aiosmtplib: aiosmtplib is an asynchronous SMTP client for use with asyncio
CVE-2026-55558 is fixed in aiosmtplib on 1 platform by DEBIAN-CVE-2026-55558 (version 5.1.2-1), released August 21, 2026. It is not reported as exploited. Patch Radar priority: Routine.
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.
Where it's fixed
| Product | Fixed in | Fix | Released |
|---|---|---|---|
| Linux | |||
| aiosmtplib · Debian 14 | 5.1.2-1 | DEBIAN-CVE-2026-55558 | Aug 21, 2026 |
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