{
  "affected": [
    {
      "affectedData": [
        {
          "product": "lettre",
          "vendor": "lettre",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 0.10.1, \u003c 0.11.22"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "lettre is a a mailer library for Rust. Starting in version 0.10.1 and prior to version 0.11.22, an inverted-boolean bug in lettre\u0027s `boring-tls` integration silently disables TLS hostname verification for callers using the default (strict) configuration. An on-path attacker presenting any chain-valid certificate for any domain can intercept SMTP submission, including PLAIN/LOGIN credentials and message contents, against any lettre user built with the `boring-tls` feature. Other TLS backends (`native-tls`, `rustls`) are unaffected. Version 0.11.22 patches the issue."
    }
  ],
  "id": "CVE-2026-46428",
  "lastModified": "2026-07-21T16:17:11.147",
  "metrics": {
    "cvssMetricV40": [
      {
        "cvssData": {
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "Safety": "NOT_DEFINED",
          "attackComplexity": "LOW",
          "attackRequirements": "PRESENT",
          "attackVector": "NETWORK",
          "availabilityRequirement": "NOT_DEFINED",
          "baseScore": 9.1,
          "baseSeverity": "CRITICAL",
          "confidentialityRequirement": "NOT_DEFINED",
          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirement": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnAvailabilityImpact": "NOT_DEFINED",
          "modifiedVulnConfidentialityImpact": "NOT_DEFINED",
          "modifiedVulnIntegrityImpact": "NOT_DEFINED",
          "privilegesRequired": "NONE",
          "providerUrgency": "NOT_DEFINED",
          "subAvailabilityImpact": "NONE",
          "subConfidentialityImpact": "LOW",
          "subIntegrityImpact": "LOW",
          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
          "version": "4.0",
          "vulnAvailabilityImpact": "NONE",
          "vulnConfidentialityImpact": "HIGH",
          "vulnIntegrityImpact": "HIGH",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-46428",
          "options": [
            {
              "exploitation": "poc"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "total"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-07-21T15:48:51.405919Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-07-20T16:17:01.347",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/lettre/lettre/commit/f5efffc88360dbdbfcef80f465e42d5bce68ca35"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/lettre/lettre/releases/tag/v0.11.22"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/lettre/lettre/security/advisories/GHSA-4pj9-g833-qx53"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://rustsec.org/advisories/RUSTSEC-2026-0141.html"
    },
    {
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "url": "https://github.com/lettre/lettre/security/advisories/GHSA-4pj9-g833-qx53"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-295"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}

{
  "affected": [
    {
      "package": {
        "ecosystem": "crates.io",
        "name": "lettre"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0.10.1"
            },
            {
              "fixed": "0.11.22"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-46428"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-295"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-28T14:29:10Z",
    "nvd_published_at": "2026-07-20T16:17:01Z",
    "severity": "CRITICAL"
  },
  "details": "### Summary\nAn inverted-boolean bug in lettre\u0027s `boring-tls` integration silently\ndisables TLS hostname verification for callers using the default (strict)\nconfiguration. An on-path attacker presenting any chain-valid certificate\nfor any domain can intercept SMTP submission, including PLAIN/LOGIN\ncredentials and message contents, against any lettre user built with the\n`boring-tls` feature. Other TLS backends (`native-tls`, `rustls`) are\nunaffected.\n\n### Details\nboring\u0027s `SslConnectorBuilder::set_verify_hostname(bool)` /\n`verify_hostname(bool)` API takes a *verify* flag \u2014 `true` means enforce\nhostname verification, `false` means skip it. (Boring docs:\nhttps://docs.rs/boring/latest/boring/ssl/struct.ConnectConfiguration.html#method.set_verify_hostname)\nlettre\u0027s `TlsParametersBuilder` exposes the opposite-named flag\n`accept_invalid_hostnames: bool` \u2014 `true` means *accept invalid* (skip\nverification), `false` means verify. lettre passes this flag directly to\nboring at both TLS-upgrade sites, inverting the semantics:\nsrc/transport/smtp/client/net.rs:202 (sync)\n    .verify_hostname(*accept_invalid_hostnames)\nsrc/transport/smtp/client/async_net.rs:377 (async)\n    config.set_verify_hostname(accept_invalid_hostnames);\nConcrete behaviour under `boring-tls`:\n  * accept_invalid_hostnames = false  (the strict default)\n      \u2192 set_verify_hostname(false)  \u2192 hostname verification SKIPPED\n        (wrong; the caller asked for strict verification)\n  * accept_invalid_hostnames = true   (explicit opt-in to \"dangerous\")\n      \u2192 set_verify_hostname(true)   \u2192 hostname verification ENFORCED\n        (wrong; the caller opted into accepting any name)\nBoring\u0027s own warning on `set_verify_hostname`:\n    \"If hostname verification is not used, *any* valid certificate for\n    *any* site will be trusted for use from any other. This introduces\n    a significant vulnerability to man-in-the-middle attacks.\"\n`native-tls` (src/transport/smtp/client/tls.rs:355) uses\n`danger_accept_invalid_hostnames(self.accept_invalid_hostnames)`, whose\nflag name and semantics match lettre\u0027s \u2014 unaffected. `rustls` uses a\nseparate verifier path \u2014 unaffected.\nThe bug was introduced in PR #797 (\"Add support for boring TLS\"), commit\n985fa7e, first released in v0.10.1, and persists unchanged through v0.11.21\n(latest). Zero existing tests cover `accept_invalid_hostnames` end-to-end\nwith any backend, which is why the inversion has gone undetected.\nFix: negate the flag at both call sites. Two-line patch:\n    -    .verify_hostname(*accept_invalid_hostnames)\n    +    .verify_hostname(!*accept_invalid_hostnames)\n    -    config.set_verify_hostname(accept_invalid_hostnames);\n    +    config.set_verify_hostname(!accept_invalid_hostnames);\nI have the patch ready locally and can push to a private fork if a\ncollaborative fix branch is preferred.\n\n### PoC\nSetup (any lettre version \u003e= v0.10.1, built with the `boring-tls`\nfeature, default-strict configuration):\n1. Generate a TLS cert for any hostname the attacker controls, e.g.\n   `attacker.example`, signed by any CA in the client\u0027s trust store\n   (e.g. a Let\u0027s Encrypt cert is sufficient).\n2. Stand up an attacker SMTP server on the network path between the\n   lettre client and the intended MX. Have it present the\n   `attacker.example` cert on STARTTLS or implicit TLS.\n3. Use lettre with default-strict TLS to send to the real MX hostname,\n   e.g. `mail.example.com`:\n       use lettre::transport::smtp::client::{Tls, TlsParameters};\n       use lettre::{AsyncSmtpTransport, Tokio1Executor, Message};\n       let tls = TlsParameters::builder(\"mail.example.com\".to_owned())\n           .build_boring()\n           .unwrap();\n       let mailer = AsyncSmtpTransport::\u003cTokio1Executor\u003e::relay(\n               \"mail.example.com\",\n           )?\n           .tls(Tls::Required(tls))\n           .build();\n       mailer.send(message).await?;     // succeeds against attacker\nExpected with hostname verification: handshake fails because the\npresented leaf cert\u0027s SAN does not match `mail.example.com`.\nActual on `boring-tls`: handshake succeeds. lettre proceeds with the\nSMTP transaction, sending MAIL FROM, RCPT TO, message body, and any\nconfigured SMTP AUTH credentials to the attacker.\nSymmetric inverse PoC (proves the inversion, not just a missing check):\nconfigure `.dangerous_accept_invalid_hostnames(true)` and try to connect\nto a server whose cert SAN matches the connect domain. With the bug,\nthe handshake *fails* because lettre passes `true` to `verify_hostname`\nand boring rejects... actually wait, it succeeds (SAN matches). Better\ninverse demonstration: connect to a server whose cert SAN matches the\nconnect domain with `accept_invalid_hostnames=true`, then connect to a\nmismatched-SAN server with the same flag \u2014 the latter is rejected,\nproving the flag is inverted (the user opted into accepting any name,\nyet lettre enforces strict verification).\nA reproducer cargo project can be produced on request.\n\n### Impact\nWho is impacted: any lettre user who\n  1. enables the `boring-tls` feature, AND\n  2. relies on default-strict hostname verification\n     (i.e. does NOT call `.dangerous_accept_invalid_hostnames(true)`).\nThis is the entire \"I want strict TLS\" population on `boring-tls`. They\nget the opposite of what they asked for: a TLS handshake that ignores\nhostname mismatch.\nExploitation: an attacker on the network path between the lettre client\nand the target SMTP server, holding any chain-valid TLS certificate\n(e.g. a free Let\u0027s Encrypt cert for a domain the attacker owns), can\npresent that certificate when the lettre client connects to the intended\nMX. boring accepts the handshake, lettre proceeds, and the attacker\ngains read/write access to:\n  * SMTP envelope (MAIL FROM, RCPT TO)\n  * Message bodies, attachments, headers\n  * Any SMTP AUTH credentials (PLAIN, LOGIN, CRAM-MD5, etc.)\n  * Any DKIM signatures and DMARC alignment downstream depends on\n    can be inspected and selectively dropped\nConfidentiality and integrity impact: High. Availability impact: None\ndirect (the attacker can choose to forward or drop, but the bug itself\ndoes not deny service).\nCloudflare\u0027s email-fwdr independently identified and mitigated this\ndownstream by promoting a previously-metric-only post-handshake SAN\ncheck to a hard error (fix landed 2026-05-12). Because we\u0027re mitigated\ndownstream, we are not on a publication clock; we are reporting so the\nupstream fix can ship and other lettre+boring-tls deployments can stop\nrelying on per-caller workarounds. Happy to coordinate on embargo\ntiming.\nAffected versions: every released lettre version with `boring-tls`\nsupport, from v0.10.1 (PR #797, commit 985fa7e) through v0.11.21\n(latest). No prior advisory or fix.\nSuggested fix: two-line negation of the flag at the boring call sites\n(see Details). Patch is ready locally and can be attached to a private\ncollaboration fork.",
  "id": "GHSA-4pj9-g833-qx53",
  "modified": "2026-07-28T14:29:10Z",
  "published": "2026-07-28T14:29:10Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/lettre/lettre/security/advisories/GHSA-4pj9-g833-qx53"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-46428"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lettre/lettre/commit/f5efffc88360dbdbfcef80f465e42d5bce68ca35"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/lettre/lettre"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lettre/lettre/releases/tag/v0.11.22"
    },
    {
      "type": "WEB",
      "url": "https://rustsec.org/advisories/RUSTSEC-2026-0141.html"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:L/SI:L/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "lettre has TLS hostname verification disabled when using Boring TLS backend"
}