The FBI is warning iPhone and Android users against sending text messages to each other without encryption due to a recent Chinese cyber attack.

On Wednesday, Deputy National Security Adviser Anne Neuberger announced that at least eight US telecommunications companies had been compromised by a Chinese hacking operation known as “Salt Typhoon”. The operation had access to private text messages and phone conversations with an undisclosed number of Americans.

Hackers are believed to have successfully intercepted communications of senior government officials and high-ranking political figures. However, Neuberger said the government does not believe “any classified communications” have been made available.

In response to the attack, the FBI and the Cybersecurity and Infrastructure Security Agency (CISA) issued guidance aimed at identifying hackers and preventing future cyberattacks.

Because communications between iPhone and Android users are not encrypted by default, agency officials urged Americans to stop sending texts between the two operating systems or use third-party apps with end-to-end encryption.

Why aren’t text messages end-to-end encrypted?

SMS messaging first appeared in the early 1990s, when there were far fewer concerns about the security of text messages. At this point, changing SMS to include end-to-end encryption would likely be impractical or impossible.

However, modern text messages often do not use SMS and are transmitted using other systems such as Apple iMessage and Google Messages.

Are iMessage, WhatsApp, Signal and Google Messages encrypted?

Text messages between iPhone users and between Android users include end-to-end encryption by default. However, messages transmitted between the two systems are not encrypted and may be vulnerable.

Both WhatsApp and Signal offer end-to-end encryption. NBC News reported Wednesday that CISA official Jeff Greene and a senior FBI official urged Americans to use an encrypted app instead of communicating between iPhone and Android devices.

“Encryption is your friend, whether it’s text messaging or if you have the capacity to use encrypted voice communications,” Greene said. “Even if an adversary is able to intercept the data, if it is encrypted, it will make it impossible.”

How can you tell if someone has encrypted messages?

Since messages with end-to-end encryption can only be read using a type of “key” that is only available to the sender and receiver, the contents of the messages remain inaccessible if hacked by a third party.

Encrypted messages between iPhone users appear in blue. On Android devices, a lock icon under a message or an indication that the message is being sent using the Rich Communication Services (RCS) protocol indicates encryption.

What is a salt typhoon?

“Salt Typhoon” is the nickname given to Microsoft’s Chinese hacking operation. The group was also referred to using the nicknames “FamousSparrow” and “Ghost Emperor”.

While U.S. intelligence officials linked the group to the Chinese government, Liu Pengyu, a spokesman for the Chinese embassy in Washington, denied involvement earlier this week, insisting that “the U.S. must stop its own cyber attacks against other countries and refrain from using cyber attacks.” smear security and slander China.”

Update 12/5/24, 6:45 PM ET: This article has been updated with additional information.