{"id":"CVE-2026-39821","url":"https://www.cyber-defence.io/tools/cve/CVE-2026-39821","generatedAt":"2026-10-28T00:17:03.374Z","title":"This behavior can lead to privilege escalation in programs using the idna package.","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\".","published":"2026-05-22T16:16:20.000Z","lastModified":"2026-09-17T12:18:06.000Z","status":"Modified","sourceIdentifier":"security@golang.org","cvss":{"version":"3.1","score":9.6,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N"},"cwe":["CWE-1289"],"affected":["golang/net"],"epss":{"score":0.01834,"percentile":0.78762,"date":"2026-10-26","history":[{"date":"2026-10-25","score":0.01834,"percentile":0.78763}]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"high","text":"High impact if exploited, but EPSS currently rates exploitation as unlikely (1.83%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit."},"changes":[],"references":[{"url":"https://go.dev/cl/767220","tags":["Issue Tracking"],"source":"security@golang.org"},{"url":"https://go.dev/issue/78760","tags":["Issue Tracking"],"source":"security@golang.org"},{"url":"https://groups.google.com/g/golang-announce/c/94pEornpRlI","source":"security@golang.org"},{"url":"https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8","tags":["Mailing List"],"source":"security@golang.org"},{"url":"https://pkg.go.dev/vuln/GO-2026-5026","tags":["Vendor 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BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}