CVE-2026-47066: Hackney Hackney Has An Infinite Loop Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-6cp8-v795-jr2j",
  "modified" : "2026-06-26T21:53:07Z",
  "published" : "2026-06-26T21:53:07Z",
  "aliases" : [ "CVE-2026-47066" ],
  "summary" : "Hackney has an infinite loop on non-token byte at start of an Alt-Svc entry",
  "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",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "hackney"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.0.0"
      }, {
        "fixed" : "4.0.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "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" : "WEB",
    "url" : "https://github.com/benoitc/hackney/commit/e548aba1f97ffa3f4750da7b772998fb78c01894"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-47066.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/benoitc/hackney"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-47066"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-835" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-26T21:53:07Z",
    "nvd_published_at" : "2026-05-25T15:16:21Z"
  }
}