CVE-2026-39821 | Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna

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".

Published: 2026-05-22 Last update: 2026-05-29 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-39821 is rated Moderate Risk (44/100): CVSS Critical severity, with low exploitation likelihood (EPSS 0.05%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-39821

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-05-28 0.04% 0.05% +0.01%
2 2026-05-23 — 0.04% —

Full EPSS history (2 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-39821

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
9.6 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N)
Service keeps running; no real outage angle.
 3.1 5.8 134c704f-9b21-4f2e-91b3-4a467353bcc0

Weakness enumeration for CVE-2026-39821

GitHub Security Advisory for CVE-2026-39821

GHSA-w2q5-6q6x-x959 · Severity: critical — The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an...

OS Trackers for CVE-2026-39821

vendor priority summary link
debian not yet assigned CVE-2026-39821 not yet assigned priority: Debian including 1 source packages (golang-golang-x-net), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): open 3, resolved 2. https://security-tracker.debian.org/tracker/CVE-2026-39821
suse high — https://www.suse.com/security/cve/CVE-2026-39821/
ubuntu medium CVE-2026-39821 medium priority: Ubuntu including 1 source packages (golang-golang-x-net-dev), 8 status rows across 8 suites (bionic, focal, jammy, noble, questing, resolute, upstream, xenial): DNE 4, not-affected 2, released 2. https://ubuntu.com/security/CVE-2026-39821

Affected software / configurations for CVE-2026-39821

Vendor Product Version Raw CPE
golang net < 0.55.0 cpe:2.3:a:golang:net:*:*:*:*:*:go:*:*

References for CVE-2026-39821

cvelogic Threat Intelligence