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 multi-threaded
programs.
Alternate Terms
TOCTTOU:
The TOCCTOU acronym expands to "Time Of Check To Time Of Use". Usage
varies between TOCTOU and TOCTTOU.
Time of Introduction
Implementation
Applicable Platforms
Languages
All
Common Consequences
Scope
Effect
Access Control
The attacker can gain access to otherwise unauthorized
resources.
Access Control
Authorization
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
The resource in question, or other resources (through the corrupted
one), may be changed in undesirable ways by a malicious user.
Accountability
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
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)
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)
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.
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
Description
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.
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.
Do not rely on user-specified input to determine what path to format.
Architecture and Design
Limit the interleaving of operations on files from multiple processes.
Limit the spread of time (cycles) between the check and use of a
resource.
Implementation
Recheck the resource after the use call to verify that the action was
taken appropriately.
Architecture and Design
Ensure that some environmental locking mechanism can be used to
protect resources effectively.
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.
Description
