UBUNTU-CVE-2026-55558

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Source
https://ubuntu.com/security/CVE-2026-55558
Import Source
https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-55558.json
JSON Data
https://api.osv.dev/v1/vulns/UBUNTU-CVE-2026-55558
Upstream
Published
2026-08-20T15:17:00Z
Modified
2026-09-02T19:36:26Z
Severity
  • 5.9 (Medium) CVSS_V3 - CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N CVSS Calculator
  • Ubuntu - medium
Summary
[none]
Details

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.

References

Affected packages

Ubuntu:22.04:LTS / aiosmtplib

Package

Name
aiosmtplib
Purl
pkg:deb/ubuntu/aiosmtplib?arch=source&distro=jammy

Affected ranges

Type
ECOSYSTEM
Events
Introduced
0 Unknown introduced version / All previous versions are affected

Affected versions

1.*
1.1.4-1
1.1.4-2
1.1.6-1

Ecosystem specific 

{
    "binaries": [
        {
            "binary_name": "python3-aiosmtplib",
            "binary_version": "1.1.6-1"
        }
    ]
}

Database specific

source 
"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-55558.json"

Ubuntu:26.04:LTS / aiosmtplib

Package

Name
aiosmtplib
Purl
pkg:deb/ubuntu/aiosmtplib?arch=source&distro=resolute

Affected ranges

Type
ECOSYSTEM
Events
Introduced
0 Unknown introduced version / All previous versions are affected

Affected versions

4.*
4.0.0-1
4.0.2-1
5.*
5.0.0-1
5.1.0-1

Ecosystem specific 

{
    "binaries": [
        {
            "binary_name": "python3-aiosmtplib",
            "binary_version": "5.1.0-1"
        }
    ]
}

Database specific

source 
"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-55558.json"