CVE-2026-55558 json
aiosmtplib: STARTTLS response injection
published 2026-08-20 2 mo ago modified 2026-10-18 21 d ago also GHSA-vxj7-4xrp-5vr4 , PYSEC-2026-3805 , EUVD-2026-63376
- cvss
- 5.9
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.4%
- EPSS 0.4% chance of exploitation in 30 days, percentile 34%
- public exploits
- 0
- none known
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.
Patch / fix
- pypi aiosmtplib 5.1.2
- aiosmtplib Debian 14 5.1.2-1
debian advisories DEBIAN-CVE-2026-55558
- python-aiosmtplib openSUSE Tumbleweed 5.1.2-1.1
opensuse advisories openSUSE-SU-2026:11569-1
CISA SSVC
- Exploitation
- none
- Automatable
- no
- Technical impact
- partial
- Assessed
- 2 mo ago
Exploit prediction
0.40% chance of exploitation in the next 30 days, higher than 34% of all scored CVEs (FIRST EPSS, 2026-11-08).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 5.9 | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N |
| GitHub | 3.1 | 5.9 | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N |
| OSV | 3.1 | 5.9 | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| pypi | aiosmtplib | <5.1.2 |
5.1.2 |
Affected products
- cole aiosmtplib
Classification
- Weakness
-
- CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component
References
-
github.com
/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9
web
-
github.com
/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4
web
-
github.com
/cole/aiosmtplib/releases/tag/v5.1.2
web
-
nvd.nist.gov
/vuln/detail/CVE-2026-55558
advisory
-
github.com
/cole/aiosmtplib
package
-
pypi.org
/project/aiosmtplib
package
-
github.com
/advisories/GHSA-vxj7-4xrp-5vr4
advisory
Sources
- CVE List CVE-2026-55558
- GitHub Advisory GHSA-vxj7-4xrp-5vr4
- NVD CVE-2026-55558
- OSV PYSEC-2026-3805
Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.