Arbitrary Code Injection Affecting aiosmtplib package, versions <5.1.2-1
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-DEBIAN14-AIOSMTPLIB-19166041
- published22 Aug 2026
- disclosed20 Aug 2026
Introduced: 20 Aug 2026
NewCVE-2026-55558 (opens in a new tab)How to fix?
Upgrade Debian:14 aiosmtplib to version 5.1.2-1 or higher.
NVD Description
Note: Versions mentioned in the description apply only to the upstream aiosmtplib package and not the aiosmtplib package as distributed by Debian.
See How to fix? for Debian:14 relevant fixed versions and status.
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.