If it is know that secret agencies are using Signal, then it is almost certain that other agencies are working to exploit that. An obvious attack on Signal is

If it is know that secret agencies are using Signal, then it is almost certain that other agencies are working to exploit that. An obvious attack on Signal is to get one of your people a job working there, or to bribe/blackmail and existing employee, and have them install a backdoor or other exploitable code (maybe a secret weakening of the encryption?).