Impact The "matrix-sdk-crypto" crate before 0.16.1 is missing a check for the sender's user ID when decrypting an Olm-encrypted to-device message containing the "sender_device_keys" property. This could be exploited to spoof the sender of an encrypted to-device message, but only if the attacker colludes with (or is) the homeserver operator. Patches This issue is fixed in "matrix-sdk-crypto" 0.16.1. Workarounds There are no known workarounds for the issue. References This issue was fixed in https://github.com/matrix-org/matrix-rust-sdk/pull/6553. For more information If you have any questions or comments about this advisory, please email us at "security at matrix.org" (mailto:security@matrix.org).