CVE-2026-69210 Http4s: WebSocket decoder accepts negative length, causing infinite decode loop
A remote attacker can cause a denial of service in Http4s by sending a WebSocket frame with a negative length, triggering an infinite decode loop.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-69210?
CVE-2026-69210 is a high-severity denial of service vulnerability in http4s, published on 15 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, WebSocket FrameTranscoder.bodyLength rejects extended payload lengths above Integer.MAX_VALUE but permits negative 64-bit lengths. A remote client that completes a WebSocket handshake through an Ember server can send such a frame, causing the decoder to return an empty frame without advancing its input. The decode loop then runs indefinitely, pins a worker at full CPU, and grows an ArrayBuffer without bound, resulting in denial of service. This issue is fixed in versions 0.23.35 and 1.0.0-M47.
Which versions are affected by CVE-2026-69210?
How do I fix CVE-2026-69210?
No fix has been published for CVE-2026-69210 yet. Limit exposure of the affected software until one is available.
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-2026-69210 being exploited?
There is no known exploitation of CVE-2026-69210 in the wild and no public exploit code.
Sources
Also known as GHSA-3j86-pj9g-jchr.