CVE-2026-54556Network attack vector

Heap exhaustion via oversized HPACK expansion

Published Aug 26, 2026 · Updated Aug 26, 2026

HTTP/2 denial of service in http4s Ember 0.23.34 and earlier and 1.0.0-M1 through 1.0.0-M45 allows remote attackers to exhaust memory. The Hpack wrapper joins HEADERS and CONTINUATION fragments, decodes them into one retained List, and omits indexed headers from maxHeaderSize accounting. Exposure requires the Ember client or server backend with HTTP/2 enabled and an untrusted peer; concurrent compressed header blocks exhaust the heap and crash the process.

CVSS severity8.2
High
EPSS probability0.30%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP/2 denial of service in http4s Ember 0.23.34 and earlier and 1.0.0-M1 through 1.0.0-M45 allows remote attackers to exhaust memory. The Hpack wrapper joins HEADERS and CONTINUATION fragments, decodes them into one retained List, and omits indexed headers from maxHeaderSize accounting. Exposure requires the Ember client or server backend with HTTP/2 enabled and an untrusted peer; concurrent compressed header blocks exhaust the heap and crash the process.

The record

CVE
CVE-2026-54556
Published
Aug 26, 2026
Updated
Aug 26, 2026
Vendor
http4s
Product
http4s-ember-core
Classifications
CWE-409, T1499.003
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 26, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 26, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 0.23.35
  2. Affected versionversion=>= 1.0.0-M1, < 1.0.0-M46

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

http4s · http4s-ember-coreversion=< 0.23.35; version=>= 1.0.0-M1, < 1.0.0-M46
http4s · http4sversion=< 0.23.35; version=>= 1.0.0-M1, < 1.0.0-M46

Published CVSS scores

8.2GitHubCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-409Improper Handling of Highly Compressed Data (Data Amplification)
T1499.003Application Exhaustion Flood

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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