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

Description (CNA)

(*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 (CNA)

Vendor / product	Versions
Go standard library / crypto/x509	affected 0 before 1.25.11; affected 1.26.0-0 before 1.26.4

CWE

No CWE identifiers supplied.

References

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.

redhat-SADP

A flaw was found in the `crypto/x509` package of `golang`. This vulnerability allows a remote attacker to cause a Denial of Service (DoS) by presenting a specially crafted X.509 certificate with a large number of DNS Subject Alternative Name (SAN) entries. The certificate verification process, specifically the `VerifyHostname` function, incurs excessive computational overhead due to repeated string operations when processing these entries. This can lead to a significant performance degradation or unresponsiveness of systems validating such certificates.