How to Repair Firefox error with MOZILLA PKIX ERROR MITM DETECTED

Firefox was one of the world’s most-used browsers. They worked constantly to make the browser a safer place for users. One of the recent updates has centered on notifying its users of any MITM attacks.


Firefox 61, Mozilla introduces MITM Attack Warning, called “MOZILLA PKIX ERROR MITM DETECTED,” warning users that a program is attempting to initiate a man-in-the-middle SSL Attack. In Firefox 65, Mozilla has explained that this error can be caused by software, like an antivirus program.

A Statement on MITM Attack

A man-in-the-middle ( MITM ) attack means a program attaches its own certificate to the browser as a certificate authority ( CA) so that it can eavesdrop or sniff, the encrypted SSL communication between the user and an encrypted SSL website. This allows the program to view traffic between your browser and the site, including passwords, financial information entered or any other data.

MITM attack may sound scary but for legitimate reasons, such as giving antivirus programs the ability to scan the encrypted traffic for malicious content or HTTP debugging tools, it can be used.

There are adware and malware protocols that use the system so they can inject advertisements or steal information that has been transmitted.

Firefox has changed the message that is used to describe a MOZILLA PKIX ERROR MITM DETECTED error to make it easier to understand and allow users to see what certificates may attempt to perform a MiTM attack.

Update Mozilla Firefox to 65

Earlier in Firefox 64 and below, the browser would show an error stating “Warning: Potential Security Risk Ahead” when a certificate is being used in an MITM attack. Which did not provide any real information about which certificate causes the error, as it would be useless for most users.

A new error message has been added in Firefox 65, which is much more informative and includes information about the certificate which is found to be performing the MITM attack. This will allow a user to check whether it is a program they are deliberately using, such as antivirus software or a web debugger such as Fiddler.

Using their own certificates is common for antivirus software so it can scan SSL traffic for malicious scripts and behaviour, so it is useful that Firefox now includes information about this AV feature in the error message.


If Firefox starts to view MOZILLA PKIX ERROR MITM DETECTED errors, you have a program that tries to insert its own certificates so that they can eavesdrop into the encrypted traffic on the site. The problem is that Firefox does not trust this mark, so this error will continue to show up.

Third-party antivirus apps can typically interfere with stable connections to Firefox. We recommend that you uninstall your third-party software and use the Windows security software on offer.