CVE-2026-39821
CRITICALDescription
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 to fix
This vulnerability affects an unusually large number of packages. The highest-confidence fixes are shown above; the full list is longer. Check the referenced advisories for complete coverage.
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N/E:U
Exploit Intelligence
Moderate risk: more likely to be exploited than 48% of all known CVEs.
References
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Related Vulnerabilities
Other CWE-1289 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2024-45179 | High | 7.2 | 2.6% | - | - |
| CVE-2026-48710 | Medium | 6.5 | 1.8% | - | Fix |
| CVE-2026-47729 | Medium | 6.5 | 1.5% | - | Fix |
| CVE-2025-62718 | Critical | 9.9 | 1.2% | - | Fix |
| CVE-2026-33515 | Medium | 5.3 | 1.0% | - | Fix |
| CVE-2022-0675 | Medium | 5.6 | 0.9% | - | Fix |
Common questions
How do I fix CVE-2026-39821?
Published advisories record a fix for 423 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.
Is CVE-2026-39821 being actively exploited?
Not that we know of. CVE-2026-39821 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.66% is the estimated probability that it will be exploited in the next 30 days. That is higher than 48% of all scored CVEs.
How severe is CVE-2026-39821?
CVE-2026-39821 has a CVSS v3 base score of 10.0, rated critical. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-39821 affect?
Published advisories record a fix for golang-golang-x-net (Debian), aap-metrics-utility (Rocky), aap-metrics-utility (Red Hat / RHEL), ansible-automation-platform-installer (Red Hat / RHEL), and 419 more. The list on this page is capped, so there may be more.
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Markdown
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-08-10.