NL Portal Backend Libraries: Unauthenticated form resolver forwards the privileged Objecten-API token to a caller-supplied URL (SSRF)

NL Portal Backend Libraries provide backend components for Dutch government portals that interact with residents, customers, suppliers, and partner organizations. From 1.1.0 until 3.0.4, and in 3.1.0, the public getFormDefinitionByObjectenApiUrl and deprecated getFormDefinitionById GraphQL resolvers are reachable without CommonGroundAuthentication and forward a caller-influenced URL through ObjectenApiService.getObjectByUrl to ObjectsApiClient.getObjectByUrl, which attaches the privileged Objecten-API token. The host-equality check restricts requests to the configured Objecten-API host but does not constrain the scheme, port, or path, allowing an unauthenticated caller to induce a token-bearing request to a chosen endpoint on that host. Standard deployments are not exposed to cross-host SSRF, token capture requires attacker-controlled routing on the configured host, and typed deserialization limits returned data to objects shaped like form definitions. The 3.0 release line is fixed in version 3.0.4. The advisory does not identify a fixed 3.1 release.