| Name | CVE-2026-39821 |
| 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". |
| Source | CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more) |
Vulnerable and fixed packages
The table below lists information on source packages.
| Source Package | Release | Version | Status |
|---|
| golang-golang-x-net (PTS) | bullseye | 1:0.0+git20210119.5f4716e+dfsg-4 | vulnerable |
| bookworm | 1:0.7.0+dfsg-1 | vulnerable |
| trixie | 1:0.27.0-2 | vulnerable |
| forky | 1:0.56.0-1 | fixed |
| sid | 1:0.57.0-1 | fixed |
The information below is based on the following data on fixed versions.
| Package | Type | Release | Fixed Version | Urgency | Origin | Debian Bugs |
|---|
| golang-golang-x-net | source | (unstable) | 1:0.55.0-1 | | | |
Notes
[trixie] - golang-golang-x-net <no-dsa> (Minor issue)
[bookworm] - golang-golang-x-net <postponed> (Limited support, minor issue; IDNA Punycode validation)
[bullseye] - golang-golang-x-net <postponed> (Limited support, minor issue)
https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8
https://github.com/golang/go/issues/78760
https://github.com/golang/net/commit/8c4c965e028475082408749b50ed7a686df0d265 (v0.54.0)