CVE-2026-55558 in aiosmtplib
Summary
by MITRE • 08/20/2026
aiosmtplib is an asynchronous SMTP client for use with asyncio. Prior to 5.1.2, SMTPProtocol.start_tls in src/aiosmtplib/protocol.py consumes the server's 220 response and starts the TLS handshake without clearing SMTPProtocol._buffer. An active network attacker can place attacker-chosen SMTP response lines after the plaintext 220 response in the same network segment. The method then calls loop.start_tls; those bytes survive the transport upgrade and are parsed as the first response from inside the TLS session, desynchronizing subsequent SMTP command and response pairs. Connections using start_tls=True or opportunistic STARTTLS are affected, while connections using use_tls=True are not. This issue is fixed in version 5.1.2.
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Analysis
by VulDB Data Team • 08/20/2026
The vulnerability identified in aiosmtplib prior to version 5.1.2 represents a critical protocol desynchronization flaw within the asynchronous SMTP client implementation. The library, designed for integration with Python's asyncio framework, facilitates communication over Simple Mail Transfer Protocol by managing network connections and TLS upgrades. The specific defect resides in the SMTPProtocol.start_tls method located in src/aiosmtplib/protocol.py. Under normal operational conditions, this function is responsible for initiating a Transport Layer Security handshake to encrypt subsequent communications. However, the implementation fails to properly manage internal state buffers during this transition phase, leading to data leakage from the plaintext session into the encrypted channel.
The technical root cause of this vulnerability lies in how the library handles server responses and buffer management prior to upgrading the transport layer. When an SMTP client initiates a STARTTLS command or connects with start_tls enabled, it receives a 220 Ready response from the mail server indicating that the connection is ready for TLS upgrade. The flawed implementation consumes this specific 220 response line but neglects to clear the internal _buffer attribute of the SMTPProtocol object. This buffer retains any additional data received during or immediately after the consumption of the initial handshake message. Consequently, when loop.start_tls is invoked to establish the encrypted tunnel, these residual bytes are not discarded. Instead, they persist across the transport upgrade boundary and become part of the input stream for the newly established TLS session.
This architectural oversight allows an active network attacker positioned within the same network segment to exploit this desynchronization through a man-in-the-middle attack vector. By injecting arbitrary SMTP response lines immediately following the legitimate 220 server response, the attacker ensures that these malicious bytes are stored in the uncleared buffer. Upon upgrading to TLS, the client interprets these injected responses as valid replies from the mail server within the encrypted session. This results in a complete desynchronization of the SMTP command-response sequence. The client expects specific protocol states based on its previous actions, but receives unexpected data due to the injection, causing parsing errors or incorrect state transitions that can disrupt email delivery or potentially allow for further exploitation depending on how subsequent commands are handled by the application layer consuming this library.
The operational impact of this vulnerability is significant for applications relying on aiosmtplib for automated email transmission using opportunistic STARTTLS or explicit start_tls=True configurations. Connections established with use_tls=True, which likely enforce TLS from the outset without an upgrade phase, remain unaffected as they do not undergo the problematic transition logic. For affected versions, the desynchronization can lead to failed message deliveries, where emails are silently dropped due to protocol errors, or more severe consequences such as information leakage if the application proceeds with commands under false assumptions about server state. In high-volume email environments, this could result in substantial data loss and operational downtime while debugging the underlying communication failure.
From a classification perspective, this vulnerability aligns closely with CWE-362, which describes concurrent execution using shared resources with inadequate synchronization, specifically manifesting here as improper buffer management during protocol state transitions. It also relates to CWE-480 regarding incorrect use of parentheses or precedence in logical operations if the filtering logic is viewed through that lens, but more accurately fits CWE-173: Improper Handling of Unexpected Data Type due to the ingestion of unexpected SMTP responses into a TLS context. In terms of adversary tactics, this flaw facilitates MITM (Man-in-the-Middle) attacks as defined by ATT&CK technique T1042, allowing attackers to manipulate network traffic without breaking encryption immediately, thereby evading detection that might trigger on certificate errors or connection drops.
Mitigation for organizations utilizing aiosmtplib requires an immediate upgrade to version 5.1.2 or later, where the developers have corrected the buffer handling logic during TLS upgrades. The fix ensures that all data received prior to and including the STARTTLS response is properly cleared before the transport layer switches to encryption, preventing residual plaintext bytes from contaminating the secure session. For environments unable to upgrade immediately, implementing network-level controls such as strict firewall rules or intrusion detection systems can help mitigate exposure by blocking unauthorized SMTP traffic on relevant ports. Additionally, developers should review their email sending logic for error handling around STARTTLS failures to ensure that desynchronization events are caught and logged rather than causing silent application failures.