On websites which are supposed to be secure (i.e. the URL begins with "https://"), Firefox must verify that the certificate presented by the website is valid. If the certificate cannot be validated, Firefox will stop the connection to the website and instead show a "Your connection is not secureThis Connection is Untrusted" error message. This article will explain why you might see error code "SEC_ERROR_UNKNOWN_ISSUER" on websites and how to troubleshoot it.

During a secure connection a website needs to provide a certificate issued by a trusted certificate authority in order to ensure that the user is connected to the intended target and the connection is encrypted. If you get a "Your connection is not secureThis Connection is Untrusted" error page and see the error code "SEC_ERROR_UNKNOWN_ISSUER" after you click on AdvancedTechnical Details, this means that the certificate provided was issued by a certificate authority that is not known by Firefox and therefore cannot be trusted by default.

If you receive the error on multiple secure sites

When you see this problem on multiple unrelated HTTPS-sites, this indicates that something on your system or network is intercepting your connection and injecting certificates in a way that is not trusted by Firefox. The most common causes for this would be security software scanning encrypted connections or malware listening in, replacing legitimate website certificates with their own.

Anti-Virus Products

Generally, if your security product contains a feature to scan encrypted connections, you could try to reinstall the security product, which might trigger the software to place its certificates into the Firefox trust store again. Here are some more solutions for particular security products:

Avast

In your Avast security product you can disable the interception of secure connections like this:

1. Open the dashboard of your Avast application.
2. Go to Settings > Active Protection and click Customize next to Web Shield.
3. Uncheck the Enable HTTPS Scanning setting and confirm this ny clicking OK.

More Information about this feature is available on: https://blog.avast.com/2015/05/25/explaining-avasts-https-scanning-feature/

Bitdefender

In your Bitdefender security product you can disable the interception of secure connections like this:

1. Open the dashboard of your Bitdefender application.
2. Click on Modules (2016 version) or Protection (2015 version).
3. Click on Web Protection.
4. Toggle off the Scan SSL setting.

For corporate Bitdefender products, please refer to: http://www.bitdefender.com/support/how-to-enable-ssl-https-scanning-in-cloud-security-for-endpoints-1117.html

Eset

In your Eset security product you can try to disable and reenable SSL/TLS protocol filtering, or generally disable the interception of secure connections like described in their support article on the subject at http://support.eset.com/kb3126/.

Kaspersky

In your Kaspersky security product you can disable the interception of secure connections like this:

1. Open the dashboard of your Kaspersky application.
2. Click on Settings on the bottom left.
3. Click Additional and then Network.
4. Check the Do not scan encrypted connections option and confirm this change (2016 version) or uncheck the Scan encrypted connections option (2015 version).

Family Safety settings in Windows accounts

In Microsoft Windows accounts protected by Family Safety settings, secure connections on popular websites like Google, Facebook and Youtube might be intercepted and their certificates replaced by one issued by Microsoft in order to filter and record search activity.

See http://windows.microsoft.com/en-us/windows/family-features-remove-uninstall-faq on how to turn off these family features for accounts. In case you want to manually install the missing certificates for affected accounts, you can refer to the Microsoft support article at https://support.microsoft.com/en-us/k.../2965142#bookmark-2.

Monitoring/Filtering in corporate networks

Some traffic monitoring/filtering products, that are used in corporate environments might intercept encrypted connections by replacing a website's certificate with their own, possibly triggering errors on secure HTTPS-sites. If you suspect this might be the case, please get in contact with your IT department in order to inquire how to properly configure firefox to work in such an environment.

Malware

Some forms of malware intercepting encrypted web traffic can cause this error message - refer to the article Troubleshoot Firefox issues caused by malware on how to deal with malware problems.

If you receive the error on one particular site only

When you see this problem on one particular site only, this type of error indicates that the web server is not configured properly: The website's certificate might not have been issued by a trusted certificate authority itself and no complete certificate chain to a trusted authority was provided either (a so-called "intermediate certificate" is missing). You should get in contact the owners of the website and inform them of the error.

In case the website allows for it, you can add an exception in order to visit the site, in spite its certificate is not being trusted by default:

1. On the warning page, click Advanced.
2. Click Add Exception.... The Add Security Exception dialog will appear.
3. Read the text describing the problems with the website. You can click View... in order to closer inspect the untrusted certificate as well.
4. Click Confirm Security Exception if you want to trust the site.