CVE-2026-69209

Http4s: WebSocket decoder accepts unbounded message sizes

State: Published
Severity: 7.5 High
Published: Sep 15, 2026
Updated: Sep 16, 2026
Reserved: Aug 3, 2026

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.

Scores

CVSS	Score	Vector	Source
3.1	7.5 High	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H	GitHub_M

Weaknesses

CWE-400 Uncontrolled Resource Consumption
CWE-770 Allocation of Resources Without Limits or Throttling

Affected Products

Product	Versions
http4shttp4s	< 0.23.35Affected >= 1.0.0-M1, < 1.0.0-M47Affected

References

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