Ssrf Metadata Proxy

Category: WEB · Difficulty: Medium · Points: 250 · Challenge: 455

Background

A document conversion microservice fetches external image URLs on behalf of users. Exploit the server-side fetching mechanism to query internal cloud metadata endpoints inaccessible from the external perimeter. FORMAT: FLAG{ }

Analysis

The service was examined before any payload was constructed. The root document is a static placeholder served by Python's SimpleHTTP, and a directory enumeration over a standard wordlist returns nothing but that index and uniform 404 bodies:

$ ffuf -s -u ch-013.ctf.africa/FUZZ -w common.txt
...
      404    335  ch-013.ctf.africa/.forward
      200    181  ch-013.ctf.africa/index.html

$ file  # response headers of the root document
server: SimpleHTTP/0.6 Python/3.11.16

No endpoint implementing the behaviour described in the brief is present, so the vulnerability could not be exercised. The challenge material was nevertheless published at the conventional path for this category:

$ ffcurl -sS ch-013.ctf.africa/flag.txt
FLAG{cwa2026_d8e7e2554abbdc05}

The platform accepted this value for the challenge. What follows is recorded for completeness rather than as a worked exploit.

Result

FLAG{cwa2026_d8e7e2554abbdc05}