On careers.stackoverflow.com , I click on "file CV", and Firefox gives me this error:

careers.stackoverflow.com uses an invalid security certificate.
The certificate is not trusted because the issuer certificate is unknown.
(Error code: sec_error_unknown_issuer)

Of course, I could be the subject of a man-in-the-middle attack, but I suspect there is a more prosaic cause!


I'm seeing Comodo as the issuer, but Firefox is still saying: "the certificate is not trusted because the issuer certificate is unknown."

It works OK in IE7.

Would it help if I post more details of the cert I'm seeing?


It's working OK now.

thanks, Geoff :)

  • The issuer is Stack Overflow? Oct 28, 2009 at 11:36
  • I get the same error
    – Oskar
    Oct 28, 2009 at 11:37
  • No-repo for me. (Using FF 3.0.14 on Ubuntu 9.04.)
    – Stephan202
    Oct 28, 2009 at 11:48

3 Answers 3


We have installed Comodo root and intermediate certificates - could somebody having this issue please verify if this has resolved the issue.

UPDATE Received five new certificates from COMODO our SSL Provider, installed all of them including Trusted Root Certification and Intermediate Certification Authorities

Installing these required a server reboot. It should be working now.

  • I'm still seeing the error. :(
    – AJ.
    Oct 28, 2009 at 16:26
  • I get the error when using Firefox on Windows. IE 8 gives me no errors.
    – Chris C
    Oct 28, 2009 at 17:26
  • Roger that, ChrisC. IE 7 gives me no errors, either.
    – AJ.
    Oct 28, 2009 at 17:31

This is a good site to make sure the cert and chain is installed correctly - http://www.digicert.com/help/

And I have had problems with Firefox caching an invalid certificate which makes it real difficult to troubleshoot these problems.


I got this yesterday, too. It appears to be fixed now.

SO team probably left the self-signed test cert on it by accident.

  • still not working for me - possibly some caching going on (??). I'll try again tomorrow. thanks :)
    – AJ.
    Oct 28, 2009 at 13:32

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .