The vulnerability stems from insecure handling of cross-protocol redirects in the redirect.js module. The key vulnerable code was in Redirect.prototype.onResponse where protocol changes were allowed without validation. The commit diff shows the addition of 'allowInsecureRedirect' check to gate this behavior, confirming this was the vulnerable path. The CVE description explicitly cites cross-protocol redirect handling as the exploit mechanism, and the patch directly modifies this function to add security controls.