CVE-2026-55558

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Source
https://cve.org/CVERecord?id=CVE-2026-55558
Import Source
https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-55558.json
JSON Data
https://api.osv.dev/v1/vulns/CVE-2026-55558
Aliases
Downstream
CGA (2)
CLEANSTART (2)
DEBIAN (1)
ECHO (1)
MINI (2)
openSUSE (1)
UBUNTU (1)
Related
Published
2026-08-20T14:29:58Z
Modified
2026-10-24T02:30:32Z
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
Summary
aiosmtplib: STARTTLS response injection
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.

Database specific 
{
    "cna_assigner":  "GitHub_M",
    "cwe_ids":  [
        "CWE-74"
    ],
    "osv_generated_from":  "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55558.json"
}
References

Affected packages

Git / github.com/cole/aiosmtplib

Affected ranges

Type
GIT
Repo
https://github.com/cole/aiosmtplib
Events
Introduced
0 Unknown introduced commit / All previous commits are affected
Fixed
bffc6d19fbb04a84284c8c12ef640fbae0641a9c
Fixed
9fab7ba1361dbf7622ede1315a24be805cff09c9
Database specific
Show details 
{
    "extracted_events":  [
        {
            "range":  [
                {
                    "introduced":  "0"
                },
                {
                    "fixed":  "5.1.2"
                }
            ],
            "source":  "AFFECTED_FIELD",
            "strategy":  "StringRangeExpression"
        },
        {
            "range":  [
                {
                    "fixed":  "9fab7ba1361dbf7622ede1315a24be805cff09c9"
                }
            ],
            "source":  "REFERENCES_COMMIT"
        },
        {
            "original_tag":  "v5.1.2",
            "range":  [
                {
                    "fixed":  "bffc6d19fbb04a84284c8c12ef640fbae0641a9c"
                }
            ],
            "source":  "REFERENCES_TAG"
        }
    ]
}

Affected versions

v0.*
v0.1
v0.1.1
v0.1.2
v0.1.3
v0.1.4
v0.1.5
v0.1.6
v0.1.7
v1.*
v1.0.0
v1.0.1
v1.0.2
v1.0.3
v1.0.4
v1.1.0
v1.1.0b0
v1.1.1
v1.1.2
v1.1.3
v1.1.4
v1.1.5
v1.1.5.dev0
v1.1.6
v1.1.7
v2.*
v2.0.0
v2.0.0.post0
v2.0.0b0
v2.0.0b1
v2.0.1
v3.*
v3.0.0
v3.0.1
v3.0.2
v4.*
v4.0.0
v4.0.1
v4.0.2
v5.*
v5.0.0
v5.1.0
v5.1.1

Database specific

source 
"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-55558.json"