To provide web application scan results for external endpoints in the AppExchange Security Review, you need to use a Dynamic Application Security Test (DAST) scanner, such as ZAP, Burp Suite, HCL AppScan, or WebInspect.
Make sure the external endpoints are within the scope of the security review, which includes endpoints that authenticate users or transfer Salesforce data. Include the DAST scan reports in your security review submission. If the external endpoints are not owned by you, you must obtain the necessary permissions from the third-party owners before conducting the scans.