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WhatsApp's 8-Month-Old Flaw Allows Interception Of Encrypted Messages

WhatsApp conversations are supposed to be impenetrable. But a vulnerability discovered by security researcher Tobias Boelter could allow the company, or anyone compelling it, to intercept communications that were supposed to be encrypted from end to end. Facebook--which owns WhatsApp--said this is expected behavior and doesn't seem to be planning a change to its setup any time soon.

WhatsApp’s Signal-based E2E Encryption

In April 2016, WhatsApp announced support for “full” end-to-end encryption for messages and voice calls on all platforms. Much like the Incognito Mode on Google’s Allo and the Secret Conversations in Facebook Messenger that followed it, WhatsApp's encryption relied on  the Signal protocol, which was developed by cryptography experts and privacy activists at the Open Whisper Systems non-profit organization.

WhatsApp seems to have added a little extra something in to the mix. Instead of telling users a message couldn't be delivered, like OWS' Signal app, the service will instead makes sure a message arrives on a recipient's device using a new encryption key generated by WhatsApp. End-to-end encryption is only supposed to use private keys stored on client devices; this setup undermines the whole purpose of building the Signal protocol into WhatsApp.

Uncovering WhatsApp’s E2E Encryption Flaw

Boelter said this vulnerability could allow entire conversations to be intercepted. The flaw has existed since WhatsApp announced support for end-to-end encryption in April, and Facebook was notified about it then. Yet the company hasn't fixed--or expressed any desire to fix--the issue since its disclosure. Here's what a Facebook spokesperson said in response to a request for comment:

Over 1 billion people use WhatsApp today because it is simple, fast, reliable, and secure. [...] At WhatsApp, we’ve always believed that people’s conversations should be secure and private. Last year, we gave all our users a better level of security by making every message, photo, video, file and call end-to-end encrypted by default. As we introduce features like end-to-end encryption, we focus on keeping the product simple and take into consideration how it's used every day around the world.In WhatsApp's implementation of the Signal Protocol, we have a “Show Security Notifications” setting (option under Settings > Account > Security) that notifies you when a contact's security code has changed. We know the most common reasons this happens are because someone has switched phones or reinstalled WhatsApp. This is because in many parts of the world, people frequently change devices and SIM cards. In these situations, we want to make sure people's messages are delivered, not lost in transit.

Facebook seems to say this so-called feature is necessary because people often change their SIM cards in other parts of the world, and they would therefore not receive messages after their cards are changed. Senders could still be warned if a message couldn't be sent, however, so they could just find their intended recipient's most recent contact information and re-send the message that way. That would be less convenient but more secure.

Keep in mind that this is only an issue between the time you change your SIM card and you turn on the phone again with the new SIM card. Once you’re in WhatsApp again, you will continue to receive the messages, even with the new SIM card and even if the security keys are not verified again. Also, dual-SIM smartphones are already popular in many of the countries WhatsApp talks about, so changing SIM cards may not be as bothersome as before.

Mitigating The Issue

Facebook’s representative said that if users are concerned about this issue they should enable the Show Security Notifications feature in their WhatsApp settings. This would tell users when WhatsApp changed the previously “trusted” key of the sender.

However, in the same way WhatsApp interfered with end-to-end encryption to generate its own keys, it could probably also hide those notifications. Therefore it’s recommended that you authenticate someone's identity with the security codes found on their WhatsApp contact pages. It’s preferable to scan a QR code when you meet someone in person, but failing that, you could also share the QR code over WhatsApp or another secure chat application.

Backdoor Or Vulnerability?

The researcher who discovered the flaw initially has a hard time figuring out whether this was an honest mistake, something that was actually intended to be a feature, or whether it was an intentional backdoor for law enforcement. As we’ve seen recently with the passage of the Investigatory Powers Act, it may now be possible for the UK government to force communications providers to backdoor encryption where it’s "technically feasible."

It’s hard to say one way or another. Even if it was meant to be a backdoor, it would probably be masked as a feature. The best backdoors are those that can be plausibly denied if discovered.

As Echoworx senior director Jacob Ginsberg said in a statement, WhatsApp has played a big role in making end-to-end encryption a mainstream feature people want in their chat applications, and it deserves some credit for that. (Open Whisper Systems still deserves the most credit for inventing a strong E2E encryption protocol that would even have a chance of being adopting by all of these apps in the first place.) Ginsberg said:

The fact that Facebook has known about this vulnerability since April is doubly damning. Not only could this be seen by many as supporting on-going government data collection interventions, it means their talk of encryption and privacy has been nothing more than lip service. The company needs to actively address its security measures. [...] These revelations add to the severe lack of clarity around encryption backdoors, most recently brought about by the IP Bill. While we know consumers are willing to trade their personal information for access to seemingly free services such as WhatsApp, now is the time to ask the question ‘at what cost to personal privacy?’ and start taking privacy more seriously.

If WhatsApp values end-to-end encryption, then it needs to stand by it, and it can’t compromise it to enable niche “features”. This issue casts doubt on how much users can trust WhatsApp and Facebook, especially after the companies went against their initial promise not to share user data. Now people may also have to wonder if WhatsApp is starting to backtrack on uncompromised end-to-end encryption, too. How WhatsApp addresses a known, eight-month-old vulnerability in its end-to-end encryption system could help answer that question.

Boelter presented the vulnerability in a short talk at the recent Chaos Computer Club (CCC) conference (minute 48:05):

Lucian Armasu
Lucian Armasu is a Contributing Writer for Tom's Hardware US. He covers software news and the issues surrounding privacy and security.