current user and group,
set the effective gid and uid to that of the current user and group when
executing this statement.
Phase: Architecture and Design
Limit the interleaving of operations on files from multiple processes.
Phases: Implementation; Architecture and Design
If you cannot perform operations atomically and you must share access
to the resource between multiple processes or threads, then try to limit
the amount of time (CPU cycles) between the check and use of the
resource. This will not fix the problem, but it could make it more
difficult for an attack to succeed.
Phase: Implementation
Recheck the resource after the use call to verify that the action was
taken appropriately.
Phase: Architecture and Design
Ensure that some environmental locking mechanism can be used to
protect resources effectively.
Phase: Implementation
Ensure that locking occurs before the check, as opposed to afterwards,
such that the resource, as checked, is the same as it is when in
use.
[REF-17] Michael Howard, David LeBlanc
and John Viega. "24 Deadly Sins of Software Security". "Sin 13: Race Conditions." Page 205. McGraw-Hill. 2010.
[REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 9, "TOCTOU", Page 527.. 1st Edition. Addison Wesley. 2006.
Description
