MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade_CVE-2026-41319

{
    "lastseen": "",
    "description": "MailKit is a cross-platform mail client library built on top of MimeKit. A STARTTLS Response Injection vulnerability in versions prior to 4.16.0 allows a Man-in-the-Middle attacker to inject arbitrary protocol responses across the plaintext-to-TLS trust boundary, enabling SASL authentication mechanism downgrade (e.g., forcing PLAIN instead of SCRAM-SHA-256). The internal read buffer in `SmtpStream`, `ImapStream`, and `Pop3Stream` is not flushed when the underlying stream is replaced with `SslStream` during STARTTLS upgrade, causing pre-TLS attacker-injected data to be processed as trusted post-TLS responses. Version 4.16.0 patches the issue.",
    "published": "2026-04-24T03:07:24.335Z",
    "modified": "2026-04-24T03:07:24.335Z",
    "type": "cve",
    "title": "MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade",
    "source": "GitHub_M",
    "references": "https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr",
    "id": "CVE-2026-41319",
    "bulletinFamily": "",
    "cwe": [
        "CWE-74"
    ],
    "cvelist": null,
    "sourceData": "jstedfast MailKit < 4.16.0",
    "sourceHref": "",
    "cvss": {
        "score": 6.5,
        "severity": "MEDIUM",
        "vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/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": "",
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            "privilegesRequired": "",
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            "confidentialityImpact": "",
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            "availabilityImpact": ""
        }
    },
    "href": "",
    "category_name": "CVE",
    "post_link": "",
    "product": "MailKit",
    "version": "< 4.16.0",
    "vendor": "jstedfast",
    "ai_description": "",
    "ai_severity": "",
    "ai_vendor": "",
    "ai_product": "",
    "ai_version": "",
    "ai_score": 0
}