CVE-2026-39821 — CRITICAL (9.6)

Q: How severe is CVE-2026-39821?

CVE-2026-39821 has been rated CRITICAL with a CVSS base score of 9.6/10. This is considered a critical vulnerability requiring immediate attention.

Q: Is there a patch for CVE-2026-39821?

Check the references section above for vendor advisories and patch information. Affected products include: Golang Net.

Vulnerability 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".

CVSS Score

9.6

CRITICAL

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

Affected Products

Vendor	Product	Versions
Golang	Net	< 0.55.0

Related Weaknesses (CWE)

CWE-1289

References

FAQ

What is CVE-2026-39821?

CVE-2026-39821 is a vulnerability with a CVSS score of 9.6 (CRITICAL). 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"...

How severe is CVE-2026-39821?

CVE-2026-39821 has been rated CRITICAL with a CVSS base score of 9.6/10. This is considered a critical vulnerability requiring immediate attention.

Is there a patch for CVE-2026-39821?

Check the references section above for vendor advisories and patch information. Affected products include: Golang Net.