Jackson Deserialization Validation

Jackson Deserialization Validation

Vulnerability Pattern

When Jackson deserializes JSON with unknown properties, if not handled properly, malicious extra properties can bypass validation logic:

{
  "validProperty": "legitimate value",
  "": {
    "enabled": true,
    "other": "malicious"
  }
}

The empty key "" can be overlooked by simple property checks.

Jackson Configuration

1. Fail on Unknown Properties

@JsonIgnoreProperties(ignoreUnknown = false)
public class MyFilter {
    @JsonProperty
    private String function;

    // This will throw exception on unknown properties
}

2. Custom Deserialization

@JsonDeserialize(using = CustomDeserializer.class)
public class MyFilter {
    // Custom logic during deserialization
}

3. @JsonAnySetter for Validation

public class MyFilter {
    @JsonProperty
    private String function;

    @JsonAnySetter
    public void handleUnknown(String key, Object value) {
        if (key == null || key.isEmpty()) {
            throw new IllegalArgumentException("Empty property keys not allowed");
        }
        // Validate other unknown properties
    }
}

Validation Strategies

Strategy 1: Whitelist Allowed Keys

private static final Set ALLOWED_KEYS =
    Set.of("function", "type", "name");

@JsonAnySetter
public void handleProperty(String key, Object value) {
    if (!ALLOWED_KEYS.contains(key)) {
        throw new IllegalArgumentException(
            "Unknown property: " + key);
    }
}

Strategy 2: Reject Empty/Null Keys

@JsonAnySetter
public void validateKey(String key, Object value) {
    if (key == null || key.trim().isEmpty()) {
        throw new IllegalArgumentException(
            "Property keys cannot be empty or null");
    }
}

Strategy 3: Post-Deserialization Validation

@JsonProperty
private Map properties;

@PostConstruct
public void validate() {
    for (String key : properties.keySet()) {
        if (key == null || key.isEmpty()) {
            throw new IllegalArgumentException(
                "Empty keys not allowed");
        }
    }
}

Implementation in Druid

Finding Vulnerable Classes

1. Search for @JsonTypeName("javascript")
2. Look for JavaScript filter/transform specs
3. Check deserialization of filter objects

Key Locations

• JavaScriptFilter.java - Main JavaScript filter class
• Filter spec handlers in indexing-service
• Transform spec deserialization

Fix Pattern

@JsonIgnoreProperties(ignoreUnknown = false)
public class JavaScriptFilter {
    @JsonProperty
    private String function;

    @JsonAnySetter
    public void handleUnknown(String key, Object value) {
        throw new IllegalArgumentException(
            "Unknown property '" + key +
            "' in JavaScript filter specification");
    }
}

Testing the Fix

Test Case 1: Valid Request

String json = "{\"type\":\"javascript\",\"function\":\"function(){return true;}\"}";
JavaScriptFilter filter = mapper.readValue(json, JavaScriptFilter.class);
// Should succeed

Test Case 2: Empty Key (Should Fail)

String json = "{\"type\":\"javascript\",\"function\":\"...\",\"\":{\"enabled\":true}}";
assertThrows(JsonMappingException.class, () ->
    mapper.readValue(json, JavaScriptFilter.class));
// Should throw exception

Test Case 3: Null Key (Should Fail)

String json = "{\"type\":\"javascript\",null:{\"value\":\"bad\"}}";
assertThrows(JsonMappingException.class, () ->
    mapper.readValue(json, JavaScriptFilter.class));

Druid-Specific Considerations

1. Custom ObjectMapper: Druid may use custom ObjectMapper configuration
2. Multiple Filter Types: Fix may need to apply to multiple filter classes
3. Backward Compatibility: Ensure legitimate requests still work
4. Nested Specs: Check if transformSpec or other nested objects also need fixes

Recommended Approach

1. Use @JsonIgnoreProperties(ignoreUnknown = false) to detect unknown properties
2. Add @JsonAnySetter to validate property names
3. Reject any empty string keys explicitly
4. Test both via HTTP API and direct deserialization

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: cxcscmu
• Source: cxcscmu/SkillLearnBench
• License: MIT
• Homepage: https://cxcscmu.github.io/SkillLearnBench

Install and usage instructions live in the source repository linked above.