A Pentagon advisory warns against using the messaging application Signal for any communications, even unclassified ones, due to a vulnerability exploited by Russian hacking groups. This follows an incident where a journalist was inadvertently included in a Signal chat about military operations in Yemen. The advisory, dated March 18, indicates that Signal is not authorized for processing or storing non-public unclassified information, despite previous guidance allowing its use for unclassified accountability exercises. A 2023 Department of Defense memo also prohibited using mobile applications for controlled unclassified information. The accidental inclusion of a journalist in sensitive discussions is termed “spillage,” which can endanger military careers. Signal's spokesman stated that the memo does not reflect concerns about the app's inherent security but emphasizes vigilance against phishing attacks.