CVE-2026-55367

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    CVE-2026-55367

    Published: 4 Nov 2026Last Modified: 7 Nov 2026

    Summary: h2o: heap exhaustion using capsules

    Details: 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.

    References: https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55367.json, https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c, https://nvd.nist.gov/vuln/detail/CVE-2026-55367, https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0, https://github.com/h2o/h2o/pull/3754

    Affected packages

    Package

    Name:

    Purl:

    Affected ranges

    Type: GIT

    Events:

    Introduced- 0
    Fixed -4212ce114506a073505fd389db16ad2ba4e582a0

    Affected versions

    v1.7.0
    v1.7.0-beta5
    v1.7.0-beta4
    v1.7.0-beta3
    v1.7.0-beta2
    v1.7.0-beta1

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High
    CVE-2026-55367 | CVE-DB