Exemption 1: Information that is classified to protect national security.
Exemption 4: Trade secrets or commercial or financial information that is confidential or privileged.
Exemption 7(E). Would disclose techniques and procedures for law enforcement investigations or prosecutions
So, beside the national security issue, the technique may be a trade secret of the contractor whom the FBI hired to pull off the hack. Next, the hack was clearly used as a technique for a law enforcement investigation.