CVE-2024-47080: Matrix JavaScript SDK's key history sharing could share keys to malicious devices

In matrix-js-sdk versions 9.11.0 through 34.7.0, the method MatrixClient.sendSharedHistoryKeys is vulnerable to interception by malicious homeservers. The method implements functionality proposed in MSC3061 and can be used by clients to share historical message keys with newly invited users, granting them access to past messages in the room.

However, it unconditionally sends these “shared” keys to all of the invited user’s devices, regardless of whether the user’s cryptographic identity is verified or whether the user’s devices are signed by that identity. This allows the attacker to potentially inject its own devices to receive sensitive historical keys without proper security checks.

Note that this only affects clients running the SDK with the legacy crypto stack. Clients using the new Rust cryptography stack (i.e. those that call MatrixClient.initRustCrypto() instead of MatrixClient.initCrypto()) are unaffected by this vulnerability, because MatrixClient.sendSharedHistoryKeys() raises an exception in such environments.