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TI-2026-40102 Aliases CVE-2026-55558GHSA-vxj7-4xrp-5vr4

aiosmtplib: STARTTLS response injection

Priority LOW 27CWE-74Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
High
Impact type
—
Confidentiality · integrity · availability
None · High · None
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub.

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00403 34th percentileScored 2026-11-07
Fix Fixed aiosmtplib (cole): 5.1.2aiosmtplib (PyPI): 5.1.2

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
coleaiosmtplibbefore 5.1.25.1.2Patch availableCNA
—aiosmtplib (PyPI)before 5.1.25.1.2Patch availableGHSA-vxj7-4xrp-5vr4
  • aiosmtplib Vendorcole Affected before 5.1.2 Fixed in 5.1.2 Patch available CNA
  • aiosmtplib (PyPI) Vendor— Affected before 5.1.2 Fixed in 5.1.2 Patch available GHSA-vxj7-4xrp-5vr4

Remediation

  • Fix · CNA aiosmtplib (cole) Fixed in 5.1.2
  • Fix · GHSA-vxj7-4xrp-5vr4 aiosmtplib (PyPI) Fixed in 5.1.2 Advisory →

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-55558
  2. 2026-10-03 Alias added: GHSA-vxj7-4xrp-5vr4

References

Advisory 1

Other 4

  • TI-2026-38606 aiosmtplib: SMTP command injection via CR/LF in sender/recipient address CVSS 4.0MEDIUM 6.9
  • TI-2026-61498 aiosmtplib before 5.1.3 ESMTP Parameter Injection via unvalidated addresses CVSS 4.0MEDIUM 6.3

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences