Unexpected behavior with removeAdditional and minProperties

[Vladislao]

What version of Ajv are you using? Does the issue happen if you use the latest version?
ajv@8.12.0

Ajv options object

{
  removeAdditional: true
}

JSON Schema

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "additionalProperties": false,
  "minProperties": 1,
  "type": "object",
  "properties": {
    "good": {
      "type": "string"
    }
  },
  "required": []
}

Sample data

{ "bad": "will be removed" }

Your code

const Ajv = require("ajv");
const ajv = new Ajv(options);

const validate = ajv.compile(schema);

console.log(validate(data));
console.log(validate.errors);
console.log(data);

Working code sample:
https://runkit.com/vladislao/65c2a14a50acdd0009747ee5

Validation result, data AFTER validation, error messages

true
null
{}

What results did you expect?

The validation is expected to fail given the constraints of minProperties: 1 and additionalProperties: false. While it may be apparent in isolated examples that removing properties can lead to such behavior, the issue becomes less obvious when Ajv is used indirectly, such as in Fastify where Ajv is the default validation tool. This situation can easily lead to errors, allowing empty objects to slip through the validation process and potentially causing unexpected behavior and security issues in applications.

Related to fastify/fastify#5104

[Vladislao]

Just to clarify: the combination of additionalProperties: false and removeAdditional: true effectively disables the minProperties constraint, since it will be a no-brainer to submit improper data to bypass the constraint.

[jasoniangreen]

Hi @Vladislao I have confirmed the behaviour you have highlighted, thanks for bringing it to our attention. It is a tough one because it all comes down to the order of evaluation of the various parts of this schema / options. Changing it could impact thousands of projects relying on the specific current order.

There is a way to control the order of execution by using the allOf keyword. You can see this example here where I have split the minProperties from the rest of the schema to ensure it is evaluate after the property is removed.

https://runkit.com/jasoniangreen/ajv-issue-2376

I will continue to think about it however to understand if something can be improved.

[jasoniangreen]

Given the risk of changing the order of execution I think this is a wontfix.