Mail in Apple Mac OS X before 10.6.3 does not properly enforce the key usage extension during processing of a keychain that specifies multi…

Mail in Apple Mac OS X before 10.6.3 does not properly enforce the key usage extension during processing of a keychain that specifies multiple certificates for an e-mail recipient, which might make it easier for remote attackers to obtain sensitive information via a brute-force attack on a weakly encrypted e-mail message.