Several days after top national security officials accidentally included a reporter in a Signal chat about bombing Houthi sites in Yemen, a Pentagon-wide advisory warned against using the messaging app, even for unclassified information.

"A vulnerability has been identified in the Signal Messenger Application," begins the department-wide email, dated March 18 and obtained by NPR.

The memo continues, "Russian professional hacking groups are employing the 'linked devices' features to spy on encrypted conversations." It notes that Google has identified Russian hacking groups that are "targeting Signal Messenger to spy on persons of interest."

Moreover, a memo in 2023, obtained by NPR, warned of using Signal for any nonpublic official information.

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