File Security

Cybersecurity 08 Sep 2026 12 min read

Validate Uploaded Files by What You Will Do with Them

A file upload endpoint often starts with a simple rule: accept .jpg images, .pdf documents, or another small set of formats. The mistake is assuming that a filename or an HTTP Content-Type value proves what the uploaded bytes really are. Both values come from the client. They are useful hints, but they are not a security boundary. If an application accepts a file because its name ends in .jpg and later sends those bytes to an image decoder, document converter, browser, or other parser, the component consuming the file becomes the place where the real security consequences appear.

Cybersecurity Secure Coding

Cybersecurity 08 Sep 2026 10 min read

Confine Archive Extraction to a Trusted Directory

An application that accepts ZIP, TAR, or similar archives may appear to be handling one uploaded file. During extraction, however, the archive can ask the application to create many filesystem objects with names chosen by whoever created the archive. If those names are treated as trusted paths, extraction can write outside the directory the application intended to use. The consequence can be more serious than a misplaced file. Depending on the process permissions and surrounding system, an unintended write might replace application data, alter configuration, or place content where another component will later consume it.

Cybersecurity Secure Coding

Cybersecurity 08 Sep 2026 10 min read

Avoid Check-Then-Use Races in File Operations

A program often checks a file before using it. It may confirm that a path is inside an allowed directory, that the target is not a symbolic link, that the file belongs to an expected user, or that it does not already exist. The code then opens, replaces, deletes, or executes the file. The security problem is the gap between those two operations. If another actor can change the relevant filesystem state after the check but before the use, the program may validate one object and operate on another. This is a time-of-check to time-of-use race, often shortened to TOCTOU.

Cybersecurity Secure Coding