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

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

On Wednesday, Washington Post analyst Josh Rogin posted a screenshot of a new kind of attack against Signal users, where hackers pretend to be the app’s support team and warn the target that their backed-up chats and media are “at risk of permanent loss due to a sync issue.” To avoid that, the message said, the target needs to share the recovery key that is used to access their online backups in the chat with the hackers. 

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

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

Mohammed Al-Maskati⁩, the director at 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 could be more widespread and targeting other communities, or there may be different groups of hackers using the same strategy.

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

In general, this type of attack relies on phishing targets, meaning tricking them into sharing some important and private information with the hackers. In this particular case, the hackers are pretending to be 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 reach out” to users first, and will never ask for their registration code, PIN, or recovery key. That means any chat pretending to be coming from “Signal Support” is actually coming from malicious hackers. The organization has publicly warned about this exact type of attacks last month. 

While there have been several campaigns of hackers impersonating Signal support in recent months, this is a new type of attack because it specifically targets backups, which can contain a victim’s older chats, photos, and documents.  

Previous hacking campaigns targeting Signal users attempted to hijack a 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, the hackers do not get access to past messages, since the attacks rely on them re-registering the victim’s account on a device they control. Because of how Signal is designed, older messages do not appear on the new device. 

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

In that scenario, one way to see older messages would be to access a victim’s online backup, which requires the recovery key.

Last year, Signal launched Secure Backups, a new opt-in feature that lets users upload their account’s contents 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” the users’ device. Signal says users should store the recovery key securely on a notebook or inside 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.

That means only the user can 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 with the recovery key. 

Signal did not respond to a request for comment.