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

Summary

The public GraphQL resolvers getFormDefinitionByObjectenApiUrl(url) and the deprecated getFormDefinitionById(id) fetch a caller-supplied URL using the privileged Objecten-API token. Because the /graphql endpoint is permitAll() and these resolvers do not declare a CommonGroundAuthentication parameter, an unauthenticated caller can make the backend issue an outbound request carrying Authorization: Token <objecten-api-token> to a caller-influenced URL on the configured Objecten-API host. This is a constrained (same-host) server-side request forgery combined with missing authorization.

Reported responsibly and confirmed in a local lab build against the project's own WebFlux security stack. No production system was accessed.

Affected

• nl.nl-portal:form (the public resolver / entry point) together with nl.nl-portal:objectenapi (where the host guard lives).
• First shipped in 1.1.0.RELEASE (2023-10-31); the vulnerable code was introduced on 2023-08-12 (commit b2f87ca) and is present in every release since (1.1.x, 1.2.5, 1.3.0, the 3.0.x line, and 3.1.0 / next-minor, HEAD 45abcd2). Fixed in 3.0.4.RELEASE (see Fix below).

Data flow (confirmed in source)

1. form/.../graphql/FormDefinitionQuery.kt — @QueryMapping getFormDefinitionByObjectenApiUrl(@Argument url), no CommonGroundAuthentication parameter (same for getFormDefinitionById).
2. → form/.../service/ObjectsApiFormDefinitionService.kt — passes the URL through unvalidated.
3. → zgw/objectenapi/.../service/ObjectenApiService.kt getObjectByUrl(url) — the only guard is host equality (URI.create(url).host == objectsApiClientConfig.url.host); no scheme/port/path check.
4. → zgw/objectenapi/.../client/ObjectsApiClient.kt getObjectByUrl(url) via webClientWithoutBaseUrl(), which attaches the default header Authorization: Token <token> to the fully caller-supplied URL.

Reachability: /graphql is permitAll() (core/.../security/OauthSecurityAutoConfiguration.kt). Authentication is only enforced on resolvers that declare a CommonGroundAuthentication parameter; these do not, and there is no @PreAuthorize/instrumentation safety net. The project's own GraphQLEndpointAuthorizationIT lists getFormDefinitionByObjectenApiUrl as an intentionally public operation — so the unauthenticated reachability is by design; the defect is that an intentionally-public resolver forwards a privileged token to a caller-influenced URL.

Secondary (defense-in-depth): zgw/zaken-api/.../service/ZakenApiService.kt getZaakDetails calls objectsApiClient.getObjectByUrl directly, bypassing the service-level host guard. It is currently only reachable via the authenticated ZaakQuery.zaakdetails field resolver with server-derived URLs, so it is not an unauthenticated vector today — but it shows why the guard belongs in the client.

Proof of concept (lab, against the real WebFlux stack)

• An unauthenticated POST /graphql calling getFormDefinitionByObjectenApiUrl(url: ...) executes without authentication.
• With the configured Objecten-API host pointed at a mock server, an outbound request to a caller-chosen port/path on that host carried Authorization: Token <configured-token> — confirming the token is attached to caller-influenced URLs.

Impact and severity — important limitations

Assessed as Medium because two code-level facts constrain practical impact:

1. No cross-host SSRF / token exfiltration in standard deployments. The token only travels to the configured Objecten-API host. Exfiltration requires an attacker-controlled listener at that host (a different port/path routing elsewhere) — generally not the case in managed deployments. A range of URL-parser bypass payloads was tested (userinfo @, %2f/%00/%09, backslash, #/?, double-host, trailing-dot, IDN/Unicode full-stop, fraction-slash, IPv6); no parser differential was found between the java.net.URI-based guard and the Spring/Netty URI builder used by WebClient — every payload either kept the request on the configured host or was rejected (fail-closed). The lab token-leak PoC works only because the configured host there is localhost; this does not generalize to production.

2. Arbitrary PII object read is blocked by typed deserialization. The response is deserialized into ObjectsApiObject<ObjectsApiFormIoFormDefinition>, whose envelope fields and data.formDefinition are all non-nullable Kotlin properties (Jackson KotlinModule registered). An object without a top-level data.formDefinition (e.g. taken/berichten/zaakdetails) fails to deserialize (DecodingException) and returns no data. The resolver can therefore only return objects shaped like a form definition — and form definitions are intentionally public (loaded pre-login).

Escalation conditions that would raise severity toward High:

• the Objecten-API host shares infrastructure with an attacker-controllable endpoint (other port/path), enabling capture of the privileged token; or
• a URL-parser differential is later found that escapes the host guard.

Remediation

• Move the host validation out of ObjectenApiService.getObjectByUrl and into ObjectsApiClient.getObjectByUrl so the direct caller ZakenApiService.getZaakDetails is covered too, and tighten it from host-only to scheme + host + port + path-prefix. Preferably, do not accept a full URL at all: validate/extract the object UUID and rebuild the URL from the fixed configured base (reuse the existing ObjectsApiClient.getObjectById pattern, /api/v2/objects/{uuid}).
• Separately decide whether getFormDefinitionByObjectenApiUrl / getFormDefinitionById should remain unauthenticated. They are currently intentionally public (forms load before login); for a stricter posture, add a CommonGroundAuthentication parameter as in the other resolvers — noting this breaks pre-login form loading.

Credit

Reported responsibly by Ray Sabee (https://whitehatsecurity.nl), independent security researcher — GitHub @raysabee.

Fix

Fixed in 3.0.4.RELEASE (commit 39ad80f, PR #700, "rework form module"):

• The unauthenticated resolvers getFormDefinitionByObjectenApiUrl and the deprecated getFormDefinitionById were removed from both FormDefinitionQuery and the GraphQL schema.
• getFormDefinitionByName now requires a CommonGroundAuthentication parameter (no longer public).
• The URL-based service method findObjectsApiFormDefinitionByUrl(url) was removed and replaced by getObjectsApiFormDefinitionById(objectId: UUID), which fetches by UUID via the fixed /api/v2/objects/{uuid} path (no caller-supplied URL, so no SSRF) and validates the object type against the configured form-definition object type.
• Form definitions are now retrieved through the new authenticated query getFormDefinitionByTaskId(taskId) in nl.nl-portal:taak, which authorizes the caller against the task (CommonGroundAuthentication, BSN/KVK match, else 401) and derives the form-definition UUID from the task's own server-side data, not from caller input.
• No resolver feeds caller-controlled input into ObjectenApiService.getObjectByUrl anymore. The objectenapi module itself was not changed; the fix lives entirely in nl.nl-portal:form and the new nl.nl-portal:taak query.

Upgrade instructions

• Backend: upgrade nl.nl-portal:* to 3.0.4 (or later).
• Frontend: upgrade nl-portal-frontend-libraries to v3.0.3 (or later). This is required: the removed GraphQL queries (getFormDefinitionByObjectenApiUrl, getFormDefinitionById) and the now-authenticated getFormDefinitionByName are a breaking change. Frontend v3.0.3 uses the new authenticated getFormDefinitionByTaskId / getFormDefinitionByName queries.