CVE-2026-53533 aiosmtplib: SMTP command injection via CR/LF in sender/recipient address
Attackers can inject SMTP commands via CR/LF in addresses, potentially executing arbitrary commands or causing denial of service in applications using aiosmtplib.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-53533?
CVE-2026-53533 is a medium-severity command injection vulnerability in cole aiosmtplib, published on 18 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 5.1.1, SMTP.mail(), SMTP.rcpt(), SMTP.vrfy(), and SMTP.expn() send caller-supplied addresses without rejecting embedded CR or LF bytes. Data after the line break is framed as additional standalone SMTP command lines, allowing an attacker who influences an envelope sender or recipient to inject commands such as MAIL FROM, RCPT TO, RSET, DATA, or AUTH. SMTP.sendmail() and SMTP.send() without a Message object pass addresses through the affected methods, while SMTP.send_message() is not affected. Successful injection can desynchronize the command-response pipeline, hang the SMTP instance, or send an arbitrary message without requiring attacker control of the SMTP server. This issue is fixed in version 5.1.1.
Which versions are affected by CVE-2026-53533?
3 open source packages ship an affected version, including aiosmtplib.
| Package | Affected | Fixed in |
|---|---|---|
aiosmtplib PyPI | All versions | 5.1.1 |
aiosmtplib PyPI | All versions | 5.1.1 |
aiosmtplib PyPI | All versions | 5.1.1 |
Linux distribution packages (1)
python-aiosmtplib openSUSE:Tumbleweed | All versions | 5.1.1-1.1 |
How do I fix CVE-2026-53533?
Upgrade aiosmtplib to 5.1.1 or later.
Upgrade, and pass arguments to programs as a list instead of through a shell. More about Command injection.
Is CVE-2026-53533 being exploited?
There is no known exploitation of CVE-2026-53533 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-WE55503, DEBIAN-CVE-2026-53533, ECHO-da4c-abee-fa30, GHSA-v3q9-hj7j-63hq, MINI-27rx-pxm8-jj73, MINI-926w-c692-6w3r and 3 more.