send_callback()
: Review your callback URL validation — any user-controllable input reaching filesystem paths is dangerous.
: Ensure you have explicit authorization to test the system, use controlled environments, and follow responsible disclosure.
The underlying vulnerability typically manifests as a Server-Side Request Forgery (SSRF) flaw.
Stay curious, and happy coding!
: An attacker can modify their request header (e.g., using Burp Suite ) to include malicious code like .
: Only allow the application to call specific, pre-approved domains.
https://victim.com/process?callback=file:///proc/self/environ
: Run the web server with a user account that doesn't have permission to read sensitive system files like those in /proc . callback-url-file-3A-2F-2F-2Fproc-2Fself-2Fenviron
Tools like Burp Suite’s Scanner, ffuf , or custom scripts can automate this testing. For CTF challenges, the exact string callback-url-file:///proc/self/environ (or its encoded form) may be given as a hint that a callback mechanism is vulnerable.
The attack scenario typically unfolds as follows:
The goal of an attacker using this string is to trick the server into reading its own sensitive internal files and "reflecting" the contents back to the user’s screen.
: Environment variables often include data from HTTP headers, such as the User-Agent . send_callback() : Review your callback URL validation —
The ultimate Bug Bounty guide to exploiting SSRF vulnerabilities
These variables often hold secrets, configuration paths, debug flags, and internal service endpoints. If an attacker can read /proc/self/environ , they can obtain:
: In web server logs (like Nginx's access.log ), this appears as a request containing encoded sequences like %2E%2E%2F (representing ../ ) used to navigate up the directory tree. Mitigation : To prevent these attacks, developers should: Sanitize all user input. Use allow-listing for file inclusions.