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I realize there is another question with the same title, but my situation is very different.

Any ideas?

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Using Firefox, I have just discovered that both problematic sites use certificates issued by Comodo – gdelfino Jan 25 '10 at 14:22

I had this problem as well. Adding new root authority certificates from Thawte fixed the problem. Apple has documentation on how to do this here

I added the root and server certificates that I downloaded from here

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Thanks. Did you have to go through all the 8 directories and 14 subrectories double clicking on every .cer and .pem file? Did you add them to your login keychain or to the system keychain? – gdelfino Nov 27 '09 at 18:27
There are dozens of certificates in that download. Which one do I need? – gdelfino Jan 25 '10 at 14:02
up vote 1 down vote accepted

I have just solved this problem by following this procedure published by phurd99 in the Apple Support Forums:

  1. Open applications
  2. Utiliities
  3. Open Keychain
  4. Click Keychain Access and select Preferences.
  5. Click the "Reset My Default Keychain"


This solves the problem but you are left without you precious keychain (a backup is saved).

Then, using the backup and trial and error I discovered that by deleting one specific certificate, the problem is solved:

Intermediate certificate authority
Note: This certificate is signed by an untrusted issuer

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[+1] Searching for UNT-USERFirst-Hardware revealed 2 such certificates on my machine. One indeed was noted as signed by an untrusted issuer. So I just deleted this certificate and my HTTPS problems with Safari were gone! I couldn't use MobileMe on my MBA because of this. Thank you very much! – Marc Dec 12 '10 at 21:12

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