GeoServer is an open source server that allows users to share and edit geospatial data. Prior to versions 2.26.4 and 2.27.3, a GeoServer that uses `ENTITY_RESOLUTION_ALLOWLIST` may allow attacker to perform unauthenticated Server-Side Request Forgery (SSRF). This vulnerability requires that GeoServer is set up to use a proxy base URL and the `ENTITY_RESOLUTION_ALLOWLIST` (default since 2.25.0). Versions 2.26.4 and 2.27.3 contain a fix. GeoServer installations are only affected by this vulnerability if they use a proxy base URL that does not contain a URL path or end with a slash. If the proxy base URL does not contain a path, adding a slash to the end of the URL will mitigate this vulnerability.