Description
Matrix Javascript SDK is the Matrix Client-Server SDK for JavaScript. Prior to version 19.7.0, an attacker cooperating with a malicious homeserver can construct messages appearing to have come from another person. Such messages will be marked with a grey shield on some platforms, but this may be missing in others. This attack is possible due to the matrix-js-sdk implementing a too permissive key forwarding strategy on the receiving end. Starting with version 19.7.0, the default policy for accepting key forwards has been made more strict in the matrix-js-sdk. matrix-js-sdk will now only accept forwarded keys in response to previously issued requests and only from own, verified devices. The SDK now sets a `trusted` flag on the decrypted message upon decryption, based on whether the key used to decrypt the message was received from a trusted source. Clients need to ensure that messages decrypted with a key with `trusted = false` are decorated appropriately, for example, by showing a warning for such messages. This attack requires coordination between a malicious homeserver and an attacker, and those who trust your homeservers do not need a workaround.
Remediation
References
https://github.com/matrix-org/matrix-js-sdk/commit/a587d7c36026fe1fcf93dfff63588abee359be76
https://github.com/matrix-org/matrix-js-sdk/security/advisories/GHSA-6263-x97c-c4gg
https://github.com/matrix-org/matrix-js-sdk/releases/tag/v19.7.0
https://matrix.org/blog/2022/09/28/upgrade-now-to-address-encryption-vulns-in-matrix-sdks-and-clients
https://github.com/matrix-org/matrix-spec-proposals/pull/3061
https://security.gentoo.org/glsa/202210-35