TI-2026-39234 Aliases CVE-2026-54340

h2o has HTTP/2 state amplification

LOW 39CWE-400Published 2026-07-16 · Updated 2026-07-17

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: Low
Impact type: Denial of service
Confidentiality · integrity · availability: None · None · High
Fix status: Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-400.

Summary

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decoded header state can be retained by stalled HTTP/2 streams, and depending on the configuration, additional limits are needed to bound decoded header state and prevent attack. This issue has been fixed by commit 9265bdd.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00493 42nd percentileScored 2026-11-07

Fix Fixed h2o (h2o): 9265bdd

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: Yes
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
h2oai	h2o	before 9265bdd	9265bdd	Patch available	CNA

h2o Vendorh2oai Affected before 9265bdd Fixed in 9265bdd Patch available CNA

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:h2o:h2o::::::::	2026-05-29 to before 2026-06-04	Vulnerable	h2oai

cpe:2.3:a:h2o:h2o:*:*:*:*:*:*:*:* Versions 2026-05-29 to before 2026-06-04 Vulnerable ·h2oai

Remediation

Fix · CNA h2o (h2o) Fixed in 9265bdd

Timeline

2026-10-02 Threatist record created from CVE-2026-54340

References

Patch 1

https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063

Advisory 1

https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9

TI-2025-05944 Deserialization Vulnerability in h2oai/h2o-3 CVSS 3.0CRITICAL 9.8
TI-2024-09776 Jdbc Deserialization in h2oai/h2o-3 CVSS 3.0CRITICAL 9.8
TI-2024-30727 H2O.ai H2O through 3.46.0.4 allows attackers to arbitrarily set the JDBC URL, leading to… CVSS 3.1CRITICAL 9.1
TI-2026-03424 Remote Code Execution in h2oai/h2o-3 CVSS 3.1CRITICAL 9.8
TI-2024-07992 Arbitrary File Overwrite in h2oai/h2o-3 CVSS 3.0HIGH 8.2
TI-2023-06020 External Control of File Name or Path in h2oai/h2o-3 CVSS 3.1HIGH 8.2

See all 29 →

Sources

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