In github.com/prebid/prebid-server/v4 before 4.4.0, multiple bidder adapters (acuityads, bidmachine, relevantdigital, smartyads, tappx, trafficgate, zeroclickfraud) interpolate user-supplied host and subdomain parameters into outbound request URLs without validation. A remote attacker can craft bid request parameters containing path traversals or target addresses to trigger server-side request forgery (SSRF), sending HTTP requests to internal network services or unintended endpoints.