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The right way to fix “The certificate is not trusted” error in Plesk

VPS or Dedicated server owners who use Plesk sometimes receive the error “The certificate is not trusted” when accessing the Plesk panel.

This is due to Plesk using a “self-signed” certificate – which means that the certificate is not an authentic CA certificate issued by a recognized authority like Verizon or Comodo.

There are two ways to fix this error:

  1. Configure the Plesk user’s browser to accept the un-trusted certificate.
  2. Use an authentic CA certificate.

Unfortunately, the more popular way is to make visitors add an “Exception” rule in their browsers to trust the un-trusted certificate.

That is a dangerous road to take.

Let’s see why, with an example.


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Why you shouldn’t use a self-signed certificate

Here at Bobcares, we help server owners resolve Web, Mail and Database errors through our On-demand Server Administration services.

A few days back, we received a support request from a web hosting providers whose customers were seeing “The certificate is not trusted” error in Google Chrome, Firefox, and other browsers.

He wanted to display a message in Plesk interface that guides users to trust the self-signed certificate.

However, we recommended against it because:

  • Customers learn an un-safe practice – When a service provider recommends users to ignore a certificate warning, they’ll feel it’s OK to do it everywhere. That’ll make users susceptible to hacking, and might open up their website to malware. They’ll just consider the service provider to be unsafe.
  • Shows the company in poor light – HTTPS green light is becoming the norm everywhere. If a hosting provider cannot protect their own servers with a green light, their ability to secure customer accounts become immediately suspect.
  • A valid CA certificate is free of cost   A few years back, CA certificates used to cost hundreds of dollars. Not anymore. These days you can get a free certificate from Let’s Encrypt CA, which is an authentic CA recognized by all browsers.

We chose to replace the self-signed certificate with an authentic Let’s Encrypt certificate.

Here’s how we did it.


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Fixing Plesk error “The certificate is not trusted” with Let’s Encrypt

In a previous post we explained how to setup Let’s Encrypt in Linux servers.

Plesk servers provide a similar functionality with a Let’s Encrypt extension.

So, the first step was to install the extension:


1. Install Let’s Encrypt extension

For this we downloaded the zip file from Let’s Encrypt extension repository.

Then the zip file was uploaded via “Server Management >> Extensions >> Add Extension” in the admin interface.


2. Enabling Let’s Encrypt certificate

We entered the Administrator Email ID under “Tools & Settings >> Security >> SSL/TLS Certificates >> Let’s Encrypt” and clicked “Install”.

This generated a certificate and assigned it to Plesk interface.


3. Bonus – Securing the mail server

It is not enough to secure the Plesk administrator interface. Users might see certificate warning when accessing the mail services.

To secure mail services using the same certificate, we chose the “Let’s Encrypt certificate (server pool)” from the drop down list near “Certificate for securing mail” option.

At this point, both Plesk and mail services were secured with a valid CA certificate.



“The certificate is not trusted” is a common error in Plesk servers. The popular solution is to add an exclusion rule in browsers, but that is an unsafe practice that could leave customers vulnerable to malware and hacking. Today we’ve seen how to resolve this error using a valid CA certificate from Let’s Encrypt that doesn’t cost a penny.




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