How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites
On websites which are supposed to be secure (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 show a "Your connection is not secure" error message instead. This article explains why you might see the error code "SEC_ERROR_UNKNOWN_ISSUER" on websites and how to troubleshoot it.
The error occurs on multiple secure sites
In case you get this problem on multiple unrelated HTTPS-sites, it 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 are security software scanning encrypted connections or malware listening in, replacing legitimate website certificates with their own.
Antivirus 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. Try the following solutions for particular security products:
Avast
In Avast security products you can disable the interception of secure connections:
Open the dashboard of your Avast application.
Go to Settings > Active Protection and click Customize next to Web Shield.
Uncheck the Enable HTTPS Scanning setting and confirm this by clicking OK.
How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites | Troubleshooting
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How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites
ESET
In ESET security products you can try to disable and re-enable SSL/TLS protocol filtering or generally disable the interception of secure connections as described in ESET’s support article.
Kaspersky
In Kaspersky security products you can disable the interception of secure connections:
Open the dashboard of your Kaspersky application.
Click on Settings on the bottom-left.
Click Additional and then Network.
If you use a 2016 version of Kaspersky: In the Encrypted connections scanning section check the Do not scan encrypted connections option and confirm this change.
Alternatively, you can click on Advanced Settings in order to try to trigger a reinstallation of Kaspersky's certificate. In the dialog that opens click on Install certificate… and follow the on-screen instructions.
If you use a 2015 version of Kaspersky: uncheck the Scan encrypted connections option.
Finally, reboot your system for the changes to take effect.
Monitoring/filtering in corporate networks
Some traffic monitoring/filtering products used in corporate environments might intercept encrypted connections by replacing a website's certificate with their own, at the same time possibly triggering errors on secure HTTPS-sites. If you suspect this might be the case, please contact your IT department to ensure the correct configuration of Firefox to enable it working properly in such an environment, as the necessary certificate might have to be placed in the Firefox trust store first
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