Array schema in a response has no maximum number of items defined

Issue ID: schema-response-array-maxitems

Average severity: High

Description

An array schema does not specify the maximum number of items it can contain.

For more details, see the OpenAPI Specification.

Example

The following is an example of how this type of risk could look in your API definition:

{
    "get": {
        "description": "Returns all pets from the system that the user has access to",
        // ...
        "responses": {
            "200": {
                "description": "pet response",
                "schema": {
                    "type": "array",
                    "items": {
                        "$ref": "#/definitions/Pet"
                    }
                }
            }
        }
    },
    // ...
    "definitions": {
        "Pet": {
            "type": "object",
            "additionalProperties": false,
            "required": [
                "name"
            ],
            "properties": {
                "name": {
                    "type": "string"
                },
                "favfood": {
                    "type": "array",
                    "items": {
                        "type": "string"
                    }
                }
            }
        }
    }
}

Possible exploit scenario

Attackers strive to make your APIs behave in an unexpected way to learn more about your system or to cause a data breach. Good data definition quality in the schemas used in API responses allows reliably validating that the contents of outgoing API responses are as expected.

While filtering API responses does not block a specific kind of attack, it is there as a damage control mechanism in the unfortunate event that a successful attack has been conducted: it allows blocking the response and prevent attackers from retrieving data they should not access.

In the vast majority of cases (with the notable exception of Denial of Service (DoS and DDoS) attacks) attacks are conducted because attackers want to access data or resources they should not have access to. Often, this means that the structure or the size of the API response changes as a result of a successful attack, compared to a normal API response.

Validating that API responses are as expected can be achieved through proper schema validation of the API responses. The accuracy of this depends on the quality of the response schemas: the better defined your schemas are, the easier it is to detect when something is not right.

Remediation

Set the maxItems property to ensure that the schema only allows calls of reasonable size:

{
    "get": {
        "description": "Returns all pets from the system that the user has access to",
        // ...
        "responses": {
            "200": {
                "description": "pet response",
                "schema": {
                    "type": "array",
                    "items": {
                        "$ref": "#/definitions/Pet"
                    }
                }
            }
        }
    },
    // ...
    "definitions": {
        "Pet": {
            "type": "object",
            "additionalProperties": false,
            "required": [
                "name"
            ],
            "properties": {
                "name": {
                    "type": "string"
                },
                "favfood": {
                    "type": "array",
                    "maxItems": 3,
                    "items": {
                        "type": "string"
                    }
                }
            }
        }
    }
}