matrix-sdk-crypto since version 0.8.0 up to 0.11.0 does not correctly validate the sender of an encrypted event. Accordingly, a malicious homeserver operator can modify events served to clients, making those events appear to the recipient as if they were sent by another user. Although the CVSS score is 4.9 (AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:N), we consider this a High Severity security issue.
Versions of the matrix-sdk-crypto Rust crate before 0.8.0 lack a dedicated mechanism to notify that a user's cryptographic identity has changed from a verified to an unverified one, which could cause client applications relying on the SDK to overlook such changes.
The UserIdentity::is_verified() method in the matrix-sdk-crypto crate before version 0.7.2 doesn't take into account the verification status of the user's own identity while performing the check and may as a result return a value contrary to what is implied by its name and documentation.
Due to a logic bug introduced in https://github.com/matrix-org/matrix-rust-sdk/pull/2961/commits/71136e44c03c79f80d6d1a2446673bc4d53a2067, the matrix-sdk-crypto crate version 0.7.0 will sometimes log the private part of the backup key pair to Rust debug logs (using the tracing crate).
When matrix-rust-sdk before 0.6 requests a room key from our devices, it correctly accepts key forwards only if they are a response to a previous request. However, it doesn't check that the device that responded matches the device the key was requested from. This allows a malicious homeserver to insert room keys of questionable validity into the key store in some situations, potentially assisting in an impersonation attack. Note that …