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.
Affected packages
| Ecosystem | Package | Affected versions | Fixed versions |
|---|---|---|---|
| PyPI | aiosmtplib | — | 5.1.2 |
Remediation: Upgrade to 5.1.2 or later.
References
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EPSS scores provided by the FIRST.org Exploit Prediction Scoring System.