CVE-2024-34083 in aiosmtpd
Summary
by MITRE • 05/18/2024
aiosmptd is a reimplementation of the Python stdlib smtpd.py based on asyncio. Prior to version 1.4.6, servers based on aiosmtpd accept extra unencrypted commands after STARTTLS, treating them as if they came from inside the encrypted connection. This could be exploited by a man-in-the-middle attack. Version 1.4.6 contains a patch for the issue.
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Analysis
by VulDB Data Team • 08/24/2025
The vulnerability identified as CVE-2024-34083 affects aiosmtpd, a Python library that provides an asynchronous smtpd implementation based on the standard library's smtpd.py module. This library serves as a foundation for building SMTP servers that can handle email transmission asynchronously, making it a critical component in email infrastructure deployments. The flaw exists in versions prior to 1.4.6, where the server implementation fails to properly enforce security boundaries after establishing a TLS encrypted connection through the STARTTLS command. This represents a fundamental breakdown in the protocol handling mechanism that governs secure communication channels.
The technical implementation flaw stems from improper state management within the SMTP server's command processing logic. When a client establishes a secure connection using STARTTLS, the server should enforce that all subsequent commands are processed within the encrypted context and reject any commands that arrive before the encryption is properly established or that attempt to bypass the secure channel. However, the vulnerable implementation allows unencrypted commands to be accepted and processed after the STARTTLS handshake has been completed, effectively creating a window where attackers can inject commands that appear to originate from within the secure connection. This behavior violates the fundamental security principle that commands following a STARTTLS negotiation should be treated as part of the encrypted session.
The operational impact of this vulnerability is significant, particularly in environments where email servers are exposed to untrusted networks or where man-in-the-middle attacks are a concern. An attacker positioned between the client and server can exploit this weakness by intercepting the connection and sending commands that will be accepted and processed by the server as legitimate encrypted traffic. This creates a potential attack vector for credential theft, email interception, or message manipulation, as the server treats these commands with the same trust level as those originating from authenticated users within the secure channel. The vulnerability essentially undermines the security guarantees provided by the STARTTLS protocol, making it possible for attackers to perform actions that should be restricted to authenticated and encrypted sessions.
This vulnerability maps to CWE-319: Cleartext Transmission of Sensitive Information, which specifically addresses the exposure of sensitive data during transmission, and CWE-310: Cryptographic Issues, which encompasses problems in cryptographic implementation that can lead to security breaches. The attack pattern aligns with the MITRE ATT&CK framework under T1566: Phishing and T1071.004: Application Layer Protocol - Email Protocols, as it enables attackers to manipulate email server behavior through protocol-level attacks. Organizations deploying aiosmtpd should immediately upgrade to version 1.4.6 or later to remediate this vulnerability. Additional mitigations include implementing network-level security controls such as firewalls that restrict access to SMTP ports, deploying intrusion detection systems that monitor for anomalous command sequences, and ensuring that all email infrastructure components properly validate command ordering and authentication status. The patch released in version 1.4.6 addresses the core issue by implementing proper command validation that ensures all commands following STARTTLS are processed within the correct security context, thereby preventing the acceptance of unencrypted commands that would otherwise be processed as if they originated from within the encrypted session.