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    CVE-2026-55367

    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.

    Published:Nov 4, 2026
    Last Modified:Nov 4, 2026
    EPS:Nov 4, 2026
    EPSS Score:
    CVSS Score:7.5

    Affected Products

    Vendor
    H2o
    Product
    H2o

    Exploits

    No exploit reference

    Related CVEs

    CVE-2026-55368 (CVSS 8.1) — h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 48b7ae12107fd2b118e4641da4…
    CVE-2026-54340 (CVSS 7.5) — h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/…
    CVE-2026-44453 (CVSS 7.5) — h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 6b5370d, h2o is vulnerable…
    CVE-2026-44452 (CVSS 5.9) — h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 8dc37cb, when h2o receives…
    CVE-2026-55213 (CVSS 7.5) — h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit edd7a120bfc4af11ac0cbebce2…
    CVE-2026-8752 (CVSS 5.5) — A weakness has been identified in h2oai h2o-3 up to 7402. This vulnerability affects the function exec of the …
    CVE-2026-8751 (CVSS 5.5) — A security flaw has been discovered in h2oai h2o-3 up to 7402. This affects the function importBinaryModel of …
    CVE-2026-8750 (CVSS 5.5) — A vulnerability was identified in h2oai h2o-3 up to 7402. Affected by this issue is the function importFiles o…
    CVE-2026-3960 (CVSS 9.8) — A critical remote code execution vulnerability exists in the unauthenticated REST API endpoint /99/ImportSQLTa…
    CVE-2025-10769 (CVSS 2.1) — A vulnerability has been found in h2oai h2o-3 up to 3.46.08. This affects an unknown function of the file /99/…

    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