Saloon is vulnerable to SSRF and credential leakage via absolute URL in endpoint overriding base URL

Summary

Saloon is a PHP library that gives users tools to build API integrations and SDKs. Prior to version 4.0.0, when building the request URL, Saloon combined the connector's base URL with the request endpoint. If the endpoint was a valid absolute URL, the code used that URL as-is and ignored the base URL. The request—and any authentication headers, cookies, or tokens attached by the connector—was then sent to the attacker-controlled host. If the endpoint could be influenced by user input or configuration (e.g. redirect_uri, callback URL), this allowed server-side request forgery (SSRF) and/or credential leakage to a third-party host. The fix in version 4.0.0 is to reject absolute URLs in the endpoint: URLHelper::join() throws InvalidArgumentException when the endpoint is a valid absolute URL, unless explicitly allowed, requiring callers to opt-in to the functionality on a per-connector or per-request basis.