CVE-2024-47080

What it actually does

The sendSharedHistoryKeys method in the legacy crypto stack unconditionally transmits historical message keys to all devices of an invited user without verifying cryptographic identity or device signatures, enabling a malicious homeserver to inject devices and obtain those keys.

Why it matters to the business

Organizations relying on end-to-end encrypted Matrix rooms for sensitive communications risk exposure of past message history to unauthorized parties when a compromised or malicious homeserver is present, potentially violating confidentiality requirements for internal or regulated data.

How an attack unfolds

An attacker who controls or influences a Matrix homeserver waits for a legitimate invite to a room using shared history, then intercepts the unconditional key transmission performed by sendSharedHistoryKeys and registers additional devices under the invited identity to receive the historical decryption keys.

How to fix it

Upgrade matrix-js-sdk to version 34.8.0 or later (commit c9b43ab25120c0174e456639f2e3bccaedaded2d) which removes the vulnerable functionality; alternatively migrate clients to the Rust cryptography stack by calling initRustCrypto() instead of initCrypto().

How to confirm the fix worked

Inspect client initialization code for calls to initRustCrypto(), verify the installed matrix-js-sdk version is 34.8.0 or newer, and confirm that sendSharedHistoryKeys is no longer reachable in the application.

Typical attack path

1. Gain control of or compromise a Matrix homeserver
2. Trigger or observe an invite that uses shared history keys
3. Intercept the sendSharedHistoryKeys transmission
4. Inject attacker-controlled devices to receive the historical keys

How to detect it

Review application logs and network traffic for invocations of sendSharedHistoryKeys, unexpected device registrations during room invites, and any use of the legacy crypto initialization path.

• sendSharedHistoryKeys
• initCrypto()

What your security team should do

• Prioritize upgrade to matrix-js-sdk 34.8.0 or migration to the Rust crypto stack for all affected clients
• Audit codebases for any direct or indirect usage of sendSharedHistoryKeys and remove or replace it
• Monitor upstream Matrix ecosystem announcements for related client updates