---
cve: "CVE-2026-39821"
severity: "CRITICAL"
cvss: 9.6
epss: "69%"
vendor: "Go standard library"
kev: false
exploited: false
published: "2026-05-22 16:16:20"
tags: [cve, security, critical]
source: tsecurity.de CVE-Dossier
exported: "2026-09-03T05:38:29+02:00"
---

# CVE-2026-39821

> 9.6 CRITICAL

## Beschreibung

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-Vektor

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

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Netzwerk | bad |
| AC Komplexität | Gering | bad |
| PR Privilegien | Gering | warn |
| UI Interaktion | Keine | bad |
| S Scope | Verändert | bad |
| C Vertraulichkeit | Hoch | bad |
| I Integrität | Hoch | bad |
| A Verfügbarkeit | Keine | good |

## Patch verfügbar (OSV)

- 7770ec48d03fec35e378665337b4faca93c38423 (Commit)

## Referenzen

- <https://go.dev/cl/767220>
- <https://go.dev/issue/78760>
- <https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8>
- <https://groups.google.com/g/golang-announce/c/94pEornpRlI>
- <https://pkg.go.dev/vuln/GO-2026-5026>
- <https://access.redhat.com/errata/RHSA-2026:23262>
- <https://access.redhat.com/errata/RHSA-2026:23264>
- <https://access.redhat.com/errata/RHSA-2026:26546>
- <https://access.redhat.com/errata/RHSA-2026:26547>
- <https://access.redhat.com/errata/RHSA-2026:30650>
- <https://access.redhat.com/errata/RHSA-2026:30651>
- <https://access.redhat.com/errata/RHSA-2026:30853>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-39821) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_