CVE-2026-39821
Publication date 22 May 2026
Last updated 1 October 2026
Ubuntu priority
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".
Read the notes from the security team
Why is this CVE high priority?
This is a critical issue
Status
| Package | Ubuntu Release | Status |
|---|---|---|
| golang-golang-x-net | 26.04 LTS resolute |
Needs evaluation
|
| 24.04 LTS noble |
Needs evaluation
|
|
| 22.04 LTS jammy |
Needs evaluation
|
|
| 20.04 LTS focal | Not in release | |
| 18.04 LTS bionic | Not in release | |
| 16.04 LTS xenial | Not in release | |
| 14.04 LTS trusty | Not in release | |
| google-guest-agent | 26.04 LTS resolute |
Needs evaluation
|
| 24.04 LTS noble |
Needs evaluation
|
|
| 22.04 LTS jammy |
Needs evaluation
|
|
| 20.04 LTS focal |
Needs evaluation
|
|
| 18.04 LTS bionic |
Needs evaluation
|
|
| 16.04 LTS xenial |
Needs evaluation
|
|
| 14.04 LTS trusty | Not in release | |
| containerd | 26.04 LTS resolute |
Needs evaluation
|
| 24.04 LTS noble |
Needs evaluation
|
|
| 22.04 LTS jammy |
Needs evaluation
|
|
| 20.04 LTS focal |
Needs evaluation
|
|
| 18.04 LTS bionic |
Needs evaluation
|
|
| 16.04 LTS xenial |
Needs evaluation
|
|
| 14.04 LTS trusty | Ignored end of standard support | |
| adsys | 26.04 LTS resolute |
Needs evaluation
|
| 24.04 LTS noble |
Needs evaluation
|
|
| 22.04 LTS jammy |
Needs evaluation
|
|
| 20.04 LTS focal |
Needs evaluation
|
|
| juju-core | 16.04 LTS xenial |
Needs evaluation
|
| lxd | 20.04 LTS focal |
Needs evaluation
|
| 18.04 LTS bionic |
Needs evaluation
|
|
| 16.04 LTS xenial |
Needs evaluation
|
|
| golang-golang-x-net-dev | 26.04 LTS resolute | Not in release |
| 24.04 LTS noble | Not in release | |
| 22.04 LTS jammy | Not in release | |
| 20.04 LTS focal |
Fixed 1:0.0+git20190811.74dc4d7+dfsg-1ubuntu0.1~esm3
|
|
| 18.04 LTS bionic |
Fixed 1:0.0+git20170629.c81e7f2+dfsg-2ubuntu0.1~esm3
|
|
| 16.04 LTS xenial |
Not affected
|
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octagalland
google-guest-agent, adsys, juju-core and lxd contain a vendored copy of golang-golang-x-net
mdeslaur
containerd contains a vendored copy of golang-golang-x-net
Severity score breakdown
CVSS version: CVSS v3.0
Base score
9.6 · Critical
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
References
Related Ubuntu Security Notices (USN)
- USN-8416-1
- Go Networking vulnerability
- 9 June 2026