threat-modeling.md

Threat Model Analysis for dart-lang/json_serializable

Threat: Threat 1: Arbitrary Object Instantiation via Type Manipulation

• Description: An attacker crafts a malicious JSON payload that specifies a different Dart type than the one expected by the application. The attacker leverages json_serializable's reliance on type information within the JSON (e.g., a hypothetical "$type" field) to instantiate an arbitrary class. If this attacker-controlled class has side effects in its constructor or fromJson factory, those side effects are executed.
  • Impact: This can lead to arbitrary code execution if the attacker-chosen class has methods that perform sensitive actions (e.g., file system access, network connections, command execution) that are triggered during object creation or deserialization. This is a complete system compromise.
  • Affected Component: The fromJson factory method generated by json_serializable. Specifically, the code that uses type information from the JSON to determine which class to instantiate. This is the core of the vulnerability.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • a. Strict Type Whitelisting (Essential): Within the fromJson factory, hardcode a whitelist of allowed types. Do not trust any type information provided in the JSON. Reject any JSON that doesn't match an allowed type. This is the primary defense.
    • b. Avoid Polymorphism with Untrusted Input: If possible, do not use polymorphic deserialization (where the JSON determines the concrete type) with data from untrusted sources. If polymorphism is absolutely necessary, the whitelist is non-negotiable.
    • c. checked: true (Secondary Defense): While not sufficient on its own, checked: true in @JsonSerializable adds a layer of runtime type checking that can help detect some mismatches after the object has been (potentially incorrectly) instantiated.

Threat: Threat 2: Denial of Service via Deeply Nested JSON

• Description: An attacker sends a JSON payload with excessively deep nesting. json_serializable's generated fromJson methods recursively deserialize this structure.
  • Impact: This can consume excessive stack space and CPU cycles, leading to a stack overflow error or general resource exhaustion, causing the application to crash or become unresponsive (Denial of Service). This directly exploits the recursive nature of the generated code.
  • Affected Component: The recursive deserialization logic within the generated fromJson methods. This is inherent to how json_serializable handles nested objects.
  • Risk Severity: High
  • Mitigation Strategies:
    • a. Depth Limiting (Custom Deserializer/fromJson Modification): The most direct mitigation is to modify the generated fromJson factory (or use a custom JsonConverter) to track the nesting depth during deserialization. If the depth exceeds a predefined limit, throw an exception or reject the input. This directly addresses the recursive nature of the problem.
    • b. Input Size Limits (Less Direct, but Important): Enforce limits on the overall JSON size before it reaches json_serializable. While this doesn't directly address the nesting issue, it provides a general defense against excessively large inputs.

Threat: Threat 3: Denial of Service via Large String/Array/Map Fields

• Description: An attacker sends JSON with extremely large strings, arrays, or maps within fields. json_serializable will attempt to allocate memory for these large fields during deserialization.
  • Impact: This can lead to excessive memory allocation, potentially causing an out-of-memory error and a denial-of-service condition. This exploits how json_serializable handles the creation of these data structures.
  • Affected Component: The generated fromJson methods, specifically the parts that handle the assignment of values to string, array, and map fields. The code generated by json_serializable is directly responsible for allocating memory for these fields.
  • Risk Severity: High
  • Mitigation Strategies:
    • a. Field-Specific Size Limits (Custom fromJson / JsonConverter): Within custom fromJson factories or JsonConverter implementations, add checks to limit the length of strings and the size of arrays and maps. This directly addresses the problem within the json_serializable-generated code.

Threat: Threat 4: Unvalidated JsonConverter Implementation

• Description: A developer implements a custom JsonConverter and introduces a vulnerability within the fromJson or toJson methods of the converter itself. This could be due to incorrect type handling, insufficient validation, or other logic errors within the custom code.
  • Impact: The impact depends on the specific vulnerability within the JsonConverter. It could range from data corruption to arbitrary code execution (if the converter handles sensitive data or operations). This is a high risk because custom converters often handle complex or security-sensitive logic.
  • Affected Component: The custom JsonConverter implementation. This is directly related to json_serializable because the vulnerability exists within a component specifically designed to integrate with it.
  • Risk Severity: High (Potentially Critical, depending on the converter's purpose)
  • Mitigation Strategies:
    • a. Thorough Testing: Extensively test the JsonConverter with a wide range of valid and invalid inputs, including edge cases and boundary conditions.
    • b. Security-Focused Code Review: Have another developer with security expertise review the JsonConverter implementation, paying close attention to potential vulnerabilities.
    • c. Follow Best Practices: Adhere to secure coding principles when implementing the JsonConverter, including input validation, error handling, and avoiding potentially dangerous operations.