# cf.tls\_client\_auth.cert\_serial

`cf.tls_client_auth.cert_serial` String

Serial number of the certificate in the request.

This field is only filled in if the request includes a client certificate for [mTLS authentication](https://developers.cloudflare.com/ssl/client-certificates/enable-mtls/).

Example value:

```txt
"527E0F20A20EA2A4146C78390F34CE7AF0878CA4"
```

Categories:

* Request
* mTLS
