GHSA-6CP8-V795-JR2J
Vulnerability from github – Published: 2026-06-26 21:53 – Updated: 2026-06-26 21:53Summary
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.
Details
1. Parser dispatch
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, []).
2. Failed token consumption
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:
parse_token(Rest, <<>>) -> {undefined, Rest}.
This returns the input unchanged — no byte is consumed.
3. No-progress loop
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:
parse_entries(Data, Acc) % Data identical to previous iteration
Erlang tail recursion never preempts on a pure CPU loop, so the scheduler is pinned and the process never yields or returns.
4. Root cause
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.
PoC
- Start an HTTP server that responds with the header
Alt-Svc: !(any single non-token byte suffices). - Issue any HTTP GET request via hackney to that server:
erlang hackney:request(get, "http://attacker.example/", [], <<>>, []) - Observe that the call never returns; the Erlang scheduler hosting the connection process is pinned at 100% CPU indefinitely.
Alternatively, call the parser directly: hackney_altsvc:parse(<<"!">>) — the process hangs immediately.
Impact
Denial 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).
Resources
- Introduction commit: https://github.com/benoitc/hackney/commit/408e5fe20302226ea8c74dde2bcbd452d712b5b2
- Patch commit: https://github.com/benoitc/hackney/commit/e548aba1f97ffa3f4750da7b772998fb78c01894
{
"affected": [
{
"package": {
"ecosystem": "Hex",
"name": "hackney"
},
"ranges": [
{
"events": [
{
"introduced": "2.0.0"
},
{
"fixed": "4.0.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-47066"
],
"database_specific": {
"cwe_ids": [
"CWE-835"
],
"github_reviewed": true,
"github_reviewed_at": "2026-06-26T21:53:07Z",
"nvd_published_at": "2026-05-25T15:16:21Z",
"severity": "HIGH"
},
"details": "### Summary\n\n[CVE-2026-47066](https://nvd.nist.gov/vuln/detail/CVE-2026-47066) is an infinite loop (CWE-835) in hackney\u0027s 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` \u2192 `parse_entry/1` \u2192 `parse_protocol/1` \u2192 `parse_token(Data, \u003c\u003c\u003e\u003e)`. 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, \u003c\u003c\u003e\u003e) -\u003e {undefined, Rest}.\n```\n\nThis returns the *input unchanged* \u2014 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/\", [], \u003c\u003c\u003e\u003e, [])\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(\u003c\u003c\"!\"\u003e\u003e)` \u2014 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",
"id": "GHSA-6cp8-v795-jr2j",
"modified": "2026-06-26T21:53:07Z",
"published": "2026-06-26T21:53:07Z",
"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"
}
],
"schema_version": "1.4.0",
"severity": [
{
"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",
"type": "CVSS_V4"
}
],
"summary": "Hackney has an infinite loop on non-token byte at start of an Alt-Svc entry"
}
Sightings
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