Who needs to break encryption when you can impersonate your targets thanks to flaws in mobile protocols

May 10, 2016 12:45 GMT  ·  By

Mobile networking experts from security firm Positive Technologies revealed last week a new attack that uses the SS7 mobile telecommunications protocol that allows attackers to impersonate mobile users and receive messages intended for other people.

Their proof-of-concept demonstration relied only on a cheap laptop running Linux and an SDK that enabled them to interact with the SS7 protocol.

SS7 protocol flaws are known since 2014

The Signaling System No. 7 (SS7) protocol is a standard developed in 1975 that allows telco operators to interconnect fixed line and/or mobile telephone networks.

The protocol is of great importane to the telephony communications backbone but was never updated to take into account the advancements made in current mobile technologies and remained grossly outdated.

Many security experts have warned about its lack of proper security measures ever since 2014. Infamous are two talks given by researchers at the 31st Chaos Communication Congress in Germany. Positive Technologies was also one of those companies, releasing an in-depth report about the protocol's issues in December 2014.

More recently, the protocol was subjected to public criticism and will face a government-sanctioned investigation after a CBS researcher with the help of a German security firm, used SS7 weaknesses to track and spy on a US elected official.

New SS7 attack demo shows how to circumvent encrypted apps

Seeing the attention this protocol started to get from the press once again, Positive Technologies has called on its previous research and has put out a blog post in which they detail an SS7-based attack on encrypted communications carried out via apps such as WhatsApp and Telegram.

The researchers, using their Linux laptop, spoofed a mobile network node and intercepted the initial phase of a chat between two users of an encrypted app.

Because the encrypted apps they've tested use SMS authentication to identify and authenticate users participating in encrypted conversations, researchers didn't bother to break the app's encryption, but simply impersonated the second person in an encrypted communications channel.

To do this, they used loopholes in the SS7 protocol detailed in their 2014 research paper, which allows an attacker to intercept incoming SMS messages, used by the apps to identify users.

SS7 attacks can take place from anywhere in the world

"Attacks on SS7 may be conducted from anywhere, and hackers may choose other targets apart from messengers," researchers said, referring to the possibility of intercepting other content such as 2FA codes.

"It is worth noting that all the tests were performed with default settings, i.e. the mode most users apply," researchers also explained, which means that users that haven't turned on the "Show Security Notifications" setting in WhatsApp (turned off by default) won't see that their contact's original security code has changed, a clear sign that something's amiss.

For Telegram researchers were able to access the victim's account and conversations, but they weren't able to access secret chats. "But you may create a new one and write messages impersonating your victim," the researchers said.

For WhatsApp, researchers also obtained access to the WhatsApp account, but since WhatsApp doesn't store previous chat history, they couldn't access past conversations.

Their demonstration proved that surveillance agencies don't necessarily need to crack encryption to spy on users, and can very well use the existing mobile networking infrastructure to carry out such operations. The attack is not tailored for WhatsApp or Telegram, and can be used for other apps such asr Viber.

"The main purpose of the attack was to encourage the messaging applications not to rely on core functions of the operator to verify users identity, in this case “SMS”, as most of the core functions, Voice, Msgs, USSD’s etc are compromised and can be manipulated," researchers told Softpedia. "Applications like WhatsApp will have features like notifications on contacts code change, but we need to ask ourselves, whats the percentage of users who know such features, and are these features enabled by default? Features like such notifications are not “good to have”, but are “absolute must have” to prevent such kinds of identify impersonation."

UPDATE: An encryption specialist with knowledge of WhatsApp's inner workings has contacted Softpedia and pointed out that the victim of such attacks should "get a notice that their contact's security code changed." Softpedia has reached out to Positive Technologies and inquired if their attack circumvents this protection system. The title has been changed accordingly.

UPDATE 2: The article was substantially modified with new information from the researchers.

Simplified view of the attack
Simplified view of the attack

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Researchers intercepting WhatsApp conversations (third phone)
Researchers intercepting Telegram conversations (third phone)Simplified view of the attack
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