Pattern for string schema is too loose

Average severity: Medium

Description

The pattern for a string schema is too loosely defined. It does not actually limit what gets passed to the API.

Example

The following is an example of how this type of risk could look in your API definition. The defined pattern is so loose that the API effectively accepts any string of any size and value:

"post": {
  "description": "Creates a new pet in the store",
  "operationId": "addPet",
  "parameters": [
    {
      "name": "pet",
      "in": "body",
      "description": "Pet to add to the store",
      "required": true,
      "schema": {
        "type": "object",
        "required": [
          "name"
        ],
        "properties": {
        "name": {
          "type": "string",
          "pattern": ".*"
        }       
      }
    }
   ],

Possible exploit scenario

If you define too loose pattern for strings, you do not actually limit what is accepted as the input. This could open your backend server to various attacks, like SQL injections or buffer overflows.

Remediation

Set a well-defined regular expression in the pattern field of the string schema. This ensures that only strings matching the pattern get passed to your API.

For example, the API below only accepts strings that are compliant with RFC 4122:

"post": {
  "description": "Creates a new pet in the store",
  "operationId": "addPet",
  "parameters": [
    {
      "name": "pet",
      "in": "body",
      "description": "Pet to add to the store",
      "required": true,
      "schema": {
        "type": "object",
        "required": [
          "name"
        ],
        "properties": {
        "name": {
          "type": "string",
          "pattern": "/^[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89AB][0-9a-f]{3}-[0-9a-f]{12}$/i"
        }       
      }
    }
   ],

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