Jackson @JacksonInject Security

The Vulnerability Pattern

When a @JsonCreator constructor uses @JacksonInject without useInput = OptBoolean.FALSE, Jackson can fall back to JSON input to provide the injectable value. This is exploitable when:

1. The injectable ID (default: empty string "") matches a JSON property key
2. The JSON property contains a deserializable value of the expected type
3. This overrides the server-configured injected value (e.g., security settings)

Example Vulnerable Code

@JsonCreator
public SomeFilter(
    @JsonProperty("dimension") String dimension,
    @JacksonInject SecurityConfig config  // VULNERABLE: can be overridden via JSON ""
)

Exploit Pattern

{
  "dimension": "value",
  "": {"securityEnabled": false}  // Empty key "" overrides @JacksonInject default id
}

The Fix

Option 1: useInput = OptBoolean.FALSE (Recommended for Jackson 2.9+)

import com.fasterxml.jackson.annotation.OptBoolean;

@JsonCreator
public SomeFilter(
    @JsonProperty("dimension") String dimension,
    @JacksonInject(useInput = OptBoolean.FALSE) SecurityConfig config  // FIXED
)

This tells Jackson: NEVER use JSON to provide this value, ALWAYS use InjectableValues only.

Option 2: Explicit Injectable ID (prevents empty string match)

@JacksonInject("securityConfig")  // Named ID that won't match "" key
SecurityConfig config

Option 3: Server-side validation before processing

// Validate BEFORE Jackson deserialization or AFTER using Guice-injected config
@Inject
public MyService(SecurityConfig trustedConfig) {
    this.trustedConfig = trustedConfig;  // From Guice, not from Jackson
}

@JacksonInject Behavior Summary

useInput value	Behavior
OptBoolean.DEFAULT	Use InjectableValues if present; may fall back to JSON
OptBoolean.TRUE	JSON input takes precedence over InjectableValues
OptBoolean.FALSE	ONLY use InjectableValues; NEVER read from JSON

Druid-Specific Context

In Apache Druid, Jackson InjectableValues are configured via Guice's GuiceInjectableValues. The injectable ID lookup uses the parameter type's class name by default.

When @JacksonInject.value() is "" (empty string default), Jackson may use the empty string as the lookup key, which fails to find the Guice-bound value, causing fallback to JSON.

Detection

Look for @JacksonInject without useInput = OptBoolean.FALSE in @JsonCreator constructors where the injected value controls security behavior.