When the burglar alarm goes off – would you just mute it?

I’m sure you’ve all been told to ignore links like

You have wun $1,000,000 click hear to download a dodgy application to steal your money” or

We have been unable to deliver a parcel to you. Click here to rearrange delivery and pay the excess money” but how about
Your connection is not private, Net::err_cert_invalid – click here to accept it

Collect 10 points if you did not fall for any of these.

I would like to talk about the last one, about your connection is not private; because I saw a web site recommending “click here to accept”.

You get messages like this if there are problems with the certificate. Common problems are

  • Your browser does not recognise the certificate, because it has not been signed by any CA in your browser’s trust store. This looks like someone is sending you an invalid certificate. It may be your enterprise security team need to update all browsers with the missing certificate, but they should have spotted this problem not you.
  • The certificate sent from the server has “valid host addresses are…., my sys.com” and this has been sent from a different site. This could be because bad guys have intercepted your request and played “Man in the middle” between you and the server.
  • Less common, is where the certificate has parameters which your browser cannot handle. For example very old cipher specs, and your browser only supports TLS 1.2 or above.
  • It could be caused by a server using a self signed certificate. It may be acceptable to use this in a test environment, but it is not secure – as anyone could have created it. If it is signed, then you can check the digital signature.

All of these should set off the intruder alarm.

If you get a message like this – do not click on the accept button. This is just like muting the burglar alarm, and ignoring the warning – bad guys could steal your personal details – and you would not know.

Many web servers are configured with a bad certificate, for example they may specify an alternate name of, but not “localhost”. It is much easier (though wrong) to tell people to accept the certificate, than to fix the certificate.

Do not trust it

The blog post went on to say that you can download the certificate which has been sent to you and install it in your key store. I felt like screaming “NO!” and attacking the screen with a hammer!

Imaging, twenty years ago, you were a bank manager, and a man comes in, and says “I am a policeman, and we are doing a survey about your bank’s safe and where you keep the money. I would like my colleague “fingers” to go down inspect it. He has to work anonymously, which is why he is wearing a mask, and gloves, and he is wearing a black and white striped shirt. I’ll come back tomorrow”. You let “fingers” go down to the safe on his own, and say goodbye to the nice policeman. An hour later you find your safe is empty and there is no sign of the two men.

Your first mistake was to accept the policeman was a policeman. You should have checked his credentials before doing anything else. If he was fake, then any identification papers would be fake. You need to talk to your contacts in the local police station, and check if your visitor is valid or bogus (while he is sitting in your office).

Only in rare situations, such as a test environment, would you extract the certificate from what was sent to you, and install it in your key store. If you install it, then later, another request can come in from the bad guys, and be validated by the CA you just installed.

If you need a CA, you go through the proper corporate process. For example as part of the overnight maintenance on your machine, or download it securely from a known site. Emailing may not be good enough if your machine has been hacked.

So, in summary, today’s lesson is

  1. Do not “accept” bad certificates when using web site. Fix the the server.
  2. Get any additional certificates through an approved process – do not accept what the server is giving you.

In the words of the Police Constable in Dixon of Dock Green. “Evening all – mind how you go”.

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