CVE-2025-58359: frost-core: refresh shares with smaller min_signers will reduce security of group

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.