CVE-2026-39821The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only label.
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No exploitation reported and no EPSS estimate yet.
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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
- 9.6 CRITICAL (v3.1, NVD)CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
- Weakness (CWE)
- CWE-1289
- Known exploited
- Not in CISA's KEV catalog
- EPSS (exploitation in the next 30 days)
- Not loaded for this CVE
- Assigned by
- Not given
- NVD analysis
- Modified
- Record last updated
- Sep 17, 2026
- First seen by InfoSec Signals
- Oct 9, 2026
Affected products
golang net golang:net | before 0.55.0 | NVD |
References
- https://go.dev/cl/767220issue-tracking
- https://go.dev/issue/78760issue-tracking
- https://groups.google.com/g/golang-announce/c/94pEornpRlI
- https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8mailing-list
- https://pkg.go.dev/vuln/GO-2026-5026vendor-advisory
- https://access.redhat.com/errata/RHSA-2026:23262
- https://access.redhat.com/errata/RHSA-2026:23264
- https://access.redhat.com/errata/RHSA-2026:26546
- https://access.redhat.com/errata/RHSA-2026:26547
- https://access.redhat.com/errata/RHSA-2026:30650
- https://access.redhat.com/errata/RHSA-2026:30651
- https://access.redhat.com/errata/RHSA-2026:30853
- https://access.redhat.com/errata/RHSA-2026:30854
- https://access.redhat.com/errata/RHSA-2026:30855
- https://access.redhat.com/errata/RHSA-2026:33155
- https://access.redhat.com/errata/RHSA-2026:33160
- https://access.redhat.com/errata/RHSA-2026:33163
- https://access.redhat.com/errata/RHSA-2026:33173
- https://access.redhat.com/errata/RHSA-2026:33183
- https://access.redhat.com/errata/RHSA-2026:33524
- https://access.redhat.com/errata/RHSA-2026:33531
- https://access.redhat.com/errata/RHSA-2026:34342
- https://access.redhat.com/errata/RHSA-2026:34357
- https://access.redhat.com/errata/RHSA-2026:34359
- https://access.redhat.com/errata/RHSA-2026:34364
The CVE record on cve.org NVD's entry
The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.