3 Simple Ways to Fix Firefox Error Code SEC_ERROR_UNKNOWN_ISSUER

by Ola-Hassan Bolaji
Ola-Hassan Bolaji
Ola-Hassan Bolaji
A Computer Engineering graduate, he has spent most of his life reading and writing about computers. Away from computers, he is either reading or watching football! read more
Affiliate Disclosure
  • The Firefox SEC_ERROR_UNKNOWN_ISSUER error code occurs as a result of an invalid SSL certificate.
  • It can also be caused by the activities of third-party software on your PC.
  • This error code can be resolved by temporarily disabling your antivirus software.
firefox error code sec_error_unknown_issuer
Instead of fixing issues with Firefox, upgrade to a better browser: Opera
You deserve a better browser ! 350 million people use Opera daily, a fully-fledged navigation experience that comes with various built-in packages, enhanced resource consumption and great design.Here's what Opera can do:

  • Easy migration: use the Opera assistant to transfer exiting Firefox data, in just a few steps
  • Optimize resource usage: your RAM memory is used more efficiently than Firefox does
  • Enhanced privacy: free and unlimited VPN integrated
  • No ads: built-in Ad Blocker speeds up loading of pages and protects against data-mining
  • Download Opera

The Firefox SEC_ERROR_UNKNOWN_ISSUER error code shows up when the browser can’t trust a website’s SSL certificate.

Whenever you try to visit a website on Firefox, it checks the SSL certificate from the site. This certificate must be valid and be from a trusted certificate authority.

If there is a problem with the website’s security certificate, it shows this error code to warn you of the potential risks of the site. This is all part of its security feature to protect you from dangerous websites.

However, if you face the error code SEC_ERROR_UNKNOWN_ISSUER in Firefox but not in Chrome, especially on trusted websites, it means the issue is far beyond just the SSL certicate. trusted websites like Google.

Plus, this is not the first time an error like this has prevented users from accessing sites on Firefox. People have encountered some of the errors below at one point or another on the browser.

Like we did with the errors above, we have also provided practicable fixes to this error. Read on to find out more!


The error code SEC_ERROR_UNKNOWN_ISSUER occurs on Firefox due to a conflict between the browser and the website certificate. But other factors could cause it.

Below are some of the popular ones:

  • A website’s SSL certificate is not from a trusted certificate authority. This can also be that the certificate has been self-signed.
  • A website’s certificate is obtained from a provider whose root certificate has been removed from the root store.
  • Presence of malware on PC or the website.
  • Activities of third-party software.

It is important not to bypass or ignore the Firefox SEC_ERROR_UNKNOWN_ISSUER issue if it is due to the first two factors above. However, if you face the problem on a trusted website, you can make exceptions.


1. Perform virus scan

1.1 Use the built-in virus scanner

  1. Press the Windows key and type Virus & threat.
  2. Select the Virus & threat protection option.
    virus and threat sec_error_unknown_issuer
  3. Click Scan options.
    scan options
  4. Select the Full scan or Microsoft Defender Offline scan.
  5. Click the Scan now button.
    scan now firefox error code sec_error_unknown_issuer

If a virus infects your system, it might prevent Firefox from finding a website’s certificate. This stops it from establishing a secure connection, leading to the error code SEC_ERROR_UNKNOWN_ISSUER.

You might also notice that the Firefox security warning keeps popping up no matter what you try. In this case, your best bet is to perform a virus scan and remove the malware. After the scan, restart your PC and relaunch Firefox to check if the error persists.

1.2. Use a third-party antivirus software

While your PC’s built-in malware and virus protection is pretty reliable, Microsoft is not a security company. Hence, there is room for improvement.

One software that gives you the all-around security you need is the ESET NOD32 antivirus. It is an award-winning cybersecurity technology that uses cutting-edge expertise and advanced machine learning to protect your digital life.

Expert Tip: Some PC issues are hard to tackle, especially when it comes to corrupted repositories or missing Windows files. If you are having troubles fixing an error, your system may be partially broken. We recommend installing Restoro, a tool that will scan your machine and identify what the fault is.
Click here to download and start repairing.

What’s more, it offers malware, ransomware, and phishing protection to keep you protected from threats of different kinds. Add its privacy and banking protection, and you know you are always secured online.

If you need an antivirus that keeps your devices running fast and detects viruses before they manifest, with a bonus of a free trial and cheap pricing, ESET NOD32 is what you are looking for.


2. Disable third-party antivirus temporarily

  1. Press the Windows key + X and select Task Manager.
    task manager
  2. Click Startup in the top menu bar.
  3. Right-click on your third-party antivirus program and select Disable. Note that the app used in the diagram is for explanation only and should be replaced with your antivirus as needed.
    disable startup firefox error code sec_error_unknown_issuer
  4. Restart your PC to effect the changes.

The Firefox error code SEC_ERROR_UNKNOWN_ISSUER and sometimes the Firefox did not connect: Potential security issue can be due to some of the security features of your third-party antivirus software.

Most modern antivirus programs offer special web shielding properties that can block access to insecure sites.

If you have such software and trust the site you want to visit, disable the antivirus temporarily. To enable the app later, you only need to repeat the steps above and select Enable instead of Disable in the last step.

3. Switch to another browser

If the fixes above prove futile in solving the error code SEC_ERROR_UNKNOWN_ISSUER on Firefox, your best bet is to try a new browser in Opera.

Opera is a secure browser that keeps you safe online from hackers, tracking, and data theft. It warns you of suspicious web pages after checking the page against a database of malware and phishing websites.

It also supports the SSL v2, SSL v3, and TLS 1.0 protocols to ensure the website is truly secure. However, these features can be disabled if you don’t need them.

All these, plus its built-in VPN, ad blocker, and anti-tracking feature, rank it as one of the most secure browsers, albeit without the frequent error messages.

Get Opera

The error code SEC_ERROR_UNKNOWN_ISSUER on Firefox is not as bad as it is made to look until it starts blocking trust sites. In this guide, you have everything you need to fix the error effortlessly.

Did Firefox encounter an unexpected problem on your Windows PC? Our comprehensive guide contains detailed steps for fixing it.

Feel free to let us know the solution that helped you fix the error code in the comments section below.

idee restoro Still having issues? Fix them with this tool:
  1. Download this PC Repair Tool rated Great on TrustPilot.com (download starts on this page).
  2. Click Start Scan to find Windows issues that could be causing PC problems.
  3. Click Repair All to fix issues with Patented Technologies (Exclusive Discount for our readers).

Restoro has been downloaded by 0 readers this month.

This article covers:Topics: