CVE-2026-55558 (GCVE-0-2026-55558)

Vulnerability from cvelistv5 – Published: 2026-08-20 14:29 – Updated: 2026-08-20 20:00
VLAI
Title
aiosmtplib: STARTTLS response injection
Summary
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.
Severity
5.9 (Medium)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
SSVC
Exploitation: none Automatable: no Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-08-20 20:00 UTC
CWE
  • CWE-74 - Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
Assigner
References
Impacted products
Vendor Product Version CPE status
cole aiosmtplib Affected: < 5.1.2
guessed Create a notification for this product.
Show details on NVD website
To clipboard 

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