Schema does not define the type of JSON values
The API defines a schema for a JSON payload, but the schema does not actually limit what is accepted. This can happen for two reasons:
- The schema is for an object (a set of properties) but it does not have
object. The properties are matched to the ones defined in the schema, but no limit to their types is enforced.
- Some of the properties themselves do not have
typedefined so any type is accepted for them.
For more details, see the OpenAPI Specification.
The following is an example of how this type of risk could look in your API definition. The schema
NewPet is defined in
#/definitions and referenced as the
schema for the API method. However, the type of the schema is not set to
object and the type is missing from the property
1post: 2 description: Creates a new pet in the store 3 operationId: addPet 4 parameters: 5 name: pet 6 in: body 7 description: pet to add to the system 8 required: true 9 schema: 10 $ref: "#/definitions/NewPet" 11# ... 12NewPet: 13 required: 14 - name 15 properties: 16 name: 17 description: Pet name 18 type: string 19 tag: 20 description: Pet tag 21
Possible exploit scenario
If you do not clearly define the schema and you leave properties of a JSON payload empty, you effectively allow attackers to pass in any data. This means that you are opening your backend to various attacks, such as SQL injection.
This also lets attackers to try various unexpected inputs. Unexpected inputs may cause the backend server to crash or behave in an unexpected way. This in turn may cause the server to potentially leak stack trace that can be used for further attacks, or even data.
If no restrictions to the set of properties in the JSON payload are enforced, the API might also accept more fields than expected. The received payloads could be blindly transformed into an object and stored, overwriting sensitive internal data. For more details, see API6:2019 — Mass assignment in OWASP API Security Top 10.
Make sure you define the types of all properties in the JSON payload and correctly set the type for objects to enforce limitations to what the schema accepts.
1post: 2 description: Creates a new pet in the store 3 operationId: addPet 4 parameters: 5 name: pet 6 in: body 7 description: pet to add to the system 8 required: true 9 schema: 10 $ref: "#/definitions/NewPet" 11# ... 12NewPet: 13 type: object 14 required: 15 - name 16 properties: 17 name: 18 type: string 19 description: Pet name 20 tag: 21 type: string 22 description: Pet tag 23
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