What is Time of Check Time of Use (TOCTOU)?

Understanding Time of Check Time of Use (TOCTOU)

A class of software bugs caused by changes in a system between the checking of a condition and the use of the results of that check. TOCTOU is a race condition vulnerability where there is a time gap between when a security decision is made (checking a condition) and when that decision is applied (using the result), allowing an attacker to modify something in between these two events. This vulnerability affects file operations, permission checks, and other security enforcement mechanisms. TOCTOU vulnerabilities are addressed in secure coding standards like CERT/SEI and OWASP guidelines. Organizations prevent TOCTOU vulnerabilities through secure programming practices, code reviews, security testing, and proper locking mechanisms. For example, a secure financial application might prevent TOCTOU vulnerabilities in its transaction processing by implementing atomic operations, file locking mechanisms, transaction isolation, proper synchronization techniques, and privilege dropping, ensuring that security checks and subsequent actions cannot be separated by malicious intervention. Related terms: Race condition, Atomicity, Concurrency, File descriptor, File privilege, Secure coding, Time of check, Time of use, Synchronization, Privilege dropping.