{"id":"CVE-2026-41319","state":"PUBLISHED","title":"MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade","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.","publishedAt":"2026-04-24T03:07:24.335Z","updatedAt":"2026-04-25T01:46:35.002Z","severity":{"score":6.5,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N","source":"GitHub_M"},"assigner":"GitHub_M","products":["jstedfast MailKit"],"affected":[{"vendor":"jstedfast","product":"MailKit","versions":[{"version":"< 4.16.0","status":"affected"}]}],"references":["https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr"],"weaknesses":["CWE-74"],"ingestedAt":"2026-10-29T20:11:57.932Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/41xxx/CVE-2026-41319.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-41319","state":"PUBLISHED","datePublished":"2026-04-24T03:07:24.335Z","dateUpdated":"2026-04-25T01:46:35.002Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-04-20T14:01:46.671Z"},"containers":{"cna":{"title":"MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"jstedfast","product":"MailKit","versions":[{"version":"< 4.16.0","status":"affected"}]}],"references":[{"url":"https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr","name":"https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr","tags":["x_refsource_CONFIRM"]}],"metrics":[{"cvssV3_1":{"attackComplexity":"LOW","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":6.5,"baseSeverity":"MEDIUM","confidentialityImpact":"NONE","integrityImpact":"HIGH","privilegesRequired":"NONE","scope":"UNCHANGED","userInteraction":"REQUIRED","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N","version":"3.1"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-74","lang":"en","description":"CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')","type":"CWE"}]}],"providerMetadata":{"shortName":"GitHub_M","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateUpdated":"2026-04-24T03:07:24.335Z"},"source":{"advisory":"GHSA-9j88-vvj5-vhgr","discovery":"UNKNOWN"}},"adp":[{"title":"CISA ADP Vulnrichment","references":[{"url":"https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr","tags":["exploit"]}],"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-04-25T01:46:14.462850Z","id":"CVE-2026-41319","options":[{"Exploitation":"poc"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-04-25T01:46:35.002Z"}}]}}}