CVE-2026-39821
Publication date 22 May 2026
Last updated 7 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
Notes
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
mdeslaur
This issue affects multiple go components: - net/http - net/http/internal/http2 - golang.org/x/net/idna Not only do vendored instances of golang.org/x/net/idna need to be fixed, but the stdlib in golang needs to be fixed first before everything is rebuilt. The fix for this issue possibly introduces a bug with hosts that contain a trailing period. (See regression bug below)
Patch details
| Package | Patch details |
|---|
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
- USN-8883-1
- Go vulnerability
- 6 October 2026
- USN-8900-1
- Go Networking vulnerabilities
- 7 October 2026