VulnerabilityModified

CVE-2026-39821

This behavior can lead to privilege escalation in programs using the idna package.

CRITICAL 9.6EPSS 0.69%

Does this matter?

High impact if exploited, but EPSS currently rates exploitation as unlikely (0.69%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.

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".

CVSS 3.1: 9.6 CRITICALCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
EPSS: 0.69% probability · 51th percentile
CISA KEV: Not listed
Weakness: CWE-1289
Affected: golang/net
Source: security@golang.org

References

Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.