---
title: cf.tls_client_auth.cert_issuer_serial
description: Serial number of the direct issuer of the mTLS client certificate.
image: https://developers.cloudflare.com/core-services-preview.png
---

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

`cf.tls_client_auth.cert_issuer_serial` ` String ` 

Serial number of the direct issuer of the mTLS client certificate.

This field defaults to `""` if the connection does not use [mTLS authentication](https://developers.cloudflare.com/ssl/client-certificates/enable-mtls/).

Example value:

```
"2688201DBA77402EA87118876F2E1B24CF8B0395"
```

Categories: 
* Request
* mTLS

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