A restricted cross-site scripting vulnerability was reported in Apache HTTP Server 2.4.0-2.4.39, affecting the mod proxy error page. An attacker might make the link on the error page faulty, causing it to redirect to a URL of their choosing. This would only be exploitable if a server was set up with proxying enabled but the Proxy Error page was displayed due to a configuration error.
Update Apache to the latest version
Correctly configure the Proxy settings