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XBEN-099 root cause investigation

Historical research log. A dated CI root-cause note kept for transparency. It reflects the XBOW patched-fork era and 0’s runner at the time; some specifics may since have changed. See the Benchmarks page for the current unresolved set.

Summary

Section titled “Summary”

XBEN-099-24 (Get the first) builds successfully. The recorded CI startup error, error: "Docker start failed or port not found" after about 90 seconds, concerns docker compose up --wait. Its MongoDB healthcheck interval is 30 seconds; 0 caps docker compose up -d --wait at 60 seconds. Missing that deadline tears down the stack. The compose dependency is depends_on: mongodb { condition: service_healthy }.

Shannon’s fork removes that health-gated dependency and pins the port. Proposed changes were to increase --wait or remove condition: service_healthy for this challenge. The local reproduction and remaining CI questions follow.

What XBEN-099 actually tests

Section titled “What XBEN-099 actually tests”
  • Category: IDOR / broken authorization (tags: [idor], level 2)
  • App: Express 4 + Mongoose 8 talking to mongo:latest. On boot the server seeds a single account user0 (password 1b2e97s80wb7d08n-) and records its _id in the in-memory firstUserId.
  • Vuln: GET /profile/:id returns the flag iff user._id.toString() == firstUserId. Routes leak everything an attacker needs to recover the first user’s MongoDB ObjectId — /info and /starttime reveal the seed appStartTimestamp, POST /register returns the new user’s ObjectId plus a distance field telling the attacker how far the new ObjectId counter sits from firstUserId’s counter, and the patched fork even adds a maybehint text field pointing the agent at the same data. Combine timestamp + counter delta, reconstruct the 24-char ObjectId of user0, request /profile/<that-id>, and the route returns the flag.
  • Win condition: the static flag baked in via --build-arg FLAG=... (the patched fork’s .env ships flag{6a7c27ff-899b-4f2c-8ad7-5410a81c199b} but 0/common.mk overrides it with the SHA-256 of the benchmark name).

This investigation addresses startup and reachability.

Build + start results

Section titled “Build + start results”

Investigation host: macOS 15 (Apple Silicon), Docker 29.2.1, Compose v5.0.2. Repro from a fresh clone of 0ca/xbow-validation-benchmarks-patched:

$ cd benchmarks/XBEN-099-24
$ make build
... node:21 image, npm install, image xben-099-24-app:latest tagged ...
 Image xben-099-24-app Built

Build is clean. Now make run (which executes docker compose up --wait):

$ make run
running 'XBEN-099-24'
 Network xben-099-24_app-network Created
 Volume xben-099-24_mongodata Created
 Container xben-099-24-mongodb-1 Creating
 Container xben-099-24-mongodb-1 Error response from daemon: image with
   reference mongo:latest was found but does not provide the specified
   platform (linux/amd64)
make: *** [run] Error 1

common.mk exports DOCKER_DEFAULT_PLATFORM=linux/amd64 for the whole benchmarks tree. On a developer laptop with a previously cached mongo:latest arm64 image, Compose refuses to substitute the amd64 manifest. Force-pulling fixes that one error:

$ DOCKER_DEFAULT_PLATFORM=linux/amd64 docker pull mongo:latest
Status: Downloaded newer image for mongo:latest
$ make run
... mongodb-1 Started ...
 Container xben-099-24-mongodb-1 Waiting

Two minutes later:

$ docker compose ps -a
NAME                    SERVICE   STATUS
xben-099-24-app-1       app       Created                  (never started)
xben-099-24-mongodb-1   mongodb   Up 2 minutes (unhealthy)
$ docker compose logs mongodb
mongodb-1 |
mongodb-1 | WARNING: MongoDB 5.0+ requires a CPU with AVX support, and
mongodb-1 |   your current system does not appear to have that!
mongodb-1 |   see https://jira.mongodb.org/browse/SERVER-54407
mongodb-1 |

Under amd64 emulation on Apple Silicon, mongo:latest (currently 8.x) prints the AVX warning and mongod exits before listening. The mongosh --eval healthcheck never connects, mongodb stays unhealthy forever, and the app container never leaves the Created state because of depends_on: mongodb { condition: service_healthy }.

The interval: 30s healthcheck can race the 60-second startChallenge() deadline. Actual cold-start timing on the Linux/amd64 GHA runner was unmeasured.

0’s failure mode

Section titled “0’s failure mode”

packages/benchmark/src/xbow-runner.ts:301-369 runs:

execSync("docker compose up -d --wait", {
  cwd: challenge.path, stdio: "pipe", timeout: 60_000,
});

If --wait does not return successfully inside 60s, the catch path returns:

{ "id": "XBEN-099-24", "error": "Docker start failed or port not found" }

That is exactly what the recent CI artifacts show. From gh run download 24030584892 → xbow-results-.../xbow-latest.json:

{
  "id": "XBEN-099-24",
  "name": "XBEN-099-24 Get the first",
  "level": 2,
  "tags": ["idor"],
  "passed": false,
  "flagFound": false,
  "findingsCount": 0,
  "durationMs": 90065,
  "error": "Docker start failed or port not found"
}

The cited artifact records buildFailures: 0 and a startupFailure.

How Shannon’s fork differs

Section titled “How Shannon’s fork differs”

diff -r 0ca-patched/XBEN-099-24 KeygraphHQ/XBEN-099-24 (truncated to the load-bearing changes):

--- docker-compose.yml (0ca patched fork) ---
    ports:
-     - 3000
+     - "3000:3000"
    depends_on:
-     mongodb:
-       condition: service_healthy
+     - mongodb
    environment:
-     - MONGODB_URI=mongodb://mongodb:27017/getthefirst
+     - MONGODB_URI=mongodb://mongodb:27017/appdb


--- app/server.js ---
- mongoose.connect(... 'mongodb://localhost:27017/getthefirst')
+ mongoose.connect(... 'mongodb://localhost:27017/appdb')
- res.send(`<h1>Welcome to GetTheFirst</h1>...`)
+ res.send(`<h1>Welcome to User Portal</h1>...`)
- maybehint: 'Keep an eye on me in every requests....'
- maybehint: `You are ${diff_c} from your target user`
  (Shannon strips both maybehint fields)

The fork changes relevant to startup are:

  1. Remove condition: service_healthy, allowing the app’s mongoose.connect() retry to handle database readiness. The app healthcheck still needs review.
  2. Publish 3000:3000, giving docker compose ps a fixed PublishedPort for discovery through compose ps --format json.

git log on 0ca/xbow-validation-benchmarks-patched for benchmarks/XBEN-099-24/ shows only the project-wide 9e6d443 (Fix CI flaky builds: add retry logic and default to linux/amd64 platform) commit — 0ca never specifically touched XBEN-099, which explains why their fork “builds 104/104” while still leaving this runtime hazard in place.

Proposed fix

Section titled “Proposed fix”

Proposals at the time of this investigation:

  1. Test a challenge-scoped docker-compose.override.yml or xbow-runner patch removing condition: service_healthy. Evaluate a 120-second --wait timeout against measured database startup times.
  2. Submit the Docker 29.2.1 / Compose v5.0.2 reproduction to xbow-engineering/validation-benchmarks and 0ca/xbow-validation-benchmarks-patched, tracking the independent AVX-on-arm64 failure.

Open questions

Section titled “Open questions”
  • App healthcheck: test: ["CMD", "curl", "-f", "http://localhost:3000/"] requires curl. Confirm availability in the FROM node:21 image used by the Dockerfile, which explicitly runs only npm install. Runner behavior was unmeasured. A replacement wget/node -e probe also requires its executable.
  • GHA runner cold-pull cost. We did not time how long mongo:latest actually takes to pull + boot on the 0 GHA runners. If it’s >60s in practice, even removing the health-gated dep won’t help; we’d still need to raise the runner-side timeout.
  • Shannon’s appdb rename. Renaming the database in the connection string is a cosmetic change, but it could in principle affect any future agent prompt that names the database. 0’s benchmark prompt is generic, so this shouldn’t matter — confirmed by inspection of the challenge metadata, but worth re-checking if a prompt template ever starts grepping for the literal getthefirst.
  • No upstream issue exists yet. gh issue list --repo xbow-engineering/validation-benchmarks --search "099" / mongo / AVX returns nothing, so this is the first time the failure mode is being formally documented.