{"id":"CVE-2026-55558","aliases":["GHSA-vxj7-4xrp-5vr4"],"url":"https://o3.security/vulnerability/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.","published":"2026-08-20T14:29:58.580Z","modified":"2026-08-22T18:50:01.620980020Z","cvss":{"score":5.9,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"PyPI","name":"aiosmtplib","fixedVersion":"5.1.2"}],"fix":{"url":"https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9","label":"cole/aiosmtplib@9fab7ba"},"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"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-22T18:50:01.620980020Z"}}