ZF FROST is a Rust implementation of FROST (Flexible Round-Optimised Schnorr Threshold signatures). In versions 2.0.0 through 2.1.0, refresh shares with smaller min_signers will reduce security of group. The inability to change min_signers (i.e. the threshold) with the refresh share functionality (frost_core::keys::refresh module) was not made clear to users. 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, potentially causing a security loss to the participant's shares. This issue is fixed in version 2.2.0.

Affected: crates.io: frost-core

### 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](https://blocksec.com/) for reporting the finding