WhatsApp’s much touted end-to-end encryption capability has become the subject of considerable scrutiny following a report by The Guardian Friday of a ‘backdoor’ in the messaging service that apparently allows for encrypted messages to be intercepted and read.
The Guardian’s report is based on a vulnerability disclosure that Tobias Boelter, a PhD student and computer scientist at the University of California made last April.
Boelter claimed that he had found a serious weakness in the manner in which the latest version of WhatsApp handles messages that are sent to a recipient who might have swapped or reset their phone, or might simply have been offline when the message was sent.
In such situations Boleter said, WhatsApp holds the message and retransmits it when the recipient comes back online but using a different encryption key than the one that was originally used to encrypt the message.
This approach gives attackers an opportunity to spoof the recipient’s phone on the WhatsApps server, use their own public key, and intercept messages that were intended for the recipient.
The Guardian described Boelter’s discovery as a security backdoor that governments could use to snoop in on the communications of targeted and unsuspecting WhatsApp users.
WhatsApp’s practice of creating new encryption keys for offline users without the knowledge of the sender and then rebroadcasting the message, effectively allows Facebook to intercept and read the messages of users communicating with each other under the assumption that their messages are impenetrable, the paper warned.
In an updated blog post, Boelter Friday expressed uncertainty over whether the issue he discovered is a bug or a backdoor. “In other words: Is this flaw put deliberately into the WhatsApp messenger to allow them or the government to look at targeted messages?” he wrote. “Or is the flaw introduced through a simple programmer error? Or is it even a useful feature?”
According to Boelter, Facebook showed little interest when he first reported his discovery to them last April, but recently claimed that it was not a bug but a feature for ensuring messages got through as intended to offline users.
In an emailed statement to Dark Reading, a Facebook spokesperson described The Guardian’s claims as patently false. “The design decision referenced in the Guardian story prevents millions of messages from being lost,” the spokesperson said. “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.”
In addition, WhatsApp has a security notifications option that people can use if they want to be notified when a contact’s security code or public encryption key has changed, the WhatsApp spokesman said.
Jon Geater, chief technology officer of Thales e-Security said the concerns raised by Boelter are not as serious as they might appear.
“Indeed, there is almost no hack here. One might argue that WhatsApp has elected to provide an insecure default configuration,” Geater said in a statement. “Users who care about the authorities intercepting their messages are quite likely to turn on security notifications, and then the problem is almost entirely diminished,” he said.
Dissidents and those using WhatsApp in repressive countries might need to take additional precautions like using an out of band method to ensure that a user has really changed devices, he said.
Chris Perry, COO of Secured Communications says the most troubling aspect about the whole controversy is the lack of full disclosure by Facebook WhatsApp.
“When a company builds into its system a failsafe, such as this feature, there should be full disclosure to their customer base that it exists and how and why it would be activated,” Perry says in comments to Dark Reading.
As it is, only the sender is informed of the encryption key change and that too only if the sender has opted-in to the notification warning, he says. “There should be a warning notification for both sender and recipient that is default activated advising when they are no longer in a secure encrypted environment.”
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