CVE-2022-39249

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.

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.

## Impact 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](https://spec.matrix.org/v1.3/client-server-api/#key-requests) strategy on the receiving end. Key forwarding is a mechanism allowing clients to recover from “unable to decrypt” messages when they missed the initial key distribution, at the time the message was originally sent. Examples include accessing message history before they joined the room but also when some network/federation errors have occurred. ## Patches 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. A unique exception to this rule is with the experimental [MSC3061](https://github.com/matrix-org/matrix-spec-proposals/pull/3061), that is forwarding room keys for past messages when invited in a room configured with the proper history visibility setting. Such key forwards are parked upon receipt and are only accepted if the SDK receives an invitation for that room from the inviter in a limited time window. 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). ### Workarounds As this attack requires coordination between a malicious homeserver and an attacker, if you trust your homeserver, no particular workaround is needed. ## References Blog post: https://matrix.org/blog/2022/09/28/upgrade-now-to-address-encryption-vulns-in-matrix-sdks-and-clients ## For more information If you have any questions or comments about this advisory, e-mail us at security@matrix.org.

## Impact 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](https://spec.matrix.org/v1.3/client-server-api/#key-requests) strategy on the receiving end. Key forwarding is a mechanism allowing clients to recover from “unable to decrypt” messages when they missed the initial key distribution, at the time the message was originally sent. Examples include accessing message history before they joined the room but also when some network/federation errors have occurred. ## Patches 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. A unique exception to this rule is with the experimental [MSC3061](https://github.com/matrix-org/matrix-spec-proposals/pull/3061), that is forwarding room keys for past messages when invited in a room configured with the proper history visibility setting. Such key forwards are parked upon receipt and are only accepted if the SDK receives an invitation for that room from the inviter in a limited time window. 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). ### Workarounds As this attack requires coordination between a malicious homeserver and an attacker, if you trust your homeserver, no particular workaround is needed. ## References Blog post: https://matrix.org/blog/2022/09/28/upgrade-now-to-address-encryption-vulns-in-matrix-sdks-and-clients ## For more information If you have any questions or comments about this advisory, e-mail us at security@matrix.org.

Matrix Javascript SDK es el SDK cliente-servidor de Matrix para JavaScript. En versiones anteriores a 19.7.0, un atacante que coopere con un servidor doméstico malicioso puede construir mensajes que parezcan proceder de otra persona. Dichos mensajes estarán marcados con un escudo gris en algunas plataformas, pero éste puede faltar en otras. Este ataque es posible debido a que matrix-js-sdk implementa una estrategia de reenvío de claves demasiado permisiva en el extremo receptor. A partir de la versión 19.7.0, la política por defecto para aceptar reenvíos de claves se ha hecho más estricta en matrix-js-sdk. matrix-js-sdk ahora sólo aceptará claves reenviadas en respuesta a peticiones previamente emitidas y sólo de dispositivos propios y verificados. El SDK ahora establece un flag "trusted" en el mensaje descifrado al descifrarlo, basándose en si la clave usada para descifrar el mensaje es recibida de una fuente confiable. Los clientes deben asegurarse de que los mensajes descifrados con una clave con "trusted = false" sean decorados adecuadamente, por ejemplo, mostrando una advertencia para tales mensajes. Este ataque requiere la coordinación entre un servidor doméstico malicioso y un atacante, y aquellos que confían en sus servidores domésticos no necesitan una mitigación