Impact

All versions below 0.10.2.

While we did verify that it was us who replied,
we didn't properly cancel storing the secret if the sending device was
one of ours but was maliciously inserted by the homeserver and
unverified. We only send secret requests to verified devices in the
first place, so only the homeserver could abuse this issue.

Additionally we protected against malicious secret poisoning by
verifying that the secret is actually the reply to a request. This means
the server only has 2 places where it can poison the secrets:

• After a verification when we automatically request the secrets
• When the user manually hits the request button

It also needs to prevent other secret answers to reach the client first
since we ignore all replies after that one.

The impact of this might be quite severe. It could allow the server to
replace the cross-signing keys silently and while we might not trust
that key, we possibly could trust it in the future if we rely on the
stored secret. Similarly this could potentially be abused to make the
client trust a malicious online key backup.

Patches

Users can upgrade to version 0.10.2 to protect against this or apply 67bee15 .

Workarounds

If your version is not patched yet and you don't control your
homeserver, you can protect against this by simply not doing any
verifications of your own devices and not pressing the request button in
the settings menu.

References

• Similar issues in other matrix clients: 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:

• Open an issue in the project repo
• Discussing it in the project room on matrix: https://matrix.to/#/#nheko:nheko.im