frost-core: refresh shares with smaller min_signers will reduce security of group

Impact

It was not clear that it is not possible to change min_signers (i.e. the threshold) with the refresh share functionality (frost_core::keys::refresh module). Using a smaller value would not decrease the threshold, and attempts to sign using a smaller threshold would fail. Additionally, after refreshing the shares with a smaller threshold, it would still be possible to sign with the original threshold; however, this could cause a security loss to the participant's shares. We have not determined the exact security implications of doing so and judged simpler to just validate min_signers.

If for some reason you have done a refresh share procedure with a smaller min_signers we strongly recommend migrating to a new key.

Patches

Updating to 2.2.0 will ensure that the min_signers parameter will be validated. However it won't restore the security of groups refreshed with a smaller min_signers parameters.

Workarounds

You don't need to update if you don't use the refresh share functionality, or if you didn't try to change the min_signers parameter using the refresh share functionality.

References

Thank you BlockSec for reporting the finding