{"cve":"CVE-2026-39821","nvd":{"published":"2026-05-22T16:16:20.410","modified":"2026-09-17T12:18:05.767","status":"Modified","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\".","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N","score":9.6},"cwes":["CWE-1289"],"references":["https://go.dev/cl/767220","https://go.dev/issue/78760","https://groups.google.com/g/golang-announce/c/94pEornpRlI","https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8","https://pkg.go.dev/vuln/GO-2026-5026"],"products":["golang:net"],"source":"nvd","synced_at":"2026-10-19T22:36:28.938604137+00:00"},"epss":{"cve":"CVE-2026-39821","score":0.01834,"percentile":0.78915,"date":"2026-11-01"},"kev":null,"osv":[{"id":"GO-2026-5026","url":"https://osv.dev/vulnerability/GO-2026-5026","summary":"Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna","modified":"2026-10-16T10:42:05.164302476Z","packages":[{"ecosystem":"Go","name":"stdlib","purl":"pkg:golang/stdlib","ranges":["<1.25.13",">=1.26.0-0 <1.26.6",">=1.27.0-0 <1.27.0-rc.3"],"fixed":["1.25.13","1.26.6","1.27.0-rc.3"]},{"ecosystem":"Go","name":"golang.org/x/net","purl":"pkg:golang/golang.org/x/net","ranges":["<0.55.0"],"fixed":["0.55.0"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}