A Matrix Update Will Patch Serious End-to-End Encryption Flaws

Developers of the open source Matrix messenger protocol are releasing an update to fix critical end-to-end encryption vulnerabilities that subvert the confidentiality and authentication guarantees that have been key to the platform’s meteoric rise.

Matrix is a sprawling ecosystem of open source and proprietary chat and collaboration clients and servers that are fully interoperable. The best-known app in this family is Element, a chat client for Windows, macOS, iOS, and Android, but there’s a dizzying array of other members as well.

Matrix roughly aims to do for real-time communication what the SMTP standard does for email, which is to provide a federated protocol allowing user clients connected to different servers to exchange messages with each other. Unlike SMTP, however, Matrix offers robust end-to-end encryption, or E2EE, designed to ensure that messages can’t be spoofed and that only the senders and receivers of messages can read the contents.

Matthew Hodgson—the cofounder and project lead for Matrix and the CEO and CTO at Element, maker of the flagship Element app—said in an email that conservative estimates are that there are about 69 million Matrix accounts spread throughout some 100,000 servers. The company currently sees about 2.5 million monthly active users using its server, though he said this is also likely an underestimate. Among the hundreds of organizations announcing plans to build internal messaging systems based on Matrix are Mozilla, KDE, and the governments of France and Germany.

On Wednesday, a team published research that reports a host of vulnerabilities that undermine Matrix’s authentication and confidentiality guarantees. All of the attacks described by the researchers require the aid of a malicious or compromised homeserver that targets the users who connect to it. In some cases, there are ways for experienced users to detect that an attack is underway.

The researchers privately reported the vulnerabilities to Matrix earlier this year and agreed to a coordinated disclosure timed to Wednesday’s release by Matrix of updates that address the most serious flaws.

“Our attacks allow a malicious server operator or someone who gains control of a Matrix server to read the messages of users and to impersonate them to each other,” the researchers wrote in an email. “Matrix aims to protect against such behavior by providing end-to-end encryption, but our attacks highlight flaws in its protocol design and its flagship client implementation Element.”

Hodgson said he disagrees with the researchers’ contention that some of the vulnerabilities reside in the Matrix protocol itself and asserts they are all implementation bugs in the first generation of Matrix apps, which include Element. He said that a newer generation of Matrix apps, including ElementX, Hydrogen, and Third Room, are unaffected. There are no indications that the vulnerabilities have ever been actively exploited, he added.

Breaking Confidentiality, Attacking Verification, and More

The first two attacks provide a simple confidentiality break by exploiting the homeserver’s control over the users and devices that are permitted to join a private room. Only a person who creates a room or is deputized by the room creator is permitted to invite and admit new members, but the room management messages that make this mechanism possible aren’t required to be authenticated with the cryptographic keys of these authorized users. It’s trivial for someone with control of the homeserver to spoof such messages and, from there, admit users without the permission of the authorized users. Once admitted, the attacker gains access to decrypted communications sent in that room.

The researchers are careful to note that all new room entrants are automatically logged in an event timeline and hence can be detected by anyone in the room who manually inspects the membership list in real time. In large rooms with lots of activity, however, this kind of inspection may not be practical, the researchers said.

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