Array schema in a response has no type of items defined

Issue ID: schema-response-array-items-notype

Average severity: High

Description

One or more array schemas does not specify the type of items it can contain.

Open API Specification (OAS) v2 requires the type property for items objects. However, most OAS v2 validators do not raise an error on items that do not have type set.

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": {
                    }
                }
            }
        }
    }
}

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

Make sure your array schema includes the type property for the items the array accepts:

{
    "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"
                    }
                }
            }
        }
    }
}