The Arlington, Virginia pentagon is seen from above.
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Even with uncategorized information, a warning occurred a week ago for using messaging app signals.
“Vulnerabilities have been identified in the Signal Messenger application,” a department-wide email from NPR dated March 18th will begin.

The note states, “The Russian professional hacking group employs the “linked devices” feature to spy on encrypted conversations. ” It points out that Google has identified it A Russian hacking group “targeting signal messengers to spy on people of interest.”
In a statement to NPR, Signal spokesman Jun Harada said:

Screen grab of pages from Pentagon-wide note warnings against using messaging app signals.
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A Pentagon note states, “Note: Third-party messaging apps (signals) are permitted in the policy for uncategorized accountability/recall exercises, but are not authorized to process or store private, uncategorized information.”
The encrypted signaling app was used by the Secretary of Defense and other major national security officials within the administration, which was used earlier this month to discuss the Yemeni bombing Houthi site. Atlantic Ocean Editor Jeffrey Goldberg was accidentally added to the group, violating a very sensitive argument.
In the military, sending data classified as unstable channels is called “slippages” when considered minor, but can still be a career ender for military officers.

Back at least back to 2023, the DOD memo, also seen in NPR, bans the use of mobile applications, even “controlled, uncategorized information.”
There is little precedent for the heads of defense, national, intelligence news and national security to share such sensitive military information on forums known to be secure.
NPR’s Bobby Allyn contributed to this story.
NPR Disclosure: NPR CEO Katherine Maher will be taking the seat on the Signal Foundation’s Board of Directors.