A logic error in the room key sharing functionality of matrix-js-sdk before 12.4.1 allows a malicious Matrix homeserver† participating in an encrypted room to steal room encryption keys from affected Matrix clients participating in that room. This allows the homeserver to decrypt end-to-end encrypted messages sent by affected clients.
† Or anyone with access to the account of the original recipient of an encrypted message.
Known clients affected (via their use of vulnerable versions of matrix-js-sdk):
This was fixed in https://github.com/matrix-org/matrix-js-sdk/commit/894c24880da0e1cc81818f51c0db80e3c9fb2be9.
To prevent a homeserver from being able to steal the room keys, vulnerable clients can be taken offline or signed out. If signing out, care should be taken to either set up Secure Backup or export E2E room keys in order to preserve access to past messages.
github.com/matrix-org/matrix-js-sdk
github.com/matrix-org/matrix-js-sdk/commit/894c24880da0e1cc81818f51c0db80e3c9fb2be9
github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1
github.com/matrix-org/matrix-js-sdk/security/advisories/GHSA-23cm-x6j7-6hq3
matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing
nvd.nist.gov/vuln/detail/CVE-2021-40823