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GO-2026-4866 BIT-golang-2026-33810 CVE-2026-33810

Case-sensitive excludedSubtrees name constraints cause Auth Bypass in crypto/x509

High Go
Severity
8.2 / 10
High · CVSS 3.1
Exploit chance (EPSS)
0.34%
Chance of exploitation in the next 30 days. Higher than 27.51% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-1289 Improper Validation of Unsafe Equivalence in Input
CWE-295 Improper Certificate Validation
Fix
1.26.2
Upgrade stdlib to this version or later

Description

When verifying a certificate chain containing excluded DNS constraints, these constraints are not correctly applied to wildcard DNS SANs which use a different case than the constraint.

This only affects validation of otherwise trusted certificate chains, issued by a root CA in the VerifyOptions.Roots CertPool, or in the system certificate pool.

Affected versions

Go

PackageVulnerableFixed in
stdlib
>= 1.26.0-0, < 1.26.2
1.26.2

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 → 1.26.2

go
go get stdlib@v1.26.2 && go mod tidy

Severity breakdown

CVSS 3.1 8.2

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

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

Weakness

CWE-1289: Improper Validation of Unsafe Equivalence in Input

The product receives an input value that is used as a resource identifier or other type of reference, but it does not validate or incorrectly validates that the input is equivalent to a potentially-unsafe value.

How to prevent it
  • Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

CWE-295: Improper Certificate Validation

The product does not validate, or incorrectly validates, a certificate.

How to prevent it
  • Architecture and Design, Implementation: Certificates should be carefully managed and checked to assure that data are encrypted with the intended owner's public key.
  • Implementation: If certificate pinning is being used, ensure that all relevant properties of the certificate are fully validated before the certificate is pinned, including the hostname.

References

Advisories

Fixes and patches

Issue reports

Other links

Timeline

  1. Published
  2. Last updated by the source
  3. Latest EPSS score
AdvisorySeverityWhy 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: @1seal, Riyas from Saintgits College of Engineering, k1rnt

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