CVE-2023-33982: Three security issues found and fixed

May 24, 2023 by The Briar Team

In February of this year, security researchers at ETH Zürich notified us that they had found three security issues in the Briar app. Two of these issues were fixed in version 1.4.22 of Briar, which was released in February. The third issue was fixed in version 1.5.3, which is being released today. All users are encouraged to upgrade to version 1.5.3 of the app as soon as possible.

We have not received any reports of these bugs being exploited in the wild, and the third issue is not exploitable.

We would like to apologise for the design and implementation mistakes that led to these issues, and to thank Yuanming Song and Prof. Kenny Paterson for finding the issues and responsibly disclosing them to us.

We have requested an independent security audit of Briar’s protocol stack to ensure that no other issues remain undiscovered.

For those who are interested in the details, a description of each issue is given below. The researchers’ report can be found here.

Issue 1: Receiving invalid message would have caused app to exit

The first issue (fixed in Briar 1.4.22) would have allowed a malicious user to prevent their contacts from using Briar by repeatedly sending them invalid messages that would cause the app to exit.

Receiving a message longer than the maximum allowed length should have raised an error that would cause the invalid message to be discarded. Instead, the check raised a different type of error that would cause the app to exit.

There was no risk of memory corruption, as the length check took place in memory-safe Java code. Invalid messages were not stored or forwarded to other users.

Issue 2: Message duplication in blogs, forums and private groups

The second issue (fixed in Briar 1.4.22) would have allowed a malicious user to create duplicates of messages written by other users in blogs, forums and private groups. The duplicates would have had the same content and timestamps as the originals, but would have appeared alongside the originals as though they were separate messages from the same authors.

To explain this issue we need to give some background information about the structure of Briar messages.

Every message in Briar consists of a timestamp, a group identifier and a message body. Each message has a unique identifier that is calculated by hashing these fields.

Different features of the app use these fields in different ways. The blog, forum and private group features make use of digital signatures: the body of each message contains some content along with a digital signature by the author of the content. The signature covers the content as well as the timestamp, the group identifier and the author’s Briar identity.

When verifying the signature on a message, Briar deserialises the message body to extract the content and signature, then serialises the content, timestamp, group identifier and author’s Briar identity to recreate the exact information that was originally signed. This ensures that none of these fields can be altered without invalidating the signature.

The ETH Zürich researchers discovered that this process of deserialising the message body and then reserialising the content to verify the signature made it possible to take a message signed by another user and produce a message with a different body (and therefore a different unique identifier) that would nevertheless be identical to the original after deserialisation and reserialisation. Thus the signature from the original message would still be valid, and there would appear to be two identical messages signed by the same author.

It would not have been possible to alter the content, timestamp, group identifier or Briar identity without invalidating the signature, but the duplicate messages might have caused confusion.

This issue occurred because Briar did not check that the message body was serialised in canonical form before deserialising it, so it was possible to create multiple non-canonical representations of the same message body, which would all be converted into the same canonical form after deserialisation and reserialisation.

Fortunately, genuine messages in blogs, forums and private groups have always been created in canonical form, so the issue was solved by rejecting any messages that were not in canonical form.

Issue 3: Poorly designed cryptographic handshake

The third issue (fixed in Briar 1.5.3) involved a poorly designed cryptographic handshake. At first it appeared that design flaws in this handshake could have allowed network traffic between Briar users to be decrypted by an attacker who had successfully carried out a specific set of other attacks against those users. Fortunately, we were able to confirm that this was not possible thanks to an extra layer of cryptographic protection provided by the Tor network. The issue was not exploitable and there was no danger to users. Even so, Briar 1.5.3 replaces the insecure handshake with a more secure version.

The handshake in question is part of the Bramble Handshake Protocol (BHP), which is used when users add each other as contacts by exchanging Briar links. The purpose of BHP is to derive a shared secret that is known to both users but not to anyone else, include anyone who may be eavesdropping on the network connection used for the handshake. The shared secret is ephemeral, meaning that it should not be possible to recreate the shared secret using any information that is retained by the users after the handshake.

BHP did not meet these design criteria. An adversary who was able to eavesdrop on the connection that was used for the handshake, and was later able to compromise both users’ Briar accounts, could have decrypted network traffic between the users that was sent between the time of the handshake and the time of the account compromises.

Fortunately, we were able to confirm that the eavesdropping part of the attack was not achievable. The connections used for BHP handshakes are always made via the Tor network, using version 3 of Tor’s hidden service protocol. Through discussions with the developers of Tor we were able to confirm that this protocol uses strong encryption that would have prevented an adversary from eavesdropping on BHP handshakes. Without being able to eavesdrop, an adversary could not later decrypt past network traffic even if both users’ accounts were compromised.

The poor design of the BHP handshake was a serious mistake on our part. Fortunately, thanks to the extra layer of protection provided by Tor, users were not put at risk by this mistake.

Briar 1.5.3 replaces the insecure handshake with a more secure version, and we have requested an independent security audit of the whole protocol stack to ensure that no other mistakes of this kind were made.

About Briar

Briar is a messaging app designed for activists, journalists, and anyone else who needs a safe, easy and robust way to communicate. Unlike traditional messaging tools such as email, Twitter or Telegram, Briar doesn’t rely on a central server - messages are synchronized directly between the users’ devices. If the internet’s down, Briar can sync via Bluetooth or Wi-Fi, keeping the information flowing in a crisis. If the internet’s up, Briar can sync via the Tor network, protecting users and their relationships from surveillance.

Briar has received funding from the Small Media Foundation, the Open Internet Tools Project, Access Now, the Open Technology Fund, the Prototype Fund, Internews, the NLnet Foundation, the Next Generation Internet programme, the ISC Project and eQualit.ie.

Contact

Torsten Grote <[email protected]> [PGP key]

Michael Rogers <[email protected]> [PGP key]

Twitter: @BriarApp

Mastodon: @Briar