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I have problems connecting to https websites with Firefox. I am not sure what causes the problem. Usually I got secure connection failed, SEC_ERROR_UNKNOWN_ISSUER and similar messages even by a simple google search. The browser sometimes offers the exception, sometimes it doesn't. It appears to be more or less random, so for example by google searches a simple refresh with f5 helps, by other sites I add an exception, or I cannot access. The problem started after I updated the Eset Smart security. The new version contains a netbank transaction protection, so it checks what sites I visit. This is a new feature, maybe it has a bug, but I am not sure whether really the antivirus is the source of the problem, since everything updates in the background... I checked some tutorials and I found that maybe deleting the cert8.db by Firefox helps. It made things even worse, now for example by facebook the page does not load some of the shared images, and by microsoft support pages it does not load the css. I am not sure what to do. Any idea how to investigate and fix this?

Update 16-10-27

It turned out this is something Firefox related. I turned off protection by Smart Security and the problem still persisted. I tried out other browsers as well, and they work properly. I'd prefer not to reinstall Firefox if possible, since it contains a lot of plugins, bookmarks, and opened pages I haven't read yet, so it would take a lot of time to restore the current state. So I decided to investigate this further, maybe there is a solution without reinstalling it completely.

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    Some AntiVirus Software may act as Man-In-The-Middle by installing its own root CA and intercepting all incoming and outgoing request. Have you tried disabling ESET temporarily and see if the trouble goes away?
    – Darius
    Commented Oct 27, 2016 at 10:53
  • @Darius It does not solve the problem, I just tried it out. So I guess something is wrong with Firefox and since I deleted a cert8.db it is worse now.
    – inf3rno
    Commented Oct 27, 2016 at 14:29
  • @Darius I just checked it with different browsers. They work perfectly, so this is Firefox related.
    – inf3rno
    Commented Oct 27, 2016 at 14:37

1 Answer 1

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I imported the root certificate from the ESET to the Firefox authorities. It solved the problem. I don't understand why a turned off antivirus still overrides HTTPS messages, I'll ask ESET about this.

# exporting root certificate from ESET

Advanced Settings > 
  Web and email > 
  Protocol filtering > 
  SSL > 
  Certificates > 
  View certificate button > 
  Details tab > 
  Copy to File... > 
  DER encoded binary X.509 (.CER) > 
  Save the file somewhere on the computer with .cer extension.

# importing certificate to Firefox

Menu > 
  Options >
  Advanced > 
  Certificates mini-tab > 
  View Certificates button > 
  Authorities tab > 
  Import button >
  Import the file you exported.

More about this: https://forum.eset.com/topic/908-how-to-import-root-certificate-to-firefox/

edit:

I ended up turning off the SSL protocol filtering on ESET Smart Security.

Advanced Settings > 
  Web and email > 
  Protocol filtering > 
  SSL > 
  turn off

The previous solution did not fix all of the pages on Firefox and for example I was not always able to play youtube videos, etc. I contacted with ESET, they might fix this later.

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    Turned off anti virus may not necessarily turn off the "internet security" component which does the https intercepting. Other browser (For now I'm assuming IE and Chrome) uses the Windows Cert Store, which ESET most likely already have their root cert in the Windows Cert Store (at the time of install).
    – Darius
    Commented Oct 28, 2016 at 4:38
  • @Darius Probably. I am not sure why Firefox does not use that Cert Store.
    – inf3rno
    Commented Oct 28, 2016 at 13:08

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