High severity · published Sep 15, 2026
CVE-2026-69209
Http4s: WebSocket decoder accepts unbounded message sizes
7.5
CVSS
0.6%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.6% · higher than 50% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
HT
http4s · http4s
< 0.23.35; >= 1.0.0-M1, < 1.0.0-M47 · source: vendor
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-69209
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-69209
github.com
https://github.com/http4s/http4s/commit/d578c3c9da6193627d40fa785e612cda7fcd77b3
github.com
https://github.com/http4s/http4s/releases/tag/v0.23.35
github.com
https://github.com/http4s/http4s/releases/tag/v1.0.0-M47
github.com
https://github.com/http4s/http4s/security/advisories/GHSA-jrxx-w2m8-5hf5
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