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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
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
Which versions are affected by CVE-2026-47066?
3 open source packages ship an affected version, including hackney. It affects 1 product.
| Package | 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 include Benoitc Hackney.
How do I fix CVE-2026-47066?
Upgrade hackney to 4.0.1 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
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 Atom table exhaustion via unrecognized URL schemes in hackney
- CVE-2026-47073 Unbounded memory consumption in WebSocket client in hackney
- CVE-2026-47071 SOCKS5 TLS upgrade ignores caller timeout in hackney
- CVE-2026-47077 Unbounded body accumulation in HTTP/3 response loop in hackney
- CVE-2026-47072 CRLF injection in WebSocket upgrade request in hackney
Sources
Also known as EEF-CVE-2026-47066, GHSA-6cp8-v795-jr2j.