{"id":"CVE-2026-47066","aliases":["EEF-CVE-2026-47066","GHSA-6cp8-v795-jr2j"],"url":"https://o3.security/vulnerability/CVE-2026-47066","summary":"Infinite loop in Alt-Svc header parser in hackney","details":"### Summary\n\n[CVE-2026-47066](https://nvd.nist.gov/vuln/detail/CVE-2026-47066) is an infinite loop (CWE-835) in hackney's Alt-Svc response header parser (`src/hackney_altsvc.erl`). When an HTTP server returns an `Alt-Svc` header whose value begins with a non-token byte (e.g. `!`, `@`, `=`, `;`), the parser enters a tight tail-recursive loop that pins an Erlang scheduler at 100% CPU and permanently hangs the calling connection process. Because the parser is invoked synchronously on every HTTP response, any attacker-controlled origin can trigger the hang with a single-byte header value.\n\n### Details\n\n**1. Parser dispatch**\n\n`parse_and_cache/3` is called inside the hackney connection process on each HTTP response. It collects all `Alt-Svc` header values via `collect_altsvc_headers/1`, concatenates them, and passes the result to `parse/1`, which calls `parse_entries(Header, [])`.\n\n**2. Failed token consumption**\n\n`parse_entries/2` → `parse_entry/1` → `parse_protocol/1` → `parse_token(Data, <<>>)`. The function `parse_token/2` pattern-matches leading bytes: alphanumeric, `-`, `_`, whitespace, and comma all have explicit clauses. Any other byte (e.g. `!`) falls through to the catch-all:\n\n```erlang\nparse_token(Rest, <<>>) -> {undefined, Rest}.\n```\n\nThis returns the *input unchanged* — no byte is consumed.\n\n**3. No-progress loop**\n\n`parse_entry` propagates `{undefined, Rest}` back to `parse_entries/2`, which calls `skip_comma(Rest)`. Because the first byte is not `,`, `skip_comma` also returns `Rest` unchanged. `parse_entries` then recurses with the identical buffer:\n\n```erlang\nparse_entries(Data, Acc)  % Data identical to previous iteration\n```\n\nErlang tail recursion never preempts on a pure CPU loop, so the scheduler is pinned and the process never yields or returns.\n\n**4. Root cause**\n\n`parse_entries/2` has no guard that detects zero-byte progress after a failed `parse_entry` call and no fallback to advance past the offending byte.\n\n### PoC\n\n1. Start an HTTP server that responds with the header `Alt-Svc: !` (any single non-token byte suffices).\n2. Issue any HTTP GET request via hackney to that server:\n   ```erlang\n   hackney:request(get, \"http://attacker.example/\", [], <<>>, [])\n   ```\n3. Observe that the call never returns; the Erlang scheduler hosting the connection process is pinned at 100% CPU indefinitely.\n\nAlternatively, call the parser directly: `hackney_altsvc:parse(<<\"!\">>)` — the process hangs immediately.\n\n### Impact\n\nDenial of service via unbounded CPU consumption. Any application using hackney 2.0.0-beta.1 through 4.0.0 that connects to attacker-controlled HTTP endpoints is affected. No authentication is required; a single response header byte is sufficient to hang the connection process. Fixed in hackney 4.0.1. CVSS v4.0 score: **8.7 (HIGH)**.\n\n## Resources\n\n* Introduction commit: https://github.com/benoitc/hackney/commit/408e5fe20302226ea8c74dde2bcbd452d712b5b2\n* Patch commit: https://github.com/benoitc/hackney/commit/e548aba1f97ffa3f4750da7b772998fb78c01894","published":"2026-05-25T14:00:39.707Z","modified":"2026-10-21T02:30:53.831804007Z","cvss":null,"epss":{"score":0.00753,"percentile":0.53037,"asOf":"2026-09-13"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Hex","name":"hackney","fixedVersion":"4.0.1"}],"fix":{"url":"https://github.com/benoitc/hackney/commit/e548aba1f97ffa3f4750da7b772998fb78c01894","label":"benoitc/hackney@e548aba"},"references":[{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-47066.html"},{"type":"WEB","url":"https://github.com"},{"type":"WEB","url":"https://osv.dev/vulnerability/EEF-CVE-2026-47066"},{"type":"WEB","url":"https://repo.hex.pm"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/47xxx/CVE-2026-47066.json"},{"type":"ADVISORY","url":"https://github.com/benoitc/hackney/security/advisories/GHSA-6cp8-v795-jr2j"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-47066"},{"type":"FIX","url":"https://github.com/benoitc/hackney/commit/e548aba1f97ffa3f4750da7b772998fb78c01894"},{"type":"PACKAGE","url":"https://github.com/benoitc/hackney"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:30:53.831804007Z"}}