A Pentagon-wide advisory went out one week ago warning against using the messaging app Signal, even for unclassified information.

“A vulnerability has been identified in the Signal messenger application,” begins the department-wide email, dated March 18, 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 who are “targeting Signal Messenger to spy on persons of interest.”

In a statement to NPR, Signal spokesperson Jun Harada said, “We aren’t aware of any vulnerabilities or supposed ones that we haven’t addressed publicly.”

A screengrab of a page from a Pentagon-wide memo warning against using the messaging app Signal.

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The Pentagon memo adds, “Please note: third-party messaging apps (e.g. Signal) are permitted by policy for unclassified accountability/recall exercises but are not approved to process or store non-public unclassified information.”

The encrypted Signal app is what Defense Secretary Pete Hegseth and other leading national security officials within the administration used to discuss bombing Houthi sites in Yemen earlier this month. The Atlantic editor Jeffrey Goldberg was inadvertently added to the group and privy to the highly sensitive discussions.

In the military, sending classified data over insecure channels is called “slippage” when it’s considered minor, but even that can be a career ender for a military officer.

At least as far back as 2023, a DoD memo, also seen by NPR, prohibited use of mobile applications for even “controlled unclassified information,” which is many degrees less important than information about on-going military operations.

There’s almost no precedent for the heads of Defense, State, Intelligence and National Security to be sharing such sensitive military intelligence in a forum that was known to be unsecured.

NPR’s Bobby Allyn contributed to this story.

NPR disclosure: Katherine Maher, the CEO of NPR, chairs the board of the Signal Foundation.