GHSA-4ffr-jq9g-5ffx - Nodemailer: Quadratic backtracking in SMTP AUTH mechanism detection regexes allows pre-auth remote denial of service

## 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. ```text 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