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".
Predictions
Heuristic predictions, AS-IS, for prioritization only.
Mitigations
No mitigations published for this CVE yet.
The vendor-content worker queues fetches as references arrive (check back in a few minutes). Or — if you've already worked around this in production — publish your fix to the community-verified tier.
✚ Propose a mitigation on Community → Mitigations published via the community go through AI scoring + 2 human reviewers + 7-day silent objection window before landing here withsource_tier=community-verified.
OS impact
Debian Affected 5 releases
| Version | Status | Fixed in |
|---|---|---|
| trixie | Affected | — |
| sid | Affected | — |
| forky | Affected | — |
| bullseye | Affected | — |
| bookworm | Affected | — |
SUSE Affected 1 release
| Version | Status | Fixed in |
|---|---|---|
| — | Affected | — |
Windows Affected 1 release
| Version | Status | Fixed in |
|---|---|---|
| — | Affected | — |
Package impact
| Ecosystem | Package | Vulnerable | Fixed |
|---|---|---|---|
| Go | golang.org/x/net | <0.55.0 | 0.55.0 |
Application impact
| Vendor | Product | Versions | Fixed |
|---|---|---|---|
| golang | net | {"endExcluding":"0.55.0"} | 0.55.0 |
References
- https://go.dev/cl/767220
- https://go.dev/issue/78760
- https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8
- https://pkg.go.dev/vuln/GO-2026-5026
- https://www.suse.com/security/cve/CVE-2026-39821.html
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-39821
- https://security-tracker.debian.org/tracker/CVE-2026-39821
CWEs
CWE-1289: CWE-1289
Community-verified mitigations for this CVE will appear above when contributors publish them.
Verify integrity in audit chain (admin only). AS-IS.