The British Army's YouTube account name was changed. Screenshots also appeared to show the Army's Twitter account, its name changed, retweeting promotions for NFT projects, complete with images of a cartoon monkey.
NFTs are digital assets, including image files, that may represent real-world items. Transactions are recorded on the blockchain.
A capture of the British Army's Twitter account also shows the page was changed to promote NFTs.
It is unknown who is behind the intrusion.
The British Army confirmed the security incident and apologized "for the temporary interruption to our feed" on Sunday night.
"The breach of the Army's Twitter and YouTube accounts that occurred earlier today has been resolved and an investigation is underway," the Ministry of Defence said. "The Army takes information security extremely seriously and until their investigation is complete it would be inappropriate to comment further."
The accounts have now been restored.
In July 2020, verified Twitter accounts belonging to high-profile individuals and companies, including Elon Musk, Bill Gates, Joe Biden, Kanye West and Apple, were taken over and used to promote a cryptocurrency scam. While the security incident was contained quickly, it wasn't quick enough in that case to stop hundreds of thousands of dollars from being sent fraudulently to the perpetrator's wallet.
While it is not known how the British Army hack took place, Jake Moore, global cyber security advisor at ESET, said the potential causes were the use of profile sharing among multiple admins, risky outsourcing, and a failure to update passwords regularly.
"It can be extremely damaging for organizations and brands when their social media accounts are hacked, so it is vital that all social media admins are using multi-factor authentication and they change the password when anyone who knows it leaves," Moore said.