CVE-2022-39250

Matrix JavaScript SDK is the Matrix Client-Server software development kit
(SDK) for JavaScript. Prior to version 19.7.0, an attacker cooperating with
a malicious homeserver could interfere with the verification flow between
two users, injecting its own cross-signing user identity in place of one of
the users’ identities. This would lead to the other device
trusting/verifying the user identity under the control of the homeserver
instead of the intended one. The vulnerability is a bug in the
matrix-js-sdk, caused by checking and signing user identities and devices
in two separate steps, and inadequately fixing the keys to be signed
between those steps. Even though the attack is partly made possible due to
the design decision of treating cross-signing user identities as Matrix
devices on the server side (with their device ID set to the public part of
the user identity key), no other examined implementations were vulnerable.
Starting with version 19.7.0, the matrix-js-sdk has been modified to double
check that the key signed is the one that was verified instead of just
referencing the key by ID. An additional check has been made to report an
error when one of the device ID matches a cross-signing key. As this attack
requires coordination between a malicious homeserver and an attacker, those
who trust their homeservers do not need a particular workaround.