# 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.

- **Severity:** Medium 6.9 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Aug 2026
- **Updated:** 19 Aug 2026
- **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, PYSEC-2026-2338, UBUNTU-CVE-2026-53533, openSUSE-SU-2026:11035-1

## 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.

Weakness: CWE-93 (Improper Neutralization of CRLF Sequences), CWE-77 (Improper Neutralization of Special Elements used in a Command)

## Which versions are affected by CVE-2026-53533?

3 open source packages ship an affected version, including aiosmtplib.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `aiosmtplib` | PyPI | All versions | 5.1.1 |
| `aiosmtplib` | PyPI | All versions | 5.1.1 |
| `aiosmtplib` | PyPI | All versions | 5.1.1 |
| `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.

## Is CVE-2026-53533 being exploited?

There is no known exploitation of CVE-2026-53533 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-53533 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-53533 in the wild and no public exploit code.

Affected packages and fixed versions:
- aiosmtplib (PyPI): affected all versions; fixed in 5.1.1
- aiosmtplib (PyPI): affected all versions; fixed in 5.1.1
- aiosmtplib (PyPI): affected all versions; fixed in 5.1.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-53533/ (Markdown: https://insights.sbomify.com/CVE-2026-53533/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-53533
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-53533
- OSV: https://osv.dev/vulnerability/CVE-2026-53533

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-53533/
Summaries are generated and may contain mistakes; the official description above is the record.