Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna_CVE-2026-39821

{
    "lastseen": "",
    "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": "2026-05-22T15:01:21.462Z",
    "modified": "2026-05-22T18:07:38.019Z",
    "type": "cve",
    "title": "Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna",
    "source": "Go",
    "references": "https://go.dev/cl/767220\nhttps://go.dev/issue/78760\nhttps://groups.google.com/g/golang-announce/c/iI-mYSI0lu8\nhttps://pkg.go.dev/vuln/GO-2026-5026",
    "id": "CVE-2026-39821",
    "bulletinFamily": "",
    "cwe": [
        "CWE-1289"
    ],
    "cvelist": null,
    "sourceData": "golang.org/x/net golang.org/x/net/idna 0",
    "sourceHref": "",
    "cvss": {
        "score": 10,
        "severity": "CRITICAL",
        "vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N",
        "version": "3.1"
    },
    "cvss2": [],
    "cvss3": {
        "version": "",
        "vectorString": "",
        "baseScore": 0,
        "baseSeverity": "",
        "attackVector": "",
        "attackComplexity": "",
        "privilegesRequired": "",
        "userInteraction": "",
        "scope": "",
        "confidentialityImpact": "",
        "integrityImpact": "",
        "availabilityImpact": "",
        "cvssV3": {
            "version": "",
            "vectorString": "",
            "baseScore": 0,
            "baseSeverity": "",
            "attackVector": "",
            "attackComplexity": "",
            "privilegesRequired": "",
            "userInteraction": "",
            "scope": "",
            "confidentialityImpact": "",
            "integrityImpact": "",
            "availabilityImpact": ""
        }
    },
    "href": "",
    "category_name": "CVE",
    "post_link": "",
    "product": "golang.org/x/net/idna",
    "version": "0",
    "vendor": "golang.org/x/net",
    "ai_description": "Privilege escalation vulnerability due to incorrect handling of Punycode-encoded labels",
    "ai_severity": "Critical",
    "ai_vendor": "Google",
    "ai_product": "idna",
    "ai_version": "0",
    "ai_score": 10
}