Exploring TOCTOU Vulnerabilities without File System Access
In this article, we delve into the concept of TOCTOU (Time-of-check to time-of-use) vulnerabilities and explore whether it is possible to exploit them without having local access to file systems. We analyze a scenario involving file uploads to a web server and discuss the practicality of such timing attacks outside of lab settings.
Understanding TOCTOU Vulnerabilities
TOCTOU vulnerabilities are not specific to files or file systems; they exist whenever you have untrusted input or state that is validated before use but can be modified and used without re-validation. In the given scenario of a web server with a save image function, conditions #1 and #2 are met, but conditions #3 and #4 are likely not met. When a new version of a file is uploaded, it creates a new web request that will be processed separately. The object in memory that represents the uploaded file does not change during the execution of the function.
Vulnerability Requirements
In order for this scenario to be vulnerable to a TOCTOU attack, two changes would need to occur. Firstly, the server would need to overwrite the memory buffer or temporary file used by the first upload when the second upload happens. Secondly, the modified temporary file would need to be re-read or the variable representing the uploaded file would need to be re-read from overwritten memory. However, in practice, this is highly unlikely as the server threads processing different requests do not have access to each other’s parameters and local variables.
Practical Exploitation
Even if the scenario was theoretically vulnerable, it would be challenging to exploit in practice from the same computer due to the slower speed of network I/O compared to local processing. However, it could be possible to exploit if the validation step was slow or the server was busy and paused after validation. Additionally, it is important to note that uploading files with malicious executable contents does not achieve anything as the server will not execute them, and users downloading the files will likely not treat them as executables.