CVE catalog
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 wit
- Severity
- Medium · CVSS 5.9
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N - Published
- Updated
- Source
- [NVD] · [CVE record]
- EPSS
- 0.40% · Oct 25, 2026
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.
Vulnerable and fixed packages
Distribution-reported package versions and fixes. These do not depend on matching against the OpenFactory package catalog.
| Distribution | Package | Release | Version | Status |
|---|---|---|---|---|
| Debian | aiosmtplib | sid | 5.1.3-1 | fixed |
| Debian | aiosmtplib | forky | 5.1.3-1 | fixed |
| Debian | aiosmtplib | trixie | 4.0.0-1 | vulnerable |
| Debian | aiosmtplib | bookworm | 2.0.0-1 | vulnerable |
| Pip | aiosmtplib | Not reported | 5.1.2 | fixed |
| Opensuse | python-aiosmtplib | tumbleweed | 5.1.2-1.1 | fixed |
Fixed-version evidence
| Distribution | Package | Type | Release | Fixed version | Urgency | Origin | Bugs |
|---|---|---|---|---|---|---|---|
| Debian | aiosmtplib | source | sid | 5.1.2-1 | not yet assigned | Not reported | Not reported |
| Debian | aiosmtplib | source | forky | 5.1.2-1 | not yet assigned | Not reported | Not reported |
| Debian | aiosmtplib | source | trixie | (unfixed) | not yet assigned | Not reported | Not reported |
| Debian | aiosmtplib | source | bookworm | (unfixed) | not yet assigned | Not reported | Not reported |
| Pip | aiosmtplib | pip | Not reported | 5.1.2 | Not reported | GHSA-VXJ7-4XRP-5VR4 | Not reported |
| Opensuse | python-aiosmtplib | rpm | tumbleweed | 5.1.2-1.1 | Not reported | OPENSUSE-SU-2026:11569-1 | Not reported |
Notes
[trixie] aiosmtplib: Minor issue [bookworm] aiosmtplib: Minor issue; can be fixed with next update
OpenFactory catalog packages
[Debian 12 (Bookworm)]
- [python3-aiosmtplib]Affected
Source package: [aiosmtplib]
Details (2)
2.0.0-1· all · bookwormAffected
nvd · aiosmtplib · debian · bookworm; nvd · aiosmtplib · debian · 12
2.0.0-1· all · bookwormAffected
nvd · aiosmtplib · debian · bookworm; nvd · aiosmtplib · debian · 12
- [python3-aiosmtplib-doc]Affected
Source package: [aiosmtplib]
Details (2)
2.0.0-1· all · bookwormAffected
nvd · aiosmtplib · debian · bookworm; nvd · aiosmtplib · debian · 12
2.0.0-1· all · bookwormAffected
nvd · aiosmtplib · debian · bookworm; nvd · aiosmtplib · debian · 12
[Debian 13 (Trixie)]
- [python3-aiosmtplib]Affected
Source package: [aiosmtplib]
Details (2)
4.0.0-1· all · trixieAffected
nvd · aiosmtplib · debian · 13
4.0.0-1· all · trixieAffected
nvd · aiosmtplib · debian · 13
- [python3-aiosmtplib-doc]Affected
Source package: [aiosmtplib]
Details (2)
4.0.0-1· all · trixieAffected
nvd · aiosmtplib · debian · 13
4.0.0-1· all · trixieAffected
nvd · aiosmtplib · debian · 13
[openSUSE Tumbleweed]
- [python313-aiosmtplib]Unknown
Source package: [python-aiosmtplib]
Details (1)
5.1.2-1.1· noarch · tumbleweedUnknown · Fixed:
5.1.2-1.1Version impact unknown
nvd · python-aiosmtplib · opensuse · tumbleweed
- [python314-aiosmtplib]Unknown
Source package: [python-aiosmtplib]
Details (1)
5.1.2-1.1· noarch · tumbleweedUnknown · Fixed:
5.1.2-1.1Version impact unknown
nvd · python-aiosmtplib · opensuse · tumbleweed
Observed:
References
- [https://ftp.suse.com/pub/projects/security/csaf/opensuse-su-2026_11569-1.json]
- [https://github.com/advisories/GHSA-vxj7-4xrp-5vr4]
- [https://nvd.nist.gov/vuln/detail/CVE-2026-55558]
- [https://osv.dev/vulnerability/DEBIAN-CVE-2026-55558]
- [https://osv.dev/vulnerability/openSUSE-SU-2026:11569-1]
- [https://security-tracker.debian.org/tracker/CVE-2026-55558]
[Debian Security Tracker] · [FIRST EPSS] · [GitHub Advisory Database] · [National Vulnerability Database (NVD)] · [OSV Package Vulnerability Database] · [SUSE security advisories and VEX]