CVE-2022-39249

Name of the Vulnerable Software and Affected Versions Matrix Javascript SDK versions prior to 19.7.0

Description 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. 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 their homeservers do not need a workaround.

Recommendations Update to version 19.7.0 or later, which includes a more strict default policy for accepting key forwards. As a temporary workaround, consider decorating messages decrypted with a key with trusted = false appropriately, for example, by showing a warning for such messages. Restrict access to the key forwarding mechanism to minimize the risk of exploitation. Ensure that clients handle messages with trusted = false correctly, for example, by showing a warning for such messages.