{"id":"DEBIAN-CVE-2026-55558","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.","modified":"2026-09-19T22:47:39.346570314Z","published":"2026-08-20T15:17:31.980Z","upstream":["CVE-2026-55558"],"references":[{"type":"ADVISORY","url":"https://security-tracker.debian.org/tracker/CVE-2026-55558"}],"affected":[{"package":{"name":"aiosmtplib","ecosystem":"Debian:12","purl":"pkg:deb/debian/aiosmtplib?arch=source&distro=bookworm"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.0.0-1","3.0.1-1","3.0.2-1","3.0.2-2","4.0.0-1","4.0.1-1","4.0.1-1~exp1","4.0.2-1","5.0.0-1","5.1.0-1","5.1.0-2","5.1.0-3","5.1.2-1","5.1.3-1"],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/debian-osv/debian-cve-osv/DEBIAN-CVE-2026-55558.json"}},{"package":{"name":"aiosmtplib","ecosystem":"Debian:13","purl":"pkg:deb/debian/aiosmtplib?arch=source&distro=trixie"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["4.0.0-1","4.0.1-1","4.0.1-1~exp1","4.0.2-1","5.0.0-1","5.1.0-1","5.1.0-2","5.1.0-3","5.1.2-1","5.1.3-1"],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/debian-osv/debian-cve-osv/DEBIAN-CVE-2026-55558.json"}},{"package":{"name":"aiosmtplib","ecosystem":"Debian:14","purl":"pkg:deb/debian/aiosmtplib?arch=source&distro=forky"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.1.2-1"}]}],"versions":["4.0.0-1","4.0.1-1","4.0.1-1~exp1","4.0.2-1","5.0.0-1","5.1.0-1","5.1.0-2","5.1.0-3"],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/debian-osv/debian-cve-osv/DEBIAN-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"}]}