{"apiVersion":"1.0","identifier":"CVE-2026-55558","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.","publishedAt":"2026-08-20T15:17:31","lastModifiedAt":"2026-08-20T20:17:35","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-55558","cvssScore":5.9,"cvssVector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N","epssProbability":0.00261,"riskScore":0.6,"affectedProduct":"aiosmtplib","affectedVersions":"<5.1.2","vulnerabilityType":"Library","operatingSystems":[],"links":{"self":"https://www.redsauce.net/api/cves/CVE-2026-55558","webPages":{"es":"https://www.redsauce.net/es/cves/CVE-2026-55558","en":"https://www.redsauce.net/en/cves/CVE-2026-55558","fr":"https://www.redsauce.net/fr/cves/CVE-2026-55558","pt":"https://www.redsauce.net/pt/cves/CVE-2026-55558","de":"https://www.redsauce.net/de/cves/CVE-2026-55558","sk":"https://www.redsauce.net/sk/cves/CVE-2026-55558","el":"https://www.redsauce.net/el/cves/CVE-2026-55558"},"source":"https://nvd.nist.gov/vuln/detail/CVE-2026-55558"}}