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

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.