# CVE-2026-55558: 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.

- Web page: https://patchblox.com/security/cve/CVE-2026-55558
- Priority: Routine
- CVSS: 5.9 (medium)
- EPSS: 0.0040
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Aug 20, 2026

## Description

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

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | aiosmtplib Debian 14 | 5.1.2-1 | [DEBIAN-CVE-2026-55558](https://patchblox.com/security/data/fix/linux/debian-cve-2026-55558.md) | Aug 21, 2026 |

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Data as of 2026-11-08T06:59:42Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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