Researchers Leverage Voicemail Flaw to Compromise Messaging Apps

Italian security researchers have discovered a vulnerability that can be easily exploited to break into messaging applications such as Telegram, WhatsApp, and Signal.

According to InTheCyber security researchers, the rather old voicemail caller-ID spoofing flaw can be leveraged to steal activation codes sent by messaging apps and compromise accounts. The researchers say that networks of two of the biggest mobile operators in Italy allow this kind of attack, explaining that the attacker only needs to know the victim’s phone number to be successful.

A security firm with offices in Milan and Lugano, InTheCyber explains that the bug can be exploited in at least three different scenarios where activation codes are sent to voicemail. All three, however, require for the user to not respond, to not be reachable, or to be engaged in different conversations.

The idea behind this attack is that, when an activation code is requested for a Telegram, WhatsApp, or Signal account, the code arrives via SMS. However, if the code isn’t introduced promptly, these services resend the activation via an automated call. Thus, if the user isn’t reachable, the call ends up in voicemail, and the attacker simply needs to look here for a vulnerability to exploit.

Calls are redirected to voicemail if the user doesn’t respond, is not reachable, or is occupied, and the security researchers explain that motivated attackers can leverage all three scenarios to execute an attack. An attacker could request the activation code late at night, could send multiple Silent-SMS to determine when the phone is detached from the network, or could set up a telephone scam to keep the phone busy during the attack.

Once the activation code has reached voicemail, the attacker can spoof his caller ID to impersonate the victim and gain access to both the target voicemail and to the activation code. The researchers have created a video to offer details on how the attack works and also publicly detailed the exploit on Monday, at the 7th National Conference on Cyber ​​Warfare in Milan.

The security researchers estimate there to be roughly 32 million users at risk on the networks of Wind and H3G. The obvious solution to this issue is to turn off voicemail, though both messaging services and wireless carriers should look into the matter and plug the security flaw.