CVE-2026-55368

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    CVE-2026-55368

    Published: 4 Nov 2026Last Modified: 5 Nov 2026

    Summary: h2o: buffer overflow when using the %{error}x log directive

    Details: h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 48b7ae12107fd2b118e4641da49599e9aac8edd8, H2O's access-log formatter could write past its log-line buffer when an affected custom access-log format was configured. The allocation errors affected %{error}x, %q when the query string was empty, and numeric elements whose temporary terminating NUL reached the end of the buffer. A matching request could crash the server, while the effect of the one-byte NUL overwrite beyond a crash was not established. This vulnerability is fixed with commit 48b7ae12107fd2b118e4641da49599e9aac8edd8.

    References: https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55368.json, https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf, https://nvd.nist.gov/vuln/detail/CVE-2026-55368, https://github.com/h2o/h2o/commit/48b7ae12107fd2b118e4641da49599e9aac8edd8, https://github.com/h2o/h2o/pull/3757

    Affected packages

    Package

    Name:

    Purl:

    Affected ranges

    Type: GIT

    Events:

    Introduced- 0
    Fixed -48b7ae12107fd2b118e4641da49599e9aac8edd8

    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-2026-55368 | CVE-DB