CVE-2026-54556

HIGHPre-NVD 8.28.2—
EchelonGraph scoreMEDIUM confidence

This high-severity CVE scores 8.2 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.3%, top 77% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, secondary
8.2EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 8.2Exploit: None knownExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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.

CVSS v3
8.2
EG Score
8.2(medium)
EG Risk
52(Track)
EG Risk 52/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity82% × 45%
Exploitation0% × 40%
Automatability100% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
23%
KEV
Not listed

Published

August 26, 2026

Last Modified

September 9, 2026

Advisory Details (4)

Auto-updated Aug 26, 2026
Patch available. Sources: github, github_commit, github_release.
github_release Patch Available

v1.0.0-M47

Patch available: http4s/http4s v1.0.0-M47 (pre-release)

https://github.com/http4s/http4s/releases/tag/v1.0.0-M47
github_release Patch Available

v0.23.35

Patch available: http4s/http4s v0.23.35

https://github.com/http4s/http4s/releases/tag/v0.23.35
github_commit Patch Available

commit 6e8eccd64a6a (http4s/http4s)

Patch available: http4s/http4s v0.23.35 (contains commit 6e8eccd64a6a)

https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e
github Patch Available

Ember HTTP/2: Denial of Service via HPACK bomb · Advisory · http4s/http4s · GitHub

https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm

Vendor Advisories for CVE-2026-54556(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Affected Packages

(3 across 1 ecosystem)
Maven(3)
PackageVulnerable rangeFixed inDependents
org.http4s:http4s-ember-core_2.120.10.0-M10 ... 0.23.9 (114 versions)0.23.35—
org.http4s:http4s-ember-core_2.130.10.0-M10 ... 0.23.9 (114 versions)0.23.35—
org.http4s:http4s-ember-core_31.0.0-M22 ... 1.0.0-M46 (24 versions)1.0.0-M47—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 13× in last 7d / 47× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-09-19 01:44 UTCEG score recompute
  2. 2026-09-18 19:28 UTCEPSS rescore
  3. 2026-09-18 00:04 UTCEG score recompute
  4. 2026-09-17 19:31 UTCEPSS rescore
  5. 2026-09-16 21:32 UTCEG score recompute
  6. 2026-09-16 14:08 UTCEPSS rescore
  7. 2026-09-16 07:37 UTCEG score recompute
  8. 2026-09-16 05:15 UTCEPSS rescore
  9. 2026-09-15 05:50 UTCEG score recompute
  10. 2026-09-15 03:11 UTCEPSS rescore
  11. 2026-09-14 04:08 UTCEG score recompute
  12. 2026-09-13 16:47 UTCEPSS rescore
  13. 2026-09-13 02:28 UTCEG score recompute
  14. 2026-09-12 00:46 UTCEG score recompute
  15. 2026-09-11 14:53 UTCEPSS rescore
  16. 2026-09-11 11:55 UTCEG score recompute
  17. 2026-09-10 10:04 UTCEG score recompute
  18. 2026-09-10 09:35 UTCEPSS rescore
  19. 2026-09-09 00:51 UTCEG score recompute
  20. 2026-09-08 22:01 UTCEPSS rescore
  21. 2026-09-07 18:59 UTCEG score recompute
  22. 2026-09-07 16:01 UTCEPSS rescore
  23. 2026-09-06 17:16 UTCEG score recompute
  24. 2026-09-06 13:47 UTCEPSS rescore
  25. 2026-09-05 15:35 UTCEG score recompute
Show 22 more
  1. 2026-09-05 02:36 UTCEG score recompute
  2. 2026-09-04 13:42 UTCEG score recompute
  3. 2026-09-04 05:06 UTCEPSS rescore
  4. 2026-09-02 23:08 UTCEG score recompute
  5. 2026-09-02 14:12 UTCEPSS rescore
  6. 2026-09-01 21:24 UTCEG score recompute
  7. 2026-09-01 13:54 UTCEPSS rescore
  8. 2026-09-01 08:33 UTCEG score recompute
  9. 2026-09-01 04:40 UTCEPSS rescore
  10. 2026-08-31 06:48 UTCEG score recompute
  11. 2026-08-30 19:17 UTCEPSS rescore
  12. 2026-08-30 05:07 UTCEG score recompute
  13. 2026-08-30 01:22 UTCEPSS rescore
  14. 2026-08-29 03:23 UTCEG score recompute
  15. 2026-08-29 02:45 UTCEG score recompute
  16. 2026-08-28 23:58 UTCEG score recompute
  17. 2026-08-28 21:41 UTCEPSS rescore
  18. 2026-08-27 18:07 UTCEG score recompute
  19. 2026-08-27 14:25 UTCEPSS rescore
  20. 2026-08-26 15:50 UTCEG score recompute
  21. 2026-08-26 14:23 UTCEG score recompute
  22. 2026-08-26 14:22 UTCMITRE cvelistV5first tracked

Frequently asked(5)

What is CVE-2026-54556?
CVE-2026-54556 is a high vulnerability published on August 26, 2026. 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…
When was CVE-2026-54556 disclosed?
CVE-2026-54556 was first published in the National Vulnerability Database on August 26, 2026, with the most recent update on September 9, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-54556 actively exploited?
CVE-2026-54556 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 76.9% of all scored CVEs.
What is the CVSS score of CVE-2026-54556?
CVE-2026-54556 has a CVSS v3 base score of 8.2 (NVD).
How do I remediate CVE-2026-54556?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-54556, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2026-54556

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