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The Federal Bureau of Investigation (FBI) and the Cybersecurity and Infrastructure Security Agency (CISA) issued a warning to Americans on Tuesday in light of a major cyberattack against telecommunications companies, urging phone users to use encrypted messaging apps to avoid privacy breaches by foreign hackers.

According to NBC News, the warning comes as FBI and CISA officials confirmed during a new call on Tuesday that the Salt Typhoon hacking campaign, which is believed to be one of the worst telecommunications hacks in U.S. history, has not been fully fixed. U.S. officials previously told NBC News that China had hacked Verizon, AT&T, and Lumen Technologies in an effort to spy on Americans.

The Independent reported that Bryan Vorndran, the assistant director of the FBI’s Cyber Division” warned that “threat actors affiliated with the People’s Republic of China (PRC) are targeting commercial telecommunications providers to compromise sensitive data and engage in cyber espionage.” Vorndran said that the FBI and other intelligence agencies have issued recommendations to “enhance the visibility of network defenders and to harden devices against People’s Republic of China-affiliated exploitation.”

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NBC reported that CISA Executive Assistant Director Jeff Greene and an anonymous senior FBI official recommended that Americans use encrypted messaging apps to prevent China from accessing their communications. According to The Independent, U.S. officials encouraged Americans to use apps such as WhatsApp or Signal that ensure encryption between iPhone and Android devices instead of using traditional text messaging apps.

“Our suggestion, what we have told folks internally, is not new here: Encryption is your friend, whether it’s on text messaging or if you have the capacity to use encrypted voice communication,” Greene said. “Even if the adversary is able to intercept the data, if it is encrypted, it will make it impossible.”

In addition to using encrypted apps to communicate between Android and iPhone devices, the anonymous FBI official said, “People looking to further protect their mobile device communications would benefit from considering using a cellphone that automatically receives timely operating system updates, responsibly managed encryption and phishing resistant” authentication for social media, email, and collaboration tool accounts, according to NBC News.

According to Forbes, U.S. officials have indicated that China’s hacking operation is “ongoing and likely larger in scale than previously understood.” NBC News reported that Greene acknowledged that it was “impossible” for U.S. intelligence agencies to “predict a time frame on when we’ll have full eviction” amid the ongoing cyberattack against U.S. telecommunications.