How to quickly resolve SEC_ERROR_UNKNOWN_ISSUER error in Firefox?


The Mozilla Firefox browser always checks the validity of the certificate that the securely connected site provides (in particular, starting with https). If the certificate does not match the database or it simply cannot be verified for any reason, then Firefox issues a SEC_ERROR_UNKNOWN_ISSUER error code along with a message like “This connection is untrusted,” trying to protect the PC from a dangerous connection to the resource. There are several ways to fix it.



One of the reasons for the error is the complete absence of a certificate from the site. It may be infected with viruses, so it is not recommended to switch to it. However, if a security certificate is present, why it may not be appropriate?

The Firewall browser does not trust the certificate of the site that the user wants to access.

Error SEC_ERROR_UNKNOWN_ISSUER appeared suddenly and began to appear more and more when loading pages that are not related to each other? This means one thing: an intermediate link is being introduced into the connection, offering its own certificates that Firefox does not trust. The link may be viral software, the antivirus itself, the ad blocker and the proxy server.

If the site has a certificate and it is with it that the download fails constantly, then this means that the page has a certificate, but a self-signed one that is not in the database and is considered by default as a browser unreliable. Often the reason lies in the corruption of the file with the identifiers Cert8.db. In this case, you should resort to a particular solution method.


Before using the following three methods, it is necessary to exclude the presence of a virus in the PC. To do this, go to the antivirus and scan the system. If everything is in order, you can safely move on to the methods presented below.

How to disable SSL scanning


If anti-virus software is an intermediate link that prevents pages from loading, then disabling network scanning will help. The software will stop intercepting encrypted connections with HTTPS.

To find out for sure that the reason is an antivirus, you need to pause its work for a while. Next, check to see if the error has disappeared in the browser. If the site is working, you need to follow these instructions on the example of the Eset Smart Security Antivirus.

Enter the antivirus software by clicking on the program icon, which is located on the right in the bottom panel.

Open antivirus window

Press F5. The “Advanced Options” window opens.

To open the window with the anti-virus settings, press F5

Next, go to the section “INTERNET AND E-MAIL.”

Open the Internet and E-mail tab to disable further scanning of sites protected by a cipher.

Select the second SSL / TLC pop-up menu. Click the check mark to the right of “Enable SSL / TLC protocol filtering.”

Open the SSL / TLC menu and click on the check mark to disable filtering.

Updating the Cert8.db file in the Firefox profile folder

If the issue is not in the antivirus (disabling it does not lead to the disappearance of the warning), it is worthwhile to suspect that the Cert8.db file in the Firefox profile, which contains information about the security certificates, has been damaged. The logical way out is to delete the file manually. You should not worry about its absence, because when it is deleted, the browser will automatically create its new working version. Thus, there will be, in other words, “updating” of information on certificates.

Go to the browser menu and click on the question mark.

Open the Firefox menu and click on the question mark.

Further, the choice will fall on the section “Information for solving problems.”

Select the item “Information for problem-solving.”

Click on “Show folder” to the right of the blue column to open the profile folder. Before you start working with the folder that appears, close Mozilla.

Choose “Show Folder” to open the folder folder

In the window, find cert8.db. Right-click on a file to open a list of possible actions with it. Click “Delete.” Next, open Mozilla Firefox and check if the error appears.

Find the file cert8.db and select “Delete.”

Adding a site to the exclusion list


Often, sites that are not accessible to the general public have self-signed certificates that are not recognized by the certification authority, so there is no trust in them by default. However, they also protect data against eavesdropping and leakage.

If the crash happened only once when loading one site, you could fix the problem by adding a site with an error in the exceptions of the Firefox browser.

Warning: You should never add a certificate exception to well-known websites or sites where financial transactions are involved — in this case, an invalid certificate may indicate that a third party has compromised your connection.

In the tab with the error to find the item “Advanced” (Advanced) or “I understand the risk.” In different versions of the browser message is different, so do not panic. Next, click on “Add Exception” (Add Exception). The browser will ask you to confirm the action. Click on the appropriate button.

Click “Add Exception” in the Advanced drop-down menu to open the site.

Now you can safely go to the page without obstacles.

Video: how to easily fix the error code SEC_ERROR_UNKNOWN_ISSUER

How to avoid reappearance?

In some cases, it is possible to prevent the occurrence of an error. For this you need a number of preventive measures:

  1. Timely update of the Firefox browser and antivirus on the computer to ensure normal working conditions.
  2. Regular scanning for PC viruses, as they often cause an error in the browser.
  3. Visiting sites with a good reputation, especially those that have HTTPS with support for strong encryption, as they provide security by protecting against viruses, eavesdropping and information leaks.

The SEC_ERROR_UNKNOWN_ISSUER error, which occurs quite often when working with Firefox, has several reasons, as well as how to fix it, depending on the factor that causes it. However, the first action on this error is to restart the PC and check it for viruses using antivirus software.