---
id: CVE-2026-53533
title: aiosmtplib is an asynchronous SMTP client for use with asyncio
summary: >-
  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…
severity: medium
cwe:
  - CWE-77
  - CWE-93
vendor: aiosmtplib
product: aiosmtplib
affected:
  - aiosmtplib < 5.1.1
patched:
  - aiosmtplib 5.1.1
published: '2026-08-18'
updated: '2026-09-18'
sourceUpdated: '2026-09-18T20:09:01.757'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-53533'
references:
  - url: >-
      https://github.com/cole/aiosmtplib/commit/8eaf6efc9a8f59e2e09d3ef11246a058c46bd3ba
    label: security-advisories@github.com
  - url: 'https://github.com/cole/aiosmtplib/releases/tag/v5.1.1'
    label: security-advisories@github.com
  - url: 'https://github.com/cole/aiosmtplib/security/advisories/GHSA-v3q9-hj7j-63hq'
    label: security-advisories@github.com
  - url: 'https://github.com/cole/aiosmtplib'
  - url: 'https://github.com/advisories/GHSA-v3q9-hj7j-63hq'
tags:
  - nvd
  - osv
  - pip
  - ghsa
epss: 0.00385
epssPercentile: 0.32478
aliases:
  - GHSA-v3q9-hj7j-63hq
  - PYSEC-2026-2338
ecosystem: pip
ingestedAt: '2026-07-07T23:43:59.370Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

## Package advisory (CVE-2026-53533)

Affected packages:

- `aiosmtplib < 5.1.1`

Patched in:

- `aiosmtplib 5.1.1`

Source: https://osv.dev/vulnerability/GHSA-v3q9-hj7j-63hq