Capstone: Exploit a File Manager App With Layered Filters and Extract the Flag

You are contracted to test a company's internal file manager application. File managers execute system commands constantly: listing directories, creating folders, deleting files, compressing archives. Every one of those operations is a potential injection point. Your objective: find the injection, bypass the filters, and read the flag file at /opt/flag.txt.

Phase one: reconnaissance

The application has a "Create Directory" feature. You provide a directory name, and the backend runs mkdir /uploads/<your_input>. Test with a normal name first. Then test with a semicolon. Then a newline. Map what gets blocked.

Newline works. Semicolon is blocked. You have your operator. Now test for space and command filters.

Phase two: filter mapping

Spaces are blocked. Tabs work. The cat command or the forward slash is blocked. You need to determine which. Test the slash alone using the environment variable extraction technique.

Phase three: build the bypass payload

You need to read /opt/flag.txt without using spaces, semicolons, the word cat, or forward slashes. Combine everything: newline operator, tab for spaces, base64 encoding for the full command.

What you just demonstrated

Systematic filter identification. Operator substitution. Character replacement. Command encoding. Each technique from this course, chained together in a realistic engagement. This is not a CTF trick. This is the exact methodology used in professional penetration tests against hardened web applications.

You started this course learning what a semicolon does in a shell. You finished it chaining operator bypasses, character substitutions, and encoding techniques to extract data from a filtered application. The methodology is the same whether the target is a PHP file manager, a NodeJS API, or a Python microservice. The shell is the shell. The filters are the filters. And now you know how to beat both.