{"id":"CVE-2026-55558","summary":"aiosmtplib: STARTTLS response injection","details":"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.","aliases":["GHSA-vxj7-4xrp-5vr4"],"modified":"2026-08-22T18:50:01.620980020Z","published":"2026-08-20T14:29:58.580Z","related":["openSUSE-SU-2026:11569-1"],"database_specific":{"cna_assigner":"GitHub_M","cwe_ids":["CWE-74"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55558.json"},"references":[{"type":"WEB","url":"https://github.com/cole/aiosmtplib/releases/tag/v5.1.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55558.json"},{"type":"ADVISORY","url":"https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55558"},{"type":"FIX","url":"https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/cole/aiosmtplib","events":[{"introduced":"0"},{"fixed":"9fab7ba1361dbf7622ede1315a24be805cff09c9"},{"fixed":"bffc6d19fbb04a84284c8c12ef640fbae0641a9c"}],"database_specific":{"extracted_events":[{"introduced":"0"},{"fixed":"5.1.2"}],"source":["AFFECTED_FIELD","REFERENCES"]}}],"versions":["v5.1.1","v5.1.0","v5.0.0","v4.0.2","v4.0.1","v4.0.0","v3.0.2","v3.0.1","v3.0.0","v2.0.1","v2.0.0.post0","v2.0.0","v2.0.0b1","v2.0.0b0","v1.1.7","v1.1.6","v1.1.5","v1.1.5.dev0","v1.1.4","v1.1.3","v1.1.2","v1.1.1","v1.1.0","v1.1.0b0","v1.0.4","v1.0.3","v1.0.2","v1.0.1","v1.0.0","v0.1.7","v0.1.6","v0.1.5","v0.1.4","v0.1.3","v0.1.2","v0.1.1","v0.1"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-55558.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N"}]}