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CVE-2026-44436— Quicly is vulnerable to connection state corruption

Quick assessment

Affected
h2o quicly
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 quicly是H2O公司开源的一个 IETF QUIC 协议的实现。 H2O quicly 8b178e6之前版本存在缓冲区错误漏洞，该漏洞源于连接状态损坏，可能导致缓冲区溢出，进而导致拒绝服务攻击。

CVSS 7.5 · High EPSS 0.49% · P40

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 1

VendorProduct Version RangeStatus
h2o quicly < 8b178e6 affected
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I. Basic Information for CVE-2026-44436

Vulnerability Information
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Vulnerability Title
Quicly is vulnerable to connection state corruption
Source: CVE Program / CVE List V5
Vulnerability Description
Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit 8b178e6, Quicly is vulnerable to a Denial of Service attack through connection state corruption. In QUIC Invariants, the maximum length of a Connection ID is 255 bytes, while QUIC version 1 further restricts the maximum to 20 bytes. Quicly implements QUIC version 1 and therefore its CID buffers are limited to 20 bytes. However, to be able to respond to unknown versions of QUIC, its packet decoder accepts Connection IDs of up to 255 bytes. As its CID buffers are merely 20 bytes long, Quicly must reject QUIC version 1 packets with Connection IDs longer than that. The command line tool bundled with Quicly has had that check, however the library itself lacked such enforcement. As a consequence, when used by applications that lack their own enforcement, the connection state becoming inconsistent to buffer overrun. Fortunately, the overflow stops within the allocated chunk of memory, but nevertheless, the bug leads to assertion failures. This issue has been fixed by commit 8b178e6.
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
未进行输入大小检查的缓冲区拷贝(传统缓冲区溢出)
Source: CVE Program / CVE List V5
Vulnerability Title
H2O quicly 缓冲区错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
H2O quicly是H2O公司开源的一个 IETF QUIC 协议的实现。 H2O quicly 8b178e6之前版本存在缓冲区错误漏洞，该漏洞源于连接状态损坏，可能导致缓冲区溢出，进而导致拒绝服务攻击。
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 quicly < 8b178e6 -

II. Public POCs for CVE-2026-44436

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Qwen3.6-35B-A3B · 9275 chars
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III. Intelligence Information for CVE-2026-44436

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Patches & Fixes for CVE-2026-44436 (1)

Vendor Advisories for CVE-2026-44436 (1)

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

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-2026-54340 7.5 HIGH h2o has HTTP/2 state amplification
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: quicly
Same vendor: h2o
Same weakness: CWE-120

V. Comments for CVE-2026-44436

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