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)