Hackers are trying to steal Signal users’ backups in a new wave of phishing attacks

TechCrunch has learned that hackers are targeting Signal users in an attempt to steal their chat backups as part of a new hacking campaign.

On Wednesday, Washington Post analyst Josh Rogin posted a screenshot of a new type of attack against Signal users, in which hackers pose as the app’s support team and warn the target that their chats and backup media are “at risk of being permanently lost due to a sync issue.” To avoid this, the message says, the target needs to share the recovery key used to access online backups in chat with the hackers.

“This links your current backup to your account. Failure to do so may result in losing access to your account and all stored data,” read the message that purports to come from an account called Signal Support.

Rogin said that many anti-Chinese Communist Party activists received this malicious message.

Mohammed Al-Maskati, director of Access Now’s digital security helpline, which investigates cyberattacks against journalists, dissidents and human rights activists, told TechCrunch that two people shared similar messages with him. Al-Maskati said that the two are not Chinese activists. This suggests that the hacking campaign may be more widespread and targeting other communities, or there may be different groups of hackers using the same strategy.

It is not clear how effective the hacking campaign was. Al Maskati said stealing the victim’s recovery keys for their chat backups is only one step in the attack, and the hackers still have to take control of the victim’s account.

Generally, this type of attack relies on phishing targets, i.e. tricking them into sharing some important and private information with hackers. In this particular case, the hackers are posing as Signal’s support team to exploit the target’s trust in the app and the organization behind it.

It’s important to note that Signal says it will “never contact” users first, and will never ask them for a registration code, PIN, or recovery key. This means that any conversation that pretends to be coming from Signal Support is actually coming from malicious hackers. The organization publicly warned of this type of attack last month.

Although there have been several hacker campaigns spoofing Signal support in recent months, this is a new type of attack because it specifically targets backups, which can contain the victim’s old chats, photos, and documents.

Previous hacking campaigns targeting Signal users have attempted to steal the victim’s account and then impersonate them, often with the potential goal of stealing the victim’s contacts or starting conversations with other people as if they were the account owner. In these cases, hackers cannot access previous messages, since the attacks rely on them to re-register the victim’s account on a device they control. Because of how Signal is designed, old messages don’t appear on the new device.

Hackers can take over Signal accounts by hijacking someone’s phone number, for example. But Signal offers optional security features to protect against this, such as Enrollment Lock, which prevents attackers from linking a target’s number to a new device unless they steal the target’s PIN.

In this scenario, one way to see old messages is to access the victim’s online backup, which requires a recovery key.

Last year, Signal launched Secure Backups, a new subscription feature that allows users to upload the contents of their accounts to Signal’s servers, which are encrypted with a recovery key that the organization says is “never shared with Signal’s servers,” and “never leaves” users’ device. Signal says users should store the recovery key securely on a laptop or within a password manager.

“Without your unique recovery key, no one (including Signal) can read, decrypt, or restore any of the data in your secure backup archive,” Signal said.

This means that a user can only access their archive in a scenario where they register their account on a new phone, download the encrypted backup from Signal’s servers, and then decrypt it using the recovery key.

Signal did not respond to a request for comment.