Skip to content
Log inStart free trial

GO-2026-5026 CVE-2026-39821

Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna

Critical Go
Severity
9.6 / 10
Critical · CVSS 3.1
Exploit chance (EPSS)
1.83%
Chance of exploitation in the next 30 days. Higher than 78.92% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-1289 Improper Validation of Unsafe Equivalence in Input
Fix
Available
2 packages have a fixed version

Description

The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only label. For example, ToUnicode("xn--example-.com") incorrectly returns the name "example.com" rather than an error.

This behavior can lead to privilege escalation in programs using the idna package. For example, a program which performs privilege checks on the ASCII hostname may reject "example.com" but permit "xn--example-.com". If that program subsequently converts the ASCII hostname to Unicode, it will inadvertently permits access to the Unicode name "example.com".

Affected versions

Go

PackageVulnerableFixed in
golang.org/x/net
< 0.55.0
0.55.0
stdlib
< 1.25.13
>= 1.26.0-0, < 1.26.6
>= 1.27.0-0, < 1.27.0-rc.3
1.25.13
1.26.6
1.27.0-rc.3

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.

golang.org/x/net Go → 0.55.0

go
go get golang.org/x/net@v0.55.0 && go mod tidy

stdlib Go → 1.27.0-rc.3

go
go get stdlib@v1.27.0-rc.3 && go mod tidy

Severity breakdown

CVSS 3.1 9.6

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

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
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.

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 golang.org/x/net
GHSA-qppj-fm5r-hxr3
HTTP/2 Stream Cancellation Attack
Medium Known exploited Also affects golang.org/x/net
CVE-2020-0601
Certificate validation bypass on Windows in crypto/x509
High Known exploited Also affects stdlib
GO-2026-6866
Check all ancestor namespace transitions when rendering raw text in html in golang.org/x/net
Not rated yet Also affects golang.org/x/net
GO-2026-6860
Avoid quadratic complexity when coalescing text nodes in html in golang.org/x/net
Not rated yet Also affects golang.org/x/net
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

Credits: KC1zs4 (https://github.com/KC1zs4), Longwei Wang (https://github.com/gn00295120)

Is this in your code?

MergeHarbor checks every repository's dependencies against this database, ranks what matters by exploitability and reachability, routes it to the owning team and opens the fix pull request.

Check your repos with MergeHarbor