--- id: CVE-2026-27145 title: >- (*x509.Certificate).VerifyHostname previously called matchHostnames in a loop over all DNS Subject Alternative Name (SAN) entries summary: >- (*x509.Certificate).VerifyHostname previously called matchHostnames in a loop over all DNS Subject Alternative Name (SAN) entries. This caused strings.Split(host, ".") to execute repeatedly on the same input hostname. With a large DNS SA… severity: medium cvss: 6.5 cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H' cwe: - CWE-606 - CWE-407 vendor: Go standard library product: crypto/x509 affected: - crypto/x509 < 1.25.11 - crypto/x509 >= 1.26.0-0 < 1.26.4 patched: - stdlib 1.26.4 published: '2026-06-02' updated: '2026-09-18' sourceUpdated: '2026-09-18T13:17:43.283' source: NVD sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-27145' references: - url: 'https://go.dev/cl/783621' label: security@golang.org - url: 'https://go.dev/issue/79694' label: security@golang.org - url: 'https://groups.google.com/g/golang-announce/c/tKs3rmcBcKw' label: security@golang.org - url: 'https://pkg.go.dev/vuln/GO-2026-5037' label: security@golang.org - url: 'https://access.redhat.com/errata/RHSA-2026:23262' label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - url: 'https://access.redhat.com/errata/RHSA-2026:23264' label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - url: 'https://access.redhat.com/errata/RHSA-2026:29980' label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - 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url: 'https://access.redhat.com/security/cve/CVE-2026-27145' label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2484207' label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - url: >- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27145.json label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c tags: - nvd - exploit-available - cve.org - osv - go epss: 0.00591 epssPercentile: 0.46688 exploits: github: 1 githubRepos: - 'https://github.com/HORKimhab/CVE-2026-27145' checkedAt: '2026-09-21T15:28:35.101Z' exploitAvailable: true aliases: - GO-2026-5037 - BIT-golang-2026-27145 ecosystem: go ssvc: exploitation: none automatable: 'no' technicalImpact: partial timestamp: '2026-06-04T12:34:03.859208Z' ingestedAt: '2026-07-06T17:44:51.174Z' --- ## Overview (*x509.Certificate).VerifyHostname previously called matchHostnames in a loop over all DNS Subject Alternative Name (SAN) entries. This caused strings.Split(host, ".") to execute repeatedly on the same input hostname. With a large DNS SAN list, verification costs scaled quadratically based on the number of SAN entries multiplied by the hostname's label count. Because x509.Verify validates hostnames before building the certificate chain, this overhead occurred even for untrusted certificates. ## Remediation Refer to the linked advisories for vendor-supplied fixes and affected version ranges. ## Package advisory (CVE-2026-27145) Affected packages: - `stdlib >= 1.26.0-0, < 1.26.4` Patched in: - `stdlib 1.26.4` Source: https://osv.dev/vulnerability/GO-2026-5037