CVE-2026-54340

High

Description

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.

SeverityHigh

CVSS7.5

Exploit probability—

Product

H2o

PublishedJul 17, 2026

First seenJul 17, 2026

Weakness typeCWE-400Uncontrolled resource consumption CWE-770Allocation of resources without limits

How to fix

Update

h2o h2o2026-06-04 or later

Affected products

Product	Affected versions	Fixed in
h2o h2o	2026-05-29 ≤ v < 2026-06-04	2026-06-04

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