# CVE-2026-54556

## Summary

- **CVE ID:** CVE-2026-54556
- **Severity:** HIGH
- **CVSS Score:** 8.2 (CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N)
- **CWE:** CWE-409
- **Published:** Aug 26, 2026
- **Last Modified:** Aug 29, 2026

## Description

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, an unauthenticated HTTP/2 peer can cause an out-of-memory denial of service in the Ember backend with HTTP/2 enabled. The Hpack wrapper in ember-core/shared/src/main/scala/org/http4s/ember/core/h2/Hpack.scala concatenates HEADERS and CONTINUATION frame fragments and decodes them into a single List, but maxHeaderSize accounting does not include indexed headers or HPACK per-header overhead. A small compressed header block can therefore expand into a much larger decoded representation that remains in memory for processing. Servers exposed to untrusted HTTP/2 traffic and clients directed to an untrusted HTTP/2 server are affected, and concurrent malicious connections can exhaust the process heap. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

## Affected Products

- http4s — http4s (< 0.23.35)
- http4s — http4s (>= 1.0.0-M1, < 1.0.0-M46)
- org.http4s — http4s-ember-core_2.12 (< 0.23.35)
- org.http4s — http4s-ember-core_2.13 (>= 1.0.0-M1, < 1.0.0-M46)
- org.http4s — http4s-ember-core_2.13 (< 0.23.35)
- org.http4s — http4s-ember-core_3 (>= 1.0.0-M1, < 1.0.0-M46)
- org.http4s — http4s-ember-core_3 (< 0.23.35)

## References

- [CNA](https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm)
- [CNA](https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e)
- [CNA](https://github.com/http4s/http4s/releases/tag/v0.23.35)
- [CNA](https://github.com/http4s/http4s/releases/tag/v1.0.0-M47)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.30%
- **EPSS Percentile:** 22.6

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._