Impact
A logic error in the room key sharing functionality of Element Android (and matrix-android-sdk2) before 1.2.2 allows a malicious Matrix homeserver† participating in an encrypted room to steal room encryption keys from affected Element Android devices participating in that room. This allows the attacker to decrypt end-to-end encrypted messages previously sent by those Element Android devices.
† Or anyone with access to the account of the original recipient of an encrypted message.
Patch
This was fixed in 3a0734f.
Workarounds
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.
References
Disclosure: https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing
For more information
If you have any questions or comments about this advisory, e-mail us at security@matrix.org.
dkasak Analyst
Impact
A logic error in the room key sharing functionality of Element Android (and matrix-android-sdk2) before 1.2.2 allows a malicious Matrix homeserver† participating in an encrypted room to steal room encryption keys from affected Element Android devices participating in that room. This allows the attacker to decrypt end-to-end encrypted messages previously sent by those Element Android devices.
† Or anyone with access to the account of the original recipient of an encrypted message.
Patch
This was fixed in 3a0734f.
Workarounds
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.
References
Disclosure: https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing
For more information
If you have any questions or comments about this advisory, e-mail us at security@matrix.org.