GO-2026-5037 BIT-golang-2026-27145 CVE-2026-27145

Inefficient candidate hostname parsing in crypto/x509

Medium Go

Severity: 6.5 / 10; Medium · CVSS 3.1
Exploit chance (EPSS): 0.59%; Chance of exploitation in the next 30 days. Higher than 48.26% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-606 Unchecked Input for Loop Condition
Fix: 1.26.4; Upgrade stdlib to this version or later

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 versions

Go

Package	Vulnerable	Fixed in
stdlib	`< 1.25.11` `>= 1.26.0-0, < 1.26.4`	`1.25.11` `1.26.4`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

stdlib Go → `1.26.4`

go

go get stdlib@v1.26.4 && go mod tidy

Severity breakdown

CVSS 3.1 6.5

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H

Attack vector: Network
Attack complexity: High
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: Low
Availability: High

Weakness

CWE-606: Unchecked Input for Loop Condition

The product does not properly check inputs that are used for loop conditions, potentially leading to a denial of service or other consequences because of excessive looping.

How to prevent it

Implementation: Do not use user-controlled data for loop conditions.
Implementation: Perform input validation.

References

Fixes and patches

go.dev/cl/783621

Issue reports

go.dev/issue/79694

Timeline

Jun 2, 2026 Published
Oct 2, 2026 Last updated by the source
Oct 31, 2026 Latest EPSS score

Advisory	Severity	Why it's related
GHSA-4374-p667-p6c8 HTTP/2 rapid reset can cause excessive work in net/http	High Known exploited	Also affects stdlib
CVE-2020-0601 Certificate validation bypass on Windows in crypto/x509	High Known exploited	Also affects stdlib
RUSTSEC-2026-0382 Distrust compromised intermediate certificates in crypto/x509	Medium	Also affects stdlib
GO-2026-6613 HTTP/1 server connection desynchronization after 2xx CONNECT response in net/http	High	Also affects stdlib
GO-2026-6609 Lack of limit on size of parsed Range headers in net/http	High	Also affects stdlib
GO-2026-6608 Memory limit bypass when parsing MIME headers in net/textproto, mime/multipart	High	Also affects stdlib
GO-2026-6607 Reject malformed ECH outer extension references in crypto/tls	High	Also affects stdlib
GO-2026-6604 Root.Mkdir(All) can follow junctions out of the root on Windows in os	Critical	Also affects stdlib

Credits: Jakub Ciolek - https://ciolek.dev/

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