CVE-2026-39821
CriticalCVSS 9.6Exploitation Probability (EPSS)
Low risk49th percentile - higher than 49% of all known CVEs
Summary
The ToASCII and ToUnicode functions in the idna library incorrectly accept Punycode-encoded labels that decode to an ASCII-only label, which can lead to privilege escalation in programs using this library.
Risk Assessment
An attacker can bypass access controls based on hostnames, gaining unauthorized access to resources, posing a threat to application security.
Recommendation
Update the idna library to a version that correctly rejects such labels and implement additional hostname validation.
Original NVD description (English source)
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".