GHSA-4g23-2xm8-66gc Nodemailer: Quadratic (O(n^2)) SMTP client reply parsing lets a malicious server or MITM cause a pre-auth remote denial of service

• npm nodemailer: >= 3.0.0, < 10.0.10 → fixed in 10.0.10

Summary The SMTP client response parser reassembles multiline server replies by appending each continuation line onto a single response-queue entry and re-testing whether that entry is still a partial (continuation) reply on every line. The partial-response test runs `str.slice(str.lastIndexOf('\n') + 1)` on the growing entry, which forces a full flatten of the entire accumulated string on each line. A malicious or compromised SMTP server (or an on-path attacker on a plaintext or pre-STARTTLS hop) that streams continuation lines and never sends a completion line drives O(n^2) CPU on the single-threaded Node.js event loop before any authentication, freezing the whole process for an attacker-controlled duration. Root Cause The invariant that a server cannot cause superlinear client-side work while a reply is being reassembled is violated. In `src/smtp-connection/index.ts`, `_onData` splits inbound bytes into lines and, while the last queued entry is a continuation reply, concatenates each new line onto that same entry with `this._responseQueue[len - 1] += '\n' + lines[i]`. On every iteration it calls `isPartialResponse(lastline)`, which evaluates `/^\d+-/.test(str.slice(str.lastIndexOf('\n') + 1))`. Because the queued entry grows by one line per continuation line and is stored as a V8 rope (ConsString), each `lastIndexOf`/`slice` forces a full string flatten of length proportional to the accumulated reply. Executed once per line over n continuation lines, this is O(n^2) CPU. While the reply stays partial the function early-returns and never resets the entry, so the growth and the per-line reflatten continue unbounded. There is no response-size or line-count cap in the connection code; the only limits are time-based (`greetingTimeout`, and an inactivity `socketTimeout`), and neither bounds accumulated reply size. ```text Affected file and functions (verified at the published v10.0.9 artifact, in src/ and in the built dist/cjs and dist/esm output): • src/smtp-connection/index.ts: isPartialResponse (the /^\d+-/ test over str.slice(str.lastIndexOf('\n')+1)) • src/smtp-connection/index.ts: SMTPConnection._onData (the continuation-line append loop with no size cap) ``` Impact An attacker who is, or can impersonate, the SMTP peer the client connects to can freeze the connecting Node.js process. Because Node.js is single-threaded, event-loop starvation halts all other work in that process (every other connection, request, and timer), not just the offending SMTP session. The reply is parsed during the greeting and EHLO phases, before the client authenticates or issues any command, so no credentials on the client are required. The blocking duration scales with the volume of continuation data the attacker streams and can be sustained indefinitely by continuing to stream, and memory for the accumulated entry also grows without bound. Independent reproduction on Node.js 24 measured roughly 11.6 seconds of event-loop blocking for about 640 KB of continuation data and roughly 47 seconds for about 1.3 MB, scaling quadratically. This is a repeatable remote denial of service against the sending process. Proof of Concept `Node.js 24; a raw TCP listener acting as a hostile SMTP server; the client uses the shipped SMTPConnection (require('nodemailer/lib/smtp-connection')).` Steps: (1) start a TCP server that accepts a connection and streams continuation lines of the form `220-flood...\r\n` in large batches, never emitting a completion line such as `220 ready`; (2) point an SMTPConnection at it and observe the client event loop stall while the single response-queue entry grows and is reflattened on every line. A minimal microbenchmark of the exact parsing logic demonstrates the complexity: ```js // Reproduces isPartialResponse + the _onData append loop verbatim (Node 24). function isPartialResponse(str) { return /^\d+-/.test(str.slice(str.lastIndexOf('\n') + 1)); } function run(n) { const queue = []; let remainder = ''; function onData(chunk) { const lines = (remainder + chunk).split(/\r?\n/); remainder = lines.pop(); for (let i = 0; i `, byte or line ceilings) finds none; the only bounds are `greetingTimeout` and the inactivity-based `socketTimeout`, both time-based and neither bounding accumulated size. Disproof attempts: the greeting timeout does not help because it is time-based, fires only during the greeting phase, is cleared once the greeting completes, and its callback is delayed while `_onData` runs synchronously; a well-behaved server does not trigger the issue because a completion line resets the entry and keeps cost linear, but the modeled attacker is a hostile peer; segmented delivery does not help the defender because the response queue and line remainder persist across `_onData` calls so the accumulation and quadratic cost survive chunking; there is no safer alternate caller because `_onData` is the only data handler and the append loop is on every reply phase. Note on scope: core nodemailer connects to a configured host and does not perform MX resolution, so this is not reachable through a direct-to-MX transport; the malicious-peer, multi-tenant tenant-supplied-host, deliverability-probe, and MITM paths establish reachability. Affected Versions • `Ecosystem: npm` • `Package: nodemailer` • `Confirmed affected range: <= 10.0.9` • `Latest release checked: nodemailer 10.0.9 (npm registry; vulnerable code verified at git tag v10.0.9 and in the published dist output)` • `Fix status: 10.0.10` The quadratic reassembly shape is long-standing and predates the addressparser complexity fix family, so the exact introducing release is not positively established and the lower bound is left open; the upper bound is verified against the current published artifact, w…