CVE-2026-54340Network attack vector

Service interruption via retained decoded header state

Published Jul 16, 2026 · Updated Jul 16, 2026

Resource exhaustion in H2O before commit 9265bdd allows remote attackers to interrupt service through crafted HTTP/2 streams. The HTTP/2 HPACK request parser decodes attacker-controlled header fields without a hard field-count limit, allowing amplified header state to remain allocated while streams are deliberately stalled. No authentication or user interaction is required, but exploitation depends on HTTP/2 exposure and configurations whose existing limits do not bound the retained decoded state.

CVSS severity7.5
High
EPSS probability0.49%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Resource exhaustion in H2O before commit 9265bdd allows remote attackers to interrupt service through crafted HTTP/2 streams. The HTTP/2 HPACK request parser decodes attacker-controlled header fields without a hard field-count limit, allowing amplified header state to remain allocated while streams are deliberately stalled. No authentication or user interaction is required, but exploitation depends on HTTP/2 exposure and configurations whose existing limits do not bound the retained decoded state.

The record

CVE
CVE-2026-54340
Published
Jul 16, 2026
Updated
Jul 16, 2026
Vendor
H2O Project
Product
H2O
Classifications
CWE-770, CWE-400, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 16, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 16, 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=< 9265bdd

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

H2O Project · H2Oversion=< 9265bdd

Published CVSS scores

7.5GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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-770Allocation of Resources Without Limits or Throttling
CWE-400Uncontrolled Resource Consumption
T1499Endpoint Denial of Service

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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