CVE-2026-55558
MEDIUM
5,9
Source: [email protected]
Attack Vector: network
Attack Complexity: high
Privileges Required: none
User Interaction: none
Scope: unchanged
Confidentiality: none
Integrity: high
Availability: none
Description
AI Translation Available
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.
74
Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
IncompleteCommon Consequences
Security Scopes Affected:
Confidentiality
Access Control
Other
Integrity
Non-Repudiation
Potential Impacts:
Read Application Data
Bypass Protection Mechanism
Alter Execution Logic
Other
Hide Activities
Applicable Platforms
All platforms may be affected
https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff0…
https://github.com/cole/aiosmtplib/releases/tag/v5.1.2
https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4