GeoServer is an open source server that allows users to share and edit geospatial data. From version 2.26.0 to before 2.26.2 and before 2.25.6, an XML External Entity (XXE) vulnerability was identified. The application accepts XML input through a specific endpoint /geoserver/wms operation GetMap. However, this input is not sufficiently sanitized or restricted, allowing an attacker to define external entities within the XML request. This issue has been patched in GeoServer 2.25.6, GeoServer 2.26.3, and GeoServer 2.27.0.