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CVE-2026-39821

CRITICALCVSS 9.6/10EPSS 0.66%

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 0.66% 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
9.6/10

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

EPSS Probability
0.66%

48.1th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

VendorProductVersions
GolangNet< 0.55.0

References

Timeline

Published
Last Modified
Status
Modified

Frequently Asked Questions

What is CVE-2026-39821?
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".
How severe is CVE-2026-39821?
CVE-2026-39821 has a CVSS score of 9.6/10 (CRITICAL severity). The EPSS model estimates a 0.66% probability of exploitation in the next 30 days.
How do I fix CVE-2026-39821?
Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

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Source: NVD / NIST