GHSA-9wh6-9hq7-9688 Klever-Go: Validator registration accepts an unvalidated BLS public key → consensus liveness DoS

• Go github.com/klever-io/klever-go: < 1.7.20 → fixed in 1.7.20

Location: `core/kapp/validators/validators.go:201` (`Register`), (`genesis/checking/nodesSetupChecker.go:73`). `core/consensus/slot/bls/subslotStartSlot.go:165` `core/consensus/.../headerSignatureVerify.go:123` (`Create(...)`). Description Klever uses a BDN (Boneh-Drijvers-Neven) BLS multi-signature over BLS12-381 to finalize blocks (`crypto/signing/mcl/multisig/bls.go`, herumi/bls-go-binary). The library is initialized with only `bls.Init(bls.BLS12_381)` and no order-verification flags, so `Deserialize` does not enforce prime-order-subgroup membership subgroup safety relies on explicit `IsValidOrder()` calls in the wrappers. Runtime validator registration (`validators.Register`) stores the submitted 96-byte `BLSPublicKey` via `SetBLSPublicKey` with no curve check, no subgroup check, and no proof-of-possession (`CreateValidatorContract` carries the key but no signature proving key ownership; grep confirms no `proofOfPossession`/`VerifyProof`/`BLSSignature` verification anywhere in `core/kapp/validators/`). `CheckPublicKeyValid` — which does `IsValid && IsValidOrder && !IsZero` — runs only at genesis, not on runtime registration/update. POC Attack: (1) stake the minimum to register a validator, submitting a 96-byte `BLSPublicKey` that is not a valid G2 point (arbitrary bytes). Registration succeeds. (2) Once the validator is eligible and selected into a consensus group, every in-group node including the honest leader (`group[0]`) calls `MultiSigner().Reset(groupPubKeys, selfIndex)` at slot start, which deserializes all group keys via `PublicKeyFromByteArray` → herumi `Deserialize` deterministically fails on the malformed key → `SetSlotCanceled(true)`. The verify side (`Create(consensusPubKeys)`) fails identically. No block is produced for that round. Impact Every consensus round whose group contains the malformed-key validator is a missed slot. One eligible bad-key validator poisons roughly a `groupSize / eligibleSet` fraction of rounds → sustained liveness degradation. Where the consensus group equals the eligible set (small or early-stage networks), this is a full chain halt. Cost is the minimum validator stake, permissionless, and repeatable; no fork flag gates the missing validation. Rated High (Byzantine liveness; Critical on small validator sets). The severity scales down to a fractional missed-slot / throughput-degradation attack on a large validator set where the bad validator is only occasionally in the active group. Recommendation Enforce `CheckPublicKeyValid` (curve + prime-order subgroup + non-zero) on the submitted BLS key at runtime validator registration and config-update, and ideally require a proof-of-possession (a BLS signature over the validator's own key/identity) at registration to prove key ownership and well-formedness. Gate the stricter validation behind an epoch fork flag for reprocessing consistency.