CVE-2026-55558: Injection (CVSS 5.9)

Description

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.

Is your site exposed to CVE-2026-55558?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

5.9

MEDIUM

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS — Exploit Prediction

0.0026

Probability of exploitation

0.18%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-74 CWE-74

References

Other References

https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9 https://github.com/cole/aiosmtplib/releases/tag/v5.1.2 https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4

Frequently Asked Questions

What is CVE-2026-55558? +

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. It has a CVSS v3.1 base score of 5.9 (MEDIUM).

How severe is CVE-2026-55558? +

CVE-2026-55558 has a CVSS v3.1 score of 5.9 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0026, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-55558? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-41237

Froxlor is open source server administration software. In version 2.3.6 and earlier, the LOC record regex uses `\s+` which matches …

CVE-2024-53263

Git LFS is a Git extension for versioning large files. When Git LFS requests credentials from Git for a remote …

CVE-2025-27107

Integrated Scripting is a tool for creating scripts for handling complex operations in Integrated Dynamics. Minecraft users who use Integrated …

CVE-2025-32699

Vulnerability in Wikimedia Foundation MediaWiki, Wikimedia Foundation Parsoid.This issue affects MediaWiki: before 1.39.12, 1.42.6, 1.43.1; Parsoid: before 0.16.5, 0.19.2, 0.20.2.

CVE-2025-40710

Host Header Injection (HHI) vulnerability in the Hotspot Shield VPN client, which can induce unexpected behaviour when accessing third-party web …

CVE-2025-6785

Securing externally available CAN wires can easily allow physical access to the CAN bus, allowing possible injection of specially formed …

CVE-2026-55558