In larger companies, one way to get around this, is to go to Human Resources instead of your chain of command. Let them anonymously handle this issue. If your company doesn't take action then you can continue discussing the matter with HR until it's resolved.
If the company isn't large enough to have an employee handbook and HR then could report to an officer of the company and note that you wish to remain anonymous and that you're genuinely concerned about company security.
You could also consider requesting a meeting with officer+manager or HR+manager and disclose to both at the same time.
I don't see any company in their right mind firing you if you do this -- and are genuinely concerned for the security of your employer and it's clientele.