Vulnerability · Published Sep 15, 2026 · Assigned by GitHub_M

CVE-2026-69209Http4s: WebSocket decoder accepts unbounded message sizes

High 7.5

In plain English

A high-severity uncontrolled resource consumption in http4s from http4s. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 50% of all known vulnerabilities).

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.6%top 49.6% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, The shared WebSocket decoder permits unbounded message buffering because defragmentation accumulates fragments without a limit and FrameTranscoder accepts declared lengths up to Int.MaxValue. A remote client that completes a WebSocket handshake against an http4s-blaze-server or http4s-ember-server endpoint can exhaust server memory with oversized frames or fragmented messages. The patched decoder applies a configurable 64 MiB default limit to individual frames and defragmented messages through EmberServerBuilder.withMaxWebSocketMessageSize. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Affected products (as reported)

Vendor	Product	Versions
http4s	http4s	< 0.23.35>= 1.0.0-M1, < 1.0.0-M47

Weakness: CWE-400, CWE-770 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

https://github.com/http4s/http4s/security/advisories/GHSA-jrxx-w2m8-5hf5 x_refsource_CONFIRM
https://github.com/http4s/http4s/commit/d578c3c9da6193627d40fa785e612cda7fcd77b3 x_refsource_MISC
https://github.com/http4s/http4s/releases/tag/v0.23.35 x_refsource_MISC
https://github.com/http4s/http4s/releases/tag/v1.0.0-M47 x_refsource_MISC