Jackson @JacksonInject Security

The Vulnerability

When using @JacksonInject in Jackson @JsonCreator constructors, JSON input can override server-side injected values by default. This happens because @JacksonInject defaults to useInput = OptBoolean.DEFAULT, which allows JSON properties to take precedence over injected values.

Attack Mechanism

If a class has:

@JsonCreator
public MyClass(@JacksonInject MyConfig config) { ... }

An attacker can send JSON with "": {"enabled": true} to override the injected config value. The empty string key "" maps to inject parameters that have no explicit ID.

Combined with FAIL_ON_UNKNOWN_PROPERTIES = false (common in many frameworks), the attack payload passes silently.

Fix: useInput = OptBoolean.FALSE

Jackson 2.9+ introduced OptBoolean for @JacksonInject:

import com.fasterxml.jackson.annotation.OptBoolean;

@JsonCreator
public MyClass(
    @JacksonInject(useInput = OptBoolean.FALSE) MyConfig config
) { ... }

This prevents JSON input from overriding the injected value entirely.

Additional Defense: Constructor Validation

Add validation in the constructor to check the config before storing it:

@JsonCreator
public MyClass(
    @JacksonInject(useInput = OptBoolean.FALSE) MyConfig config
) {
    Preconditions.checkState(config.isEnabled(), "Feature is disabled");
    this.config = config;
}

Jackson Version Compatibility

• OptBoolean.FALSE for @JacksonInject(useInput=...) requires Jackson 2.9+
• Check with: <jackson.version> in pom.xml