{"cve":"CVE-2026-55558","nvd":{"published":"2026-08-20T15:17:31.980","modified":"2026-10-18T20:10:00.110","status":"Deferred","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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N","score":5.9},"cwes":["CWE-74"],"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"],"products":[],"source":"nvd","synced_at":"2026-10-19T22:49:38.260478770+00:00"},"epss":{"cve":"CVE-2026-55558","score":0.00403,"percentile":0.34257,"date":"2026-10-25"},"kev":null,"osv":[{"id":"GHSA-vxj7-4xrp-5vr4","url":"https://osv.dev/vulnerability/GHSA-vxj7-4xrp-5vr4","summary":"aiosmtplib: STARTTLS response injection","modified":"2026-09-10T12:25:32.035695728Z","packages":[{"ecosystem":"PyPI","name":"aiosmtplib","purl":"pkg:pypi/aiosmtplib","ranges":["<5.1.2"],"fixed":["5.1.2"]}]},{"id":"PYSEC-2026-3805","url":"https://osv.dev/vulnerability/PYSEC-2026-3805","summary":"aiosmtplib: STARTTLS response injection","modified":"2026-09-10T12:15:06.298327536Z","packages":[{"ecosystem":"PyPI","name":"aiosmtplib","purl":"pkg:pypi/aiosmtplib","ranges":["<5.1.2"],"fixed":["5.1.2"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}