CVE-2026-55558 aiosmtplib: STARTTLS response injection
An active network attacker can inject SMTP responses during STARTTLS negotiation, desynchronizing the protocol state and potentially exposing or modifying data.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55558?
CVE-2026-55558 is a medium-severity vulnerability in cole aiosmtplib, published on 20 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-74
Which versions are affected by CVE-2026-55558?
3 open source packages ship an affected version, including aiosmtplib.
| Package | Affected | Fixed in |
|---|---|---|
aiosmtplib PyPI | All versions | 5.1.2 |
aiosmtplib PyPI | All versions | 5.1.2 |
aiosmtplib PyPI | All versions | 5.1.2 |
Linux distribution packages (1)
python-aiosmtplib openSUSE:Tumbleweed | All versions | 5.1.2-1.1 |
How do I fix CVE-2026-55558?
Upgrade aiosmtplib to 5.1.2 or later.
Is CVE-2026-55558 being exploited?
There is no known exploitation of CVE-2026-55558 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-DL97476, CLEANSTART-2026-DR99316, DEBIAN-CVE-2026-55558, ECHO-c2e7-d06a-81e1, GHSA-vxj7-4xrp-5vr4, MINI-46rm-9j23-2r54 and 4 more.