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CVE-2026-54340— h2o has HTTP/2 state amplification

Quick assessment

Affected
h2o h2o
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

h2o是H2O公司开源的一款新一代HTTP服务器。 h2o 9265bdd之前版本存在资源管理错误漏洞，该漏洞源于HTTP/2状态放大问题，结合HPACK解压放大与Slowloris-style流停滞，导致解码后的标头状态被停滞的HTTP/2流保留，攻击者可通过网络利用此问题造成拒绝服务。

CVSS 7.5 · High EPSS 0.49% · P40

Affected Version Matrix 1

VendorProduct Version RangeStatus
h2o h2o < 9265bdd affected
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I. Basic Information for CVE-2026-54340

Vulnerability Information
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Vulnerability Title
h2o has HTTP/2 state amplification
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
未加控制的资源消耗(资源穷尽)
Source: CVE Program / CVE List V5
Vulnerability Title
h2o 资源管理错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
h2o是H2O公司开源的一款新一代HTTP服务器。 h2o 9265bdd之前版本存在资源管理错误漏洞，该漏洞源于HTTP/2状态放大问题，结合HPACK解压放大与Slowloris-style流停滞，导致解码后的标头状态被停滞的HTTP/2流保留，攻击者可通过网络利用此问题造成拒绝服务。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
h2o h2o < 9265bdd -

II. Public POCs for CVE-2026-54340

# POC Description Source Link Shenlong Link
AI-Generated POC Premium
Qwen3.6-35B-A3B · 10225 chars
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Exploit chain & mitigation
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III. Intelligence Information for CVE-2026-54340

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Patches & Fixes for CVE-2026-54340 (1)

Vendor Advisories for CVE-2026-54340 (1)

Same Patch Batch · h2o · 2026-07-16 · 7 CVEs total

CVE-2026-44436 7.5 HIGH Quicly is vulnerable to connection state corruption
CVE-2026-44435 7.5 HIGH Quicly: Remote Denial of Service via assertion failure when CRYPTO stream handshake data e
CVE-2026-44453 7.5 HIGH h2o is vulnerable to musl libc stack overflow
CVE-2026-44452 5.9 MEDIUM h2o is vulnerable to heap overrun
CVE-2026-44433 5.3 MEDIUM Quicly is vulnerable to memory exhaustion
CVE-2026-44434 5.3 MEDIUM Quicly is vulnerable to stateless reset injection

IV. Related Vulnerabilities

Same product: h2o
Same vendor: h2o
Same weakness: CWE-400

V. Comments for CVE-2026-54340

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