GHSA-wgq8-vr6r-mqxm frost-core: refresh shares with smaller min_signers will reduce security of group

• crates.io frost-core: >= 2.0.0, < 2.2.0 → fixed in 2.2.0

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