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Critical severity9.6NVD Advisory· Published May 22, 2026· Updated Aug 21, 2026

CVE-2026-39821

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".

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Affected products

4813

Patches

Vulnerability mechanics

References

127

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