SMTP session desynchronization via STARTTLS injection
Published Aug 20, 2026 · Updated Aug 20, 2026
Injection in aiosmtplib before 5.1.2 allows man-in-the-middle attackers to desynchronize SMTP sessions via STARTTLS response injection. SMTPProtocol.start_tls reads the server's 220 reply and upgrades the transport with loop.start_tls without clearing SMTPProtocol._buffer, so attacker-supplied plaintext buffered before the handshake is accepted as the first post-TLS server response. Reachability requires interception of the plaintext STARTTLS leg; implicit TLS with use_tls=True is not exposed, and the documented consequence is forged server replies with subsequent command-response misalignment rather than direct code execution.
Summary
What happened
Injection in aiosmtplib before 5.1.2 allows man-in-the-middle attackers to desynchronize SMTP sessions via STARTTLS response injection. SMTPProtocol.start_tls reads the server's 220 reply and upgrades the transport with loop.start_tls without clearing SMTPProtocol._buffer, so attacker-supplied plaintext buffered before the handshake is accepted as the first post-TLS server response. Reachability requires interception of the plaintext STARTTLS leg; implicit TLS with use_tls=True is not exposed, and the documented consequence is forged server replies with subsequent command-response misalignment rather than direct code execution.
The record
- CVE
- CVE-2026-55558
- Published
- Aug 20, 2026
- Updated
- Aug 20, 2026
- Vendor
- Cole Maclean
- Product
- aiosmtplib
- Classifications
- CWE-74
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 20, 2026CVE publishedPublication date reported by the CVE source.
- Aug 20, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=< 5.1.2
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What is affected?
Published CVSS scores
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Attacks
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Weakness, pattern, technique
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Technologies
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Your stack
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