Affected: cole aiosmtplib

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.

Affected: PyPI: aiosmtplib

### Summary

`aiosmtplib`'s `SMTP.mail()`, `SMTP.rcpt()`, `SMTP.vrfy()` and `SMTP.expn()` send the caller-supplied email address to the server without rejecting embedded CR/LF (`\r\n`) bytes. An address that contains a CR/LF is written verbatim onto the SMTP control connection, so the bytes after the CRLF are framed by the server as one or more **additional, standalone SMTP command lines**. A caller that passes an attacker-influenced sender or recipient address into `mail()`/`rcpt()` (or `vrfy()`/`expn()`) therefore allows **SMTP command injection** (CWE-93 / CWE-77): the attacker can smuggle arbitrary SMTP verbs such as `MAIL FROM`, `RCPT TO`, `RSET`, `DATA`, or `AUTH` into the session. Injected commands will cause the `SMTP` instance to hang, but all commands required to complete the envelope could be sent in one address string.

The `SMTP.sendmail()` command will pass sender and recipient addresses verbatim through to `SMTP.mail()` & `SMTP.rcpt()`, and so is also vulnerable. `SMTP.send_message()` is not affected.

### Impact

Severity: medium. Type: SMTP protocol command injection (CWE-93 — Improper Neutralization of CRLF Sequences; CWE-77 — Command Injection).

When an application built on `aiosmtplib` derives the envelope sender or any recipient from data an attacker can influence (a web form etc.) and passes it to `mail()`/`rcpt()` (directly, or via `sendmail()`/`send()` without a `Message` object), the attacker can:

- desynchronize the command/response pipeline and c…