Spin is the developer tool for building and running serverless applications powered by WebAssembly. Prior to 2.4.3, some specifically configured Spin applications that use "self" requests without a specified URL authority can be induced to make requests to arbitrary hosts via the "Host" HTTP header. The following conditions need to be met for an application to be vulnerable: 1. The environment Spin is deployed in routes requests to the Spin runtime based on the request URL instead of the "Host" header, and leaves the "Host" header set to its original value; 2. The Spin application's component handling the incoming request is configured with an "allow_outbound_hosts" list containing ""self""; and 3. In reaction to an incoming request, the component makes an outbound request whose URL doesn't include the hostname/port. Spin 2.4.3 has been released to fix this issue.