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

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.