CVE-2026-27145

MEDIUM 6.5

Inefficient candidate hostname parsing in crypto/x509

CVSS Score

6.5

EPSS Score

0.0%

EPSS Percentile

0th

(*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.

Vendor	go standard library
Product	crypto/x509
Published	Jun 2, 2026
Last Updated	Jun 4, 2026

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Affected Versions

Go standard library / crypto/x509

0 < 1.25.11 1.26.0-0 < 1.26.4

References

NVD ↗ CVE.org ↗ EPSS Data ↗

go.dev: https://go.dev/cl/783621 go.dev: https://go.dev/issue/79694 groups.google.com: https://groups.google.com/g/golang-announce/c/tKs3rmcBcKw pkg.go.dev: https://pkg.go.dev/vuln/GO-2026-5037

Credits

Jakub Ciolek - https://ciolek.dev/