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Vulnerabilities

Vulnerability · CVE-2026-39821

The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only label.

Published May 22, 2026 · catalog updated Oct 16, 2026 · The ToASCII and ToUnicode

Check if you run it

No confirmed exploitation is on record for CVE-2026-39821. EPSS is 0.69%. Act only if a listed version of The ToASCII and ToUnicode is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (The ToASCII and ToUnicode).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Critical9.6
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.69%
Remediation
Advisory link
Affected products
Go golang.org/x/net · Go stdlib · cpe golang/net

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".

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • Go golang.org/x/net0 → 0.55.0
  • Go stdlib0 → 1.25.13
  • cpe golang/netversion unspecified

How to fix CVE-2026-39821

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-39821. EPSS is 0.69%. Act only if a listed version of The ToASCII and ToUnicode is installed.

Which versions are affected?

Compare what you have installed with 0 → 0.55.0, 0 → 1.25.13. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-1289. EPSS 0.69%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. May 22, 2026 · NIST NVD

    Detected

    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 "examp

  2. Sep 17, 2026 · NIST NVD

    Updated

    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 "examp

  3. Oct 16, 2026 · OpenSSF OSV / OSV Scanner

    Fix available

    stdlib fixed in 1.25.13; golang.org/x/net fixed in 0.55.0

  4. Oct 16, 2026 · OpenSSF OSV / OSV Scanner

    Fix available

    stdlib fixed in 1.25.13

  5. Oct 16, 2026 · OpenSSF OSV / OSV Scanner

    Fix available

    golang.org/x/net fixed in 0.55.0

  6. Oct 16, 2026 · OpenSSF OSV / OSV Scanner

    Updated

    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 "examp

Sources

  • CVE.orgVulnerability recordCVE-2026-39821

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-39821

    30-day exploitation probability 0.7%

    first.org
  • Go Vulnerability DatabasePackage advisoryGO-2026-5026

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

    osv.dev
  • NIST NVDVulnerability recordCVE-2026-39821

    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 program

    nvd.nist.gov
  • OpenSSF OSV / OSV ScannerPackage advisoryGO-2026-5026

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

    osv.dev

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.