CVE-2026-55558
CVE-2026-55558 updated by NVD
Summary
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.
References
Includes data from the National Vulnerability Database (NIST). NVD data is in the public domain; this page is not endorsed by NIST.
CVE® is a registered trademark of The MITRE Corporation. CVE content reproduced under the CVE Terms of Use; copyright designation © MITRE.