Nodemailer: Improper TLS Certificate Validation in OAuth2 Token Fetch Enables Credential Interception

### Summary Nodemailer disables TLS certificate verification in its internal HTTPS fetch client through the use of rejectUnauthorized: false inside lib/fetch/index.js. As a result, OAuth2 token requests trust invalid or self-signed HTTPS certificates and transmit sensitive OAuth credentials over connections that should fail TLS validation. An attacker in a machine-in-the-middle position can intercept OAuth2 credential exchanges and capture: - OAuth client_secret - refresh_token - access tokens The issue was verified through runtime testing using a self-signed HTTPS OAuth endpoint. ### Details Root Cause The issue originates from the internal HTTPS fetch implementation used by Nodemailer for OAuth2 token retrieval and related outbound HTTPS requests. Inside: `lib/fetch/index.js` the request options contain: `rejectUnauthorized: false` This disables TLS peer certificate verification globally for the internal HTTPS client unless explicitly overridden through optional TLS configuration. As a result: - self-signed certificates are trusted - invalid CA chains are accepted - hostname validation is bypassed - attacker-controlled HTTPS endpoints are treated as trusted This violates expected HTTPS security guarantees. **Vulnerable Flow** The vulnerable execution chain is: OAuth2 Transport ↓ XOAuth2 token generation ↓ Internal HTTPS fetch client ↓ HTTPS request with rejectUnauthorized:false ↓ Attacker-controlled/self-signed endpoint trusted ↓ OAuth credentials **transmitted** ### PoC **