{
  "cveId": "CVE-2026-55558",
  "url": "https://patchblox.com/security/cve/CVE-2026-55558",
  "summary": "CVE-2026-55558 is fixed in aiosmtplib on 1 platform by DEBIAN-CVE-2026-55558 (version 5.1.2-1), released August 21, 2026. It is not reported as exploited. Patch Radar priority: Routine.",
  "title": "aiosmtplib: aiosmtplib is an asynchronous SMTP client for use with asyncio",
  "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\u0027s 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.",
  "published": "2026-08-20T15:17:31Z",
  "severity": "medium",
  "cvss": 5.9,
  "cvssVector": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N",
  "epss": 0.00403,
  "epssPercentile": 0.34777,
  "priority": "Routine",
  "priorityTier": 4,
  "exploited": false,
  "vendorExploited": false,
  "disclosed": false,
  "fixes": [
    {
      "platform": "linux",
      "product": "aiosmtplib",
      "release": "Debian 14",
      "fixedVersion": "5.1.2-1",
      "fix": "DEBIAN-CVE-2026-55558",
      "url": "https://patchblox.com/security/fix/linux/debian-cve-2026-55558",
      "released": "2026-08-21T23:37:15Z"
    }
  ],
  "noFixYet": [],
  "dataAsOf": "2026-11-08T06:59:42Z",
  "attribution": {
    "text": "Data: Patchblox Patch Radar",
    "url": "https://patchblox.com/security/patch-radar",
    "terms": "https://patchblox.com/security/patch-radar/use-the-data"
  }
}