CWE --- Time-of-check Time-of-use (TOCTOU) 例子 和 mitigation

原文地址：

https://cwe.mitre.org/data/definitions/367.html

Example 1

The following code checks a file, then updates its contents.

(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).

Mitigation (预防措施)：

 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 callto 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.