{"id":"UBUNTU-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":"2026-09-02T19:36:26.141416182Z","published":"2026-08-20T15:17:00Z","upstream":["CVE-2026-55558"],"references":[{"type":"REPORT","url":"https://ubuntu.com/security/CVE-2026-55558"},{"type":"REPORT","url":"https://www.cve.org/CVERecord?id=CVE-2026-55558"}],"affected":[{"package":{"name":"aiosmtplib","ecosystem":"Ubuntu:22.04:LTS","purl":"pkg:deb/ubuntu/aiosmtplib?arch=source&distro=jammy"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.1.4-1","1.1.4-2","1.1.6-1"],"ecosystem_specific":{"binaries":[{"binary_name":"python3-aiosmtplib","binary_version":"1.1.6-1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-55558.json"}},{"package":{"name":"aiosmtplib","ecosystem":"Ubuntu:26.04:LTS","purl":"pkg:deb/ubuntu/aiosmtplib?arch=source&distro=resolute"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["4.0.0-1","4.0.2-1","5.0.0-1","5.1.0-1"],"ecosystem_specific":{"binaries":[{"binary_name":"python3-aiosmtplib","binary_version":"5.1.0-1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-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"},{"type":"Ubuntu","score":"medium"}]}