Vulnerability-Lookup

CVE-2026-47066 (GCVE-0-2026-47066)

Vulnerability from cvelistv5 – Published: 2026-05-25 14:00 – Updated: 2026-05-27 15:40

Title

Infinite loop in Alt-Svc header parser in hackney

Summary

Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in benoitc hackney allows Excessive Allocation. The Alt-Svc response header parser in src/hackney_altsvc.erl does not guarantee forward progress. When parse_token/2 receives a non-token, non-whitespace, non-comma byte (e.g. !, @, =, ;), it returns the input unchanged. skip_comma/1 also returns the buffer unchanged when the first byte is not a comma. parse_entries/2 then recurses with identical data, creating a tight infinite tail-recursive loop that pins a scheduler at 100% CPU. The calling process never returns. The entry point parse_and_cache/3 is called synchronously in the connection process on every HTTP response. A single-byte Alt-Svc: ! response header is sufficient to trigger the hang; the header is fully controlled by any HTTP origin the client connects to. This issue affects hackney: from 2.0.0-beta.1 before 4.0.1.

Severity

8.7 (High)


                        
                          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

SSVC

Exploitation: poc Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-05-26 15:50 UTC

CWE

CWE-835 - Loop with Unreachable Exit Condition ('Infinite Loop')

Assigner

EEF mitre root Scorecard

References

4 references

URL	Tags
https://github.com/benoitc/hackney/security/advis…	vendor-advisoryrelated
https://cna.erlef.org/cves/CVE-2026-47066.html	related
https://osv.dev/vulnerability/EEF-CVE-2026-47066	related
https://github.com/benoitc/hackney/commit/e548aba…	patch

Impacted products

2 products

Vendor	Product	Version	CPE status
benoitc	hackney	Affected: 2.0.0-beta.1 , < 4.0.1 (semver) cpe:2.3:a:benoitc:hackney::::::::
benoitc	hackney	Affected: 408e5fe20302226ea8c74dde2bcbd452d712b5b2 , < e548aba1f97ffa3f4750da7b772998fb78c01894 (git) cpe:2.3:a:benoitc:hackney::::::::

Credits

Peter Ullrich Benoit Chesneau Jonatan Männchen / EEF

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-47066

Vulnerability from fkie_nvd - Published: 2026-05-25 15:16 - Updated: 2026-07-24 10:10

Severity

7.5 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Summary

References

URL	Tags
6b3ad84c-e1a6-4bf7-a703-f496b71e49db	https://cna.erlef.org/cves/CVE-2026-47066.html	Patch, Third Party Advisory
6b3ad84c-e1a6-4bf7-a703-f496b71e49db	https://github.com/benoitc/hackney/commit/e548aba1f97ffa3f4750da7b772998fb78c01894	Patch
6b3ad84c-e1a6-4bf7-a703-f496b71e49db	https://github.com/benoitc/hackney/security/advisories/GHSA-6cp8-v795-jr2j	Exploit, Patch, Vendor Advisory
6b3ad84c-e1a6-4bf7-a703-f496b71e49db	https://osv.dev/vulnerability/EEF-CVE-2026-47066	Patch, Third Party Advisory
134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/benoitc/hackney/security/advisories/GHSA-6cp8-v795-jr2j	Exploit, Patch, Vendor Advisory

Impacted products

	Vendor	Product	Version
	benoitc	hackney	*

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "collectionURL": "https://repo.hex.pm",
          "cpes": [
            "cpe:2.3:a:benoitc:hackney:*:*:*:*:*:*:*:*"
          ],
          "defaultStatus": "unaffected",
          "modules": [
            "hackney_altsvc"
          ],
          "packageName": "hackney",
          "packageURL": "pkg:hex/hackney",
          "product": "hackney",
          "programFiles": [
            "src/hackney_altsvc.erl"
          ],
          "programRoutines": [
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              "name": "hackney_altsvc:parse_entries/2"
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              "name": "hackney_altsvc:parse_entry/1"
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            },
            {
              "name": "hackney_altsvc:parse_token/2"
            },
            {
              "name": "hackney_altsvc:skip_comma/1"
            }
          ],
          "repo": "https://github.com/benoitc/hackney",
          "vendor": "benoitc",
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              "status": "affected",
              "version": "2.0.0-beta.1",
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        {
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          "cpes": [
            "cpe:2.3:a:benoitc:hackney:*:*:*:*:*:*:*:*"
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            "hackney_altsvc"
          ],
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          "packageURL": "pkg:github/benoitc/hackney",
          "product": "hackney",
          "programFiles": [
            "src/hackney_altsvc.erl"
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              "name": "hackney_altsvc:parse_entries/2"
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            },
            {
              "name": "hackney_altsvc:parse_protocol/1"
            },
            {
              "name": "hackney_altsvc:parse_token/2"
            },
            {
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          "vendor": "benoitc",
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      "lang": "en",
      "value": "Loop with Unreachable Exit Condition (\u0027Infinite Loop\u0027) vulnerability in benoitc hackney allows Excessive Allocation. The Alt-Svc response header parser in src/hackney_altsvc.erl does not guarantee forward progress. When parse_token/2 receives a non-token, non-whitespace, non-comma byte (e.g. !, @, =, ;), it returns the input unchanged. skip_comma/1 also returns the buffer unchanged when the first byte is not a comma. parse_entries/2 then recurses with identical data, creating a tight infinite tail-recursive loop that pins a scheduler at 100% CPU. The calling process never returns.\n\nThe entry point parse_and_cache/3 is called synchronously in the connection process on every HTTP response. A single-byte Alt-Svc: ! response header is sufficient to trigger the hang; the header is fully controlled by any HTTP origin the client connects to.\n\nThis issue affects hackney: from 2.0.0-beta.1 before 4.0.1."
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      "value": "Vulnerabilidad de bucle con condici\u00f3n de salida inalcanzable (\u0027Bucle Infinito\u0027) en benoitc hackney permite asignaci\u00f3n excesiva. El analizador de encabezados de respuesta Alt-Svc en src/hackney_altsvc.erl no garantiza el progreso hacia adelante. Cuando parse_token/2 recibe un byte que no es un token, no es un espacio en blanco, no es una coma (p. ej. \u0027!\u0027, \u0027@\u0027, \u0027=\u0027, \u0027;\u0027), devuelve la entrada sin cambios. skip_comma/1 tambi\u00e9n devuelve el b\u00fafer sin cambios cuando el primer byte no es una coma. parse_entries/2 luego recurre con datos id\u00e9nticos, creando un bucle recursivo de cola infinito y ajustado que fija un planificador al 100% de CPU. El proceso de llamada nunca regresa.\n\nEl punto de entrada parse_and_cache/3 se llama sincr\u00f3nicamente en el proceso de conexi\u00f3n en cada respuesta HTTP. Un encabezado de respuesta Alt-Svc: \u0027!\u0027 de un solo byte es suficiente para activar el bloqueo; el encabezado est\u00e1 totalmente controlado por cualquier origen HTTP al que se conecta el cliente.\n\nEste problema afecta a hackney: desde 2.0.0-beta.1 antes de 4.0.1."
    }
  ],
  "id": "CVE-2026-47066",
  "lastModified": "2026-07-24T10:10:00.197",
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        }
      ],
      "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
      "type": "Secondary"
    }
  ]
}

GHSA-6CP8-V795-JR2J

Vulnerability from github – Published: 2026-06-26 21:53 – Updated: 2026-06-26 21:53

Summary

Hackney has an infinite loop on non-token byte at start of an Alt-Svc entry

Details

Summary

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

Severity

8.7 (High)


                  
                    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

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{
  "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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Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

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The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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CVE-2026-47066 (GCVE-0-2026-47066)

FKIE_CVE-2026-47066

GHSA-6CP8-V795-JR2J

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