CVE-2026-55367— h2o: heap exhaustion using capsules

Quick assessment

Affected

h2o h2o

Exploitation

No confirmed in-the-wild exploitation; assess based on exposure

Recommended action

Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

H2O 是一款支持 HTTP/1.x、HTTP/2 和 HTTP/3 的 HTTP 服务器。在提交 4212ce114506a073505fd389db16ad2ba4e582a0 之前，攻击者可发送一个声明长度过大但内容不完整的 CONNECT-UDP 胶囊（Capsule），导致 H2O 在等待剩余数据时持续缓冲接收到的字节，且未对每个胶囊设置缓冲上限。任何启用了并接收 CONNECT-UDP 请求的部署环境，都可能因此耗尽服务器内存，进而丧失可用性。该漏洞已在提交 4212ce114506a073505fd

CVSS 7.5 · High Updated Nov 05, 2026

Possible ATT&CK Techniques 1 AI

T1499.003 · Application Exhaustion Flood

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Vulnerability Information

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Vulnerability Title

h2o: heap exhaustion using capsules

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 4212ce114506a073505fd389db16ad2ba4e582a0, a client could send an incomplete CONNECT-UDP Capsule with an excessively large declared length, causing H2O to keep buffering received bytes without a per-Capsule limit while waiting for the remainder. Deployments that enabled proxy.connect-udp and accepted CONNECT-UDP requests could exhaust server memory and lose availability. This vulnerability is fixed with commit 4212ce114506a073505fd389db16ad2ba4e582a0.

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

不加限制或调节的资源分配

Source: CVE Program / CVE List V5

Affected Products

Vendor	Product	Affected Versions	CPE	Subscribe
h2o	h2o	< 4212ce114506a073505fd389db16ad2ba4e582a0	-

II. Public POCs for CVE-2026-55367

#	POC Description	Source Link	Shenlong Link

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III. Intelligence Information for CVE-2026-55367

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Other References for CVE-2026-55367 (3)

https://github.com/h2o/h2o/pull/3754 x_refsource_MISC
https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c x_refsource_CONFIRM
https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0 x_refsource_MISC

https://www.cve.org/CVERecord?id=CVE-2026-55367

IV. Related Vulnerabilities

Same product: h2o

Same vendor: h2o

Same weakness: CWE-770