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What is the MOZILLA PKIX ERROR MITM DETECTED Error and how can I fix it?

Firefox has long been one of the most widely used browsers on the planet. They have been working on making the browser a safer environment for users continuously. One of the most recent modifications has been focused on making its users aware of any MITM attacks that may be taking place.


In Firefox 61, Mozilla includes a new warning for MITM attacks, which is referred to as “MOZILLA PKIX ERROR MITM DETECTED,” which informs the user that software is attempting to launch a man-in-the-middle SSL attack on their computer. The Mozilla Firefox 65 browser includes an explanation of how software, such as an antivirus programme, can be the source of this error.

A Brief on MITM Attack

A man-in-the-middle (MITM) attack occurs when a software substitutes their certificate for that of a certificate authority (CA) in the browser, allowing the programme to eavesdrop on, or sniff, the encrypted SSL connection between the browser and an SSL-protected website. This permits the application to see all of the activity between your browser and the website, including any passwords, financial information, or other information that you may have entered.

An easy-to-understand video explanation of MITM

CyberShaolin is credited for creating the video.

Although a MITM attack may appear to be dangerous, it can be used for lawful purposes such as allowing antivirus applications to analyse encrypted communication for malicious content or HTTP debugging tools, among other things.

There are adware and malware protocols that make use of this approach to inject advertisements or steal information from the transmission.

For additional information about this error, please see the link below.

To make it easier to understand and to allow users to see which certificates may be attempting to perform a MiTM attack, Firefox has changed the message that is displayed when a MOZILLA PKIX ERROR MITM DETECTED error occurs. This change was made to make it easier to understand and to allow users to see which certificates may be attempting to perform a MiTM attack.

Mozilla Firefox version 65 is now available.
Previously, when a certificate was used in a MITM attack, the browser would display an error message indicating “Warning: Potential Security Risk Ahead.” This was the case in Firefox 64 and earlier. This did not provide any meaningful information about which certificate was producing the error, making it ineffective for the vast majority of users.

There has been an addition of a new error message in Firefox 65 that is significantly more descriptive and includes information about the certificate that has been identified as being responsible for the MITM attack. This will help a user to determine whether or not the programme in question is one that they are knowingly utilising, such as antivirus software or a web debugger such as Fiddler.

To monitor SSL traffic for harmful scripts and behaviour, antivirus software frequently uses its certificates. It is therefore beneficial that Firefox now offers information about this AV function in the error message.


When Firefox continues to exhibit MOZILLA PKIX ERROR MITM DETECTED problems, this indicates that a programme is attempting to inject their certificates into the Firefox session to eavesdrop on the encrypted website traffic being transmitted by the site. Unfortunately, Firefox does not trust this certificate, and as a result, it will continue to display this error.

Third-party antivirus software, in general, has the potential to interfere with Firefox’s encrypted connections. We recommend that you uninstall any third-party software and install the security software that is provided with Windows.