CVE-2026-42246: Net imap Net-imap Vulnerable To STARTTLS Stripping Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vcgp-9326-pqcp",
  "modified" : "2026-05-14T20:48:01Z",
  "published" : "2026-05-04T22:01:52Z",
  "aliases" : [ "CVE-2026-42246" ],
  "summary" : "net-imap vulnerable to STARTTLS stripping via invalid response timing",
  "details" : "### Summary\n\nA man-in-the-middle attacker can cause `Net::IMAP#starttls` to return \"successfully\", without starting TLS.\n\n### Details\n\nWhen using `Net::IMAP#starttls` to upgrade a plaintext connection to use TLS, a man-in-the-middle attacker can inject a tagged `OK` response with an easily predictable tag.  By sending the response before the client finishes sending the command, the command completes \"successfully\" before the response handler is registered.  This allows `#starttls` to return without error, but the response handler is never invoked, the TLS connection is never established, and the socket remains unencrypted.\n\nThis allows man-in-the-middle attackers to perform a STARTTLS stripping attack, unless the client code explicitly checks `Net::IMAP#tls_verified?`.\n\n### Impact\n\nTLS bypass, leading to cleartext transmission of sensitive information.\n\n### Mitigation\n\n* Upgrade to a patched version of net-imap that raises an exception whenever `#starttls` does not establish TLS.\n* Connect to an implicit TLS port, rather than use `STARTTLS` with a cleartext port.\n  This is strongly recommended anyway:\n  * [RFC 8314](https://www.rfc-editor.org/info/rfc8314): Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access\n  * [NO STARTTLS](https://nostarttls.secvuln.info/): Why TLS is better without STARTTLS, A Security Analysis of STARTTLS in the Email Context\n* Explicitly verify `Net::IMAP#tls_verified?` is `true`, before using the connection after `#starttls`.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "net-imap"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.6.0"
      }, {
        "fixed" : "0.6.4"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.6.3"
    }
  }, {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "net-imap"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.5.0"
      }, {
        "fixed" : "0.5.14"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.5.13"
    }
  }, {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "net-imap"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.4.0"
      }, {
        "fixed" : "0.4.24"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.4.23"
    }
  }, {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "net-imap"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.3.10"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.3.9"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/ruby/net-imap/security/advisories/GHSA-vcgp-9326-pqcp"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-42246"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ruby/net-imap/commit/0ede4c40b1523dfeaf95777b2678e54cc0fd9618"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ruby/net-imap/commit/24a4e770b43230286a05aa2a9746cdbb3eb8485e"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ruby/net-imap/commit/97e2488fb5401a1783bddd959dde007d9fbce42c"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ruby/net-imap/commit/f79d35bf5833f186e81044c57c843eda30c873da"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ruby/net-imap"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ruby/net-imap/releases/tag/v0.3.10"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ruby/net-imap/releases/tag/v0.4.24"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ruby/net-imap/releases/tag/v0.5.14"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ruby/net-imap/releases/tag/v0.6.4"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/net-imap/CVE-2026-42246.yml"
  }, {
    "type" : "WEB",
    "url" : "https://nostarttls.secvuln.info"
  }, {
    "type" : "WEB",
    "url" : "https://www.rfc-editor.org/info/rfc8314"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-392", "CWE-393", "CWE-636", "CWE-754", "CWE-841" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-04T22:01:52Z",
    "nvd_published_at" : "2026-05-09T20:16:28Z"
  }
}