{"id":"CVE-2026-55558","state":"PUBLISHED","title":"aiosmtplib: STARTTLS response injection","description":"aiosmtplib is an asynchronous SMTP client for use with asyncio. Prior to 5.1.2, SMTPProtocol.start_tls in src/aiosmtplib/protocol.py consumes the server's 220 response and starts the TLS handshake without clearing SMTPProtocol._buffer. An active network attacker can place attacker-chosen SMTP response lines after the plaintext 220 response in the same network segment. The method then calls loop.start_tls; those bytes survive the transport upgrade and are parsed as the first response from inside the TLS session, desynchronizing subsequent SMTP command and response pairs. Connections using start_tls=True or opportunistic STARTTLS are affected, while connections using use_tls=True are not. This issue is fixed in version 5.1.2.","publishedAt":"2026-08-20T14:29:58.580Z","updatedAt":"2026-08-20T20:00:21.869Z","severity":{"score":5.9,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N","source":"GitHub_M"},"assigner":"GitHub_M","products":["cole aiosmtplib"],"affected":[{"vendor":"cole","product":"aiosmtplib","versions":[{"version":"< 5.1.2","status":"affected"}]}],"references":["https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4","https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9","https://github.com/cole/aiosmtplib/releases/tag/v5.1.2"],"weaknesses":["CWE-74"],"ingestedAt":"2026-10-29T20:12:24.861Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/55xxx/CVE-2026-55558.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-55558","state":"PUBLISHED","datePublished":"2026-08-20T14:29:58.580Z","dateUpdated":"2026-08-20T20:00:21.869Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-06-16T23:11:20.213Z"},"containers":{"cna":{"title":"aiosmtplib: STARTTLS response injection","descriptions":[{"lang":"en","value":"aiosmtplib is an asynchronous SMTP client for use with asyncio. Prior to 5.1.2, SMTPProtocol.start_tls in src/aiosmtplib/protocol.py consumes the server's 220 response and starts the TLS handshake without clearing SMTPProtocol._buffer. An active network attacker can place attacker-chosen SMTP response lines after the plaintext 220 response in the same network segment. The method then calls loop.start_tls; those bytes survive the transport upgrade and are parsed as the first response from inside the TLS session, desynchronizing subsequent SMTP command and response pairs. Connections using start_tls=True or opportunistic STARTTLS are affected, while connections using use_tls=True are not. This issue is fixed in version 5.1.2."}],"affected":[{"vendor":"cole","product":"aiosmtplib","versions":[{"version":"< 5.1.2","status":"affected"}]}],"references":[{"url":"https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4","name":"https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4","tags":["x_refsource_CONFIRM"]},{"url":"https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9","name":"https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9","tags":["x_refsource_MISC"]},{"url":"https://github.com/cole/aiosmtplib/releases/tag/v5.1.2","name":"https://github.com/cole/aiosmtplib/releases/tag/v5.1.2","tags":["x_refsource_MISC"]}],"metrics":[{"cvssV3_1":{"attackComplexity":"HIGH","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":5.9,"baseSeverity":"MEDIUM","confidentialityImpact":"NONE","integrityImpact":"HIGH","privilegesRequired":"NONE","scope":"UNCHANGED","userInteraction":"NONE","vectorString":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/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":"2026-08-20T14:29:58.580Z"},"source":{"advisory":"GHSA-vxj7-4xrp-5vr4","discovery":"UNKNOWN"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-08-20T20:00:16.093023Z","id":"CVE-2026-55558","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-08-20T20:00:21.869Z"}}]}}}