CVE-2026-39821
Last modified
CVE-2026-39821 is a critical-severity vulnerability rated 9.6/10 on the CVSS scale. 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. EPSS estimates a 0.34% chance of exploitation in the next 30 days.
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".
Metrics
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
Weakness Enumeration
Affected Software
| Vendor | Product | Versions |
|---|---|---|
| Golang | Net | < 0.55.0 |
References
- https://go.dev/cl/767220Issue Tracking
- https://go.dev/issue/78760Issue Tracking
- https://pkg.go.dev/vuln/GO-2026-5026Vendor Advisory
Timeline
- Published
- Last Modified
- Status
- Analyzed
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