paypal-ipn before 3.0.0 uses the `test_ipn` parameter (which is set by the PayPal IPN simulator) to determine if it should use the production PayPal site or the sandbox.
CVSS 3.0MEDIUM 5.9·Network·No auth·No interaction·EPSS 0.01129·Published 2018-05-29·Fixed in 3.0.0CWE-287