aiosmtplib TLS Handshake Desynchronization Vulnerability

Executive Summary

CVE-2026-55558 is a STARTTLS response injection vulnerability in the aiosmtplib library affecting versions 5.1.1 and earlier. When a connection is upgraded to TLS using STARTTLS, the library reads the server's 220 response and proceeds with the TLS handshake without clearing the receive buffer. This allows a man-in-the-middle attacker to inject malicious plaintext responses before the TLS handshake completes. These injected responses are then parsed as legitimate within the encrypted session, potentially enabling unauthorized access or command execution.

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Detection Guidance

Detecting this vulnerability requires monitoring for unusual SMTP traffic patterns or response injection attempts during STARTTLS handshakes. Inspect network traffic for plaintext SMTP responses following the 220 server greeting before TLS negotiation completes. Check aiosmtplib version; versions 5.1.1 and earlier are vulnerable. Use packet capture tools like tcpdump or Wireshark to analyze SMTP sessions for injected commands or responses.

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Impact Analysis

This vulnerability can allow an attacker to inject fake SMTP commands like AUTH LOGIN into the session, potentially gaining unauthorized access to email accounts or executing commands. It requires a man-in-the-middle position on the network, so attackers must be able to intercept traffic. Applications using STARTTLS with aiosmtplib are at risk if network traffic can be intercepted. Connections using implicit TLS are not affected.

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Compliance Impact

This vulnerability could lead to unauthorized access or data modification, which may violate compliance requirements for data integrity and confidentiality under standards like GDPR and HIPAA. GDPR requires protection against unauthorized access to personal data, while HIPAA mandates safeguards for electronic protected health information. Exploitation of this flaw could result in non-compliance penalties or legal consequences.

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Mitigation Strategies

• Upgrade aiosmtplib to version 5.1.2 or later immediately to patch the vulnerability.
• Switch from STARTTLS to implicit TLS (direct TLS) if possible to avoid the vulnerable handshake process.
• Restrict STARTTLS connections to trusted networks or internal SMTP servers to reduce exposure.
• Monitor SMTP traffic for signs of injection attempts or unusual response patterns during STARTTLS negotiation.

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