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

> A malicious HTTP server can cause a denial of service in applications using the hackney library by sending a crafted Alt-Svc header that triggers an infinite loop.

- **Severity:** High 8.7 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 May 2026
- **Updated:** 27 May 2026
- **Also known as:** EEF-CVE-2026-47066, GHSA-6cp8-v795-jr2j

## What is CVE-2026-47066?

CVE-2026-47066 is a high-severity denial of service vulnerability in benoitc hackney, published on 25 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-835 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2026-47066?

3 open source packages ship an affected version, including hackney. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `hackney` | Hex | From 2.0.0 | 4.0.1 |
| `hackney` | Hex | From 2.0.0 | 4.0.1 |
| `hackney` | Hex | From 2.0.0-beta.1 | 4.0.1 |

Products: Benoitc Hackney.

## How do I fix CVE-2026-47066?

Upgrade hackney to 4.0.1 or later.

## Is CVE-2026-47066 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-47066 exists.

## Other vulnerabilities in benoitc hackney

- [CVE-2026-47067](https://insights.sbomify.com/CVE-2026-47067/): Atom table exhaustion via unrecognized URL schemes in hackney (high 8.7)
- [CVE-2026-47073](https://insights.sbomify.com/CVE-2026-47073/): Unbounded memory consumption in WebSocket client in hackney (high 8.7)
- [CVE-2026-47071](https://insights.sbomify.com/CVE-2026-47071/): SOCKS5 TLS upgrade ignores caller timeout in hackney (high 8.2)
- [CVE-2026-47077](https://insights.sbomify.com/CVE-2026-47077/): Unbounded body accumulation in HTTP/3 response loop in hackney (high 8.2)
- [CVE-2026-47072](https://insights.sbomify.com/CVE-2026-47072/): CRLF injection in WebSocket upgrade request in hackney (medium 6.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-47066 and fix it if it is affected.

What it is: CVE-2026-47066 is a high-severity denial of service vulnerability in benoitc hackney, published on 25 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-47066 exists.

Affected packages and fixed versions:
- hackney (Hex): affected from 2.0.0; fixed in 4.0.1
- hackney (Hex): affected from 2.0.0; fixed in 4.0.1
- hackney (Hex): affected from 2.0.0-beta.1; fixed in 4.0.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-47066/ (Markdown: https://insights.sbomify.com/CVE-2026-47066/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-47066
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-47066
- OSV: https://osv.dev/vulnerability/CVE-2026-47066

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-47066/
Summaries are generated and may contain mistakes; the official description above is the record.