CVE-2026-55367

High

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.

SeverityHigh

CVSS7.5

Exploit probability—

Product

H2o

PublishedNov 4, 2026

First seenNov 4, 2026

Weakness typeCWE-770Allocation of resources without limits

How to fix

Sources list no fixed version or patch link for this vulnerability.

Affected products

Product	Affected versions	Fixed in
h2o h2o	< 4212ce114506a073505fd389db16ad2ba4e582a0	—

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