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CVE-2026-27145: Inefficient candidate hostname parsing in crypto/x509

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-06-02 · updated 2026-09-18
• Weakness: CWE-606

Description

(*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.

Affected products

• Go Stdlib: before 1.25.11; from 1.26.0-0 before 1.26.4 — fixed in 1.25.11, 1.26.4
• Go standard library crypto/x509: before 1.25.11; from 1.26.0-0 before 1.26.4 — fixed in 1.25.11, 1.26.4

How to fix

Upgrade to a fixed version: Go Stdlib 1.25.11, 1.26.4; Go standard library crypto/x509 1.25.11, 1.26.4. Check the vendor advisory for mitigations.

References

• https://go.dev/cl/783621
• https://go.dev/issue/79694
• https://groups.google.com/g/golang-announce/c/tKs3rmcBcKw
• https://pkg.go.dev/vuln/GO-2026-5037
• https://access.redhat.com/errata/RHSA-2026:23262
• https://access.redhat.com/errata/RHSA-2026:23264
• https://access.redhat.com/errata/RHSA-2026:29980
• https://access.redhat.com/errata/RHSA-2026:29981
• https://access.redhat.com/errata/RHSA-2026:33574
• https://access.redhat.com/errata/RHSA-2026:34357
• https://access.redhat.com/errata/RHSA-2026:34359
• https://access.redhat.com/errata/RHSA-2026:35832

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