Уязвимости типа "Время проверки — время использования" (TOCTOU) в программном обеспечении.

Class of software bugs
In software development, time of check to time of use (TOCTOU, TOCTTOU or TOC/TOU) is a class of software bugs caused by a race condition involving the checking of the state of a part of a system (such as a security credential) and the use of the results of that check. TOCTOU race conditions are common in Unix between operations on the file system, but can occur in other contexts, including local sockets and improper use of database transactions. In the early 1990s, the mail utility of BSD 4.3 UNIX had an exploitable race condition for temporary files because it used the mktemp function. Early versions of OpenSSH had an exploitable race condition for Unix domain sockets. They remain a problem in modern systems; as of 2019, a TOCTOU race condition in Docker allows root access to the filesystem of the host platform. In the 2023 Pwn²Own competition in Vancouver, a team of hackers was able to compromise the gateway in updated Tesla model 3 using this bug.

Exploiting a TOCTOU race condition requires precise timing to ensure that the attacker's operations interleave properly with the victim's. In the example above, the attacker must execute the symlink system call precisely between the access and open. For the most general attack, the attacker must be scheduled for execution after each operation by the victim, also known as "single stepping" the victim. In the case of BSD 4.3 mail utility and mktemp , and algorithmic complexity attacks. In both cases, the attacker manipulates the OS state to control scheduling of the victim. File system mazes force the victim to read a directory entry that is not in the OS cache, and the OS puts the victim to sleep while it is reading the directory from disk. Algorithmic complexity attacks force the victim to spend its entire scheduling quantum inside a single system call traversing the kernel's hash table of cached file names. The attacker creates a very large number of files with names that hash to the same value as the file the victim will look up.

Despite conceptual simplicity, TOCTOU race conditions are difficult to avoid and eliminate. One general technique is to use error handling instead of pre checking, under the philosophy of EAFP – "It is easier to ask for forgiveness than permission" rather than LBYL – "look before you leap" – in this case there is no check, and failure of assumptions to hold are signaled by an error being returned. In the context of file system TOCTOU race conditions, the fundamental challenge is ensuring that the file system cannot be changed between two system calls. In 2004, an impossibility result was published, showing that there was no portable, deterministic technique for avoiding TOCTOU race conditions when using the UNIX access and open filesystem calls. Since this impossibility result, libraries for tracking file descriptors and ensuring correctness have been proposed by researchers. An alternative solution proposed in the research community is for UNIX systems to adopt transactions in the file system or the OS kernel. Transactions provide a concurrency control abstraction for the OS, and can be used to prevent TOCTOU races. While no production UNIX kernel has yet adopted transactions, proof of concept research prototypes have been developed for Linux, including the Valor file system and the TxOS kernel. Microsoft Windows has added transactions to its NTFS file system, but Microsoft discourages their use, and has indicated that they may be removed in a future version of Windows. File locking is a common technique for preventing race conditions for a single file, but it does not extend to the file system namespace and other metadata, nor does locking work well with networked filesystems, and cannot prevent TOCTOU race conditions. For setuid binaries a possible solution is to use the seteuid system call to change the effective user and then perform the open Differences in setuid between operating systems can be problematic.