> For the complete documentation index, see [llms.txt](https://webauthn-doc.spomky-labs.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://webauthn-doc.spomky-labs.com/pure-php/advanced-behaviours/authenticator-counter.md).

# Authenticator Counter

The authenticators may have an internal counter. This feature is very helpful to detect cloned devices.

The default behavior is to reject the assertions. This might cause some troubles as it could reject the real device whilst the fake one can continue to be used.

It is therefore required to go deeper in the protection of your application by logging the error and locking the associated account.

To do so, you have to create a custom Counter Checker and inject it into your Authenticator Assertion Response Validator. The checker must implement the interface `Webauthn\Counter\CounterChecker`.

{% code lineNumbers="true" %}

```php
<?php

declare(strict_types=1);


namespace App\Service;

use App\SecuritySystem;
use Throwable;
use Webauthn\Counter\CounterChecker;
use Webauthn\CredentialRecord;

final class CustomCounterChecker implements CounterChecker
{
    public function __construct(private SecuritySystem $securitySystem)
    {
    }

    public function check(CredentialRecord $credentialRecord, int $currentCounter): void
    {
        try {
            assert($currentCounter > $credentialRecord->counter, 'Invalid counter.');
        } catch (Throwable $throwable) {
            $this->securitySystem->fakeDeviceDetected($credentialRecord);
            throw $throwable;
        }
    }
}
```

{% endcode %}

The Counter Checker service can be injected into your Ceremony Step Manager Factory.

{% code lineNumbers="true" %}

```php
<?php

declare(strict_types=1);

use Webauthn\CeremonyStep\CeremonyStepManagerFactory;

$csmFactory = new CeremonyStepManagerFactory();
$csmFactory->setCounterChecker($customCounterChecker);
```

{% endcode %}


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://webauthn-doc.spomky-labs.com/pure-php/advanced-behaviours/authenticator-counter.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.