Summary
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.
Vulnerable versions
Affected version: aiosmtplib 5.1.0 (latest at time of report) and all earlier releases.
Credit
Reported by tonghuaroot.
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 cause the aiosmtplib client to hang, resulting in a possible denial of service
- inject multiple commands in one address to send an arbitrary message
The address only needs to reach mail()/rcpt()/vrfy()/expn(); no attacker control over the SMTP server is required.
Untrusted input is inserted into a command that is later executed by the application, allowing the attacker to alter the intent of that command. Typical impact: arbitrary command execution in the application's environment.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-53533? CVE-2026-53533 is a medium-severity command injection vulnerability in aiosmtplib (pip), affecting versions <= 5.1.0. It is fixed in 5.1.1. Untrusted input is inserted into a command that is later executed by the application, allowing the attacker to alter the intent of that command.
- Which versions of aiosmtplib are affected by CVE-2026-53533? aiosmtplib (pip) versions <= 5.1.0 is affected.
- Is there a fix for CVE-2026-53533? Yes. CVE-2026-53533 is fixed in 5.1.1. Upgrade to this version or later.
- Is CVE-2026-53533 exploitable, and should I be worried? Whether CVE-2026-53533 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-53533 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-53533? Upgrade
aiosmtplibto 5.1.1 or later.