Fox News, citing a Pentagon official, has reported that the “serious national security threat” is related to space.

This, combined with CNN’s previous reporting that the threat was also a “destabilizing foreign military capability” in relation to Russia, suggests that it may be related to last week’s Soyuz-2 rocket launch, which was carrying an unknown payload for the Russian military.

The exact nature of the threat still remains unconfirmed.

https://www.nbcnews.com/politics/national-security/house-intel-chair-warns-serious-national-security-threat-ahead-planned-rcna138848

White House to brief lawmakers on 'serious national security threat' related to Russia

https://theatlasnews.co/brief/2024/02/14/serious-national-security-threat-related-to-space-per-fox/

Two sources familiar with the issue said Turner's statement was related to Russia and operations in space, without providing further details on what was described as a highly-classified matter.
https://www.reuters.com/world/us/us-congressional-intelligence-chair-warns-serious-national-security-threat-2024-02-14/