diff --git a/signer/cosigner.go b/signer/cosigner.go
index f3db4254..b1a9f116 100644
--- a/signer/cosigner.go
+++ b/signer/cosigner.go
@@ -2,9 +2,13 @@ package signer
 
 import (
 	"context"
+	"errors"
+	"fmt"
 	"time"
 
 	cometcrypto "github.com/cometbft/cometbft/crypto"
+	"github.com/cometbft/cometbft/libs/protoio"
+	cometproto "github.com/cometbft/cometbft/proto/tendermint/types"
 	"github.com/google/uuid"
 	"github.com/strangelove-ventures/horcrux/v3/signer/proto"
 )
@@ -150,3 +154,51 @@ type CosignerSetNoncesAndSignRequest struct {
 	SignBytes              []byte
 	VoteExtensionSignBytes []byte
 }
+
+func verifySignPayload(chainID string, signBytes, voteExtensionSignBytes []byte) (HRSTKey, bool, error) {
+	var vote cometproto.CanonicalVote
+	voteErr := protoio.UnmarshalDelimited(signBytes, &vote)
+	if voteErr == nil && (vote.Type == cometproto.PrevoteType || vote.Type == cometproto.PrecommitType) {
+		hrstKey := HRSTKey{
+			Height:    vote.Height,
+			Round:     vote.Round,
+			Step:      CanonicalVoteToStep(&vote),
+			Timestamp: vote.Timestamp.UnixNano(),
+		}
+
+		if hrstKey.Step == stepPrecommit && len(voteExtensionSignBytes) > 0 && vote.BlockID != nil {
+			var voteExt cometproto.CanonicalVoteExtension
+			if err := protoio.UnmarshalDelimited(voteExtensionSignBytes, &voteExt); err != nil {
+				return hrstKey, false, fmt.Errorf("failed to unmarshal vote extension: %w", err)
+			}
+			if voteExt.ChainId != chainID {
+				return hrstKey, false, fmt.Errorf("vote extension chain ID %s does not match chain ID %s", voteExt.ChainId, chainID)
+			}
+			if voteExt.Height != hrstKey.Height {
+				return hrstKey, false,
+					fmt.Errorf("vote extension height %d does not match block height %d", voteExt.Height, hrstKey.Height)
+			}
+			if voteExt.Round != hrstKey.Round {
+				return hrstKey, false,
+					fmt.Errorf("vote extension round %d does not match block round %d", voteExt.Round, hrstKey.Round)
+			}
+			return hrstKey, true, nil
+		}
+
+		return hrstKey, false, nil
+	}
+
+	var proposal cometproto.CanonicalProposal
+	proposalErr := protoio.UnmarshalDelimited(signBytes, &proposal)
+	if proposalErr == nil {
+		return HRSTKey{
+			Height:    proposal.Height,
+			Round:     proposal.Round,
+			Step:      stepPropose,
+			Timestamp: proposal.Timestamp.UnixNano(),
+		}, false, nil
+	}
+
+	return HRSTKey{}, false,
+		fmt.Errorf("failed to unmarshal sign bytes into vote or proposal: %w", errors.Join(voteErr, proposalErr))
+}
diff --git a/signer/file.go b/signer/file.go
index b0eae6f6..d4379dbc 100644
--- a/signer/file.go
+++ b/signer/file.go
@@ -200,7 +200,7 @@ func (pv *FilePV) GetPubKey() (crypto.PubKey, error) {
 	return pv.Key.PubKey, nil
 }
 
-func (pv *FilePV) Sign(block Block) ([]byte, []byte, time.Time, error) {
+func (pv *FilePV) Sign(chainID string, block Block) ([]byte, []byte, time.Time, error) {
 	height, round, step := block.Height, int32(block.Round), block.Step
 	signBytes, voteExtensionSignBytes := block.SignBytes, block.VoteExtensionSignBytes
 
@@ -211,13 +211,18 @@ func (pv *FilePV) Sign(block Block) ([]byte, []byte, time.Time, error) {
 		return nil, nil, block.Timestamp, err
 	}
 
+	_, hasVoteExtensions, err := verifySignPayload(chainID, signBytes, voteExtensionSignBytes)
+	if err != nil {
+		return nil, nil, block.Timestamp, err
+	}
+
 	// Vote extensions are non-deterministic, so it is possible that an
 	// application may have created a different extension. We therefore always
 	// re-sign the vote extensions of precommits. For prevotes and nil
 	// precommits, the extension signature will always be empty.
 	// Even if the signed over data is empty, we still add the signature
 	var extSig []byte
-	if block.Step == stepPrecommit && len(voteExtensionSignBytes) > 0 {
+	if hasVoteExtensions {
 		extSig, err = pv.Key.PrivKey.Sign(voteExtensionSignBytes)
 		if err != nil {
 			return nil, nil, block.Timestamp, err
diff --git a/signer/io.go b/signer/io.go
new file mode 100644
index 00000000..45237a8a
--- /dev/null
+++ b/signer/io.go
@@ -0,0 +1,23 @@
+package signer
+
+import (
+	"io"
+
+	"github.com/cometbft/cometbft/libs/protoio"
+	cometprotoprivval "github.com/cometbft/cometbft/proto/tendermint/privval"
+)
+
+// ReadMsg reads a message from an io.Reader
+func ReadMsg(reader io.Reader) (msg cometprotoprivval.Message, err error) {
+	const maxRemoteSignerMsgSize = 1024 * 10
+	protoReader := protoio.NewDelimitedReader(reader, maxRemoteSignerMsgSize)
+	_, err = protoReader.ReadMsg(&msg)
+	return msg, err
+}
+
+// WriteMsg writes a message to an io.Writer
+func WriteMsg(writer io.Writer, msg cometprotoprivval.Message) (err error) {
+	protoWriter := protoio.NewDelimitedWriter(writer)
+	_, err = protoWriter.WriteMsg(&msg)
+	return err
+}
diff --git a/signer/local_cosigner.go b/signer/local_cosigner.go
index 10875b88..4559974f 100644
--- a/signer/local_cosigner.go
+++ b/signer/local_cosigner.go
@@ -214,14 +214,14 @@ func (cosigner *LocalCosigner) sign(req CosignerSignRequest) (CosignerSignRespon
 		return res, err
 	}
 
-	// This function has multiple exit points.  Only start time can be guaranteed
-	metricsTimeKeeper.SetPreviousLocalSignStart(time.Now())
-
-	hrst, err := UnpackHRST(req.SignBytes)
+	hrst, hasVoteExtensions, err := verifySignPayload(chainID, req.SignBytes, req.VoteExtensionSignBytes)
 	if err != nil {
 		return res, err
 	}
 
+	// This function has multiple exit points.  Only start time can be guaranteed
+	metricsTimeKeeper.SetPreviousLocalSignStart(time.Now())
+
 	existingSignature, err := ccs.lastSignState.existingSignatureOrErrorIfRegression(hrst, req.SignBytes)
 	if err != nil {
 		return res, err
@@ -249,7 +249,7 @@ func (cosigner *LocalCosigner) sign(req CosignerSignRequest) (CosignerSignRespon
 	}
 
 	var voteExtNonces []Nonce
-	if len(req.VoteExtensionSignBytes) > 0 {
+	if hasVoteExtensions {
 		voteExtNonces, err = cosigner.combinedNonces(
 			cosigner.GetID(),
 			uint8(cosigner.config.Config.ThresholdModeConfig.Threshold),
@@ -268,7 +268,7 @@ func (cosigner *LocalCosigner) sign(req CosignerSignRequest) (CosignerSignRespon
 		sig, err = ccs.signer.Sign(nonces, req.SignBytes)
 		return err
 	})
-	if len(req.VoteExtensionSignBytes) > 0 {
+	if hasVoteExtensions {
 		eg.Go(func() error {
 			var err error
 			voteExtSig, err = ccs.signer.Sign(voteExtNonces, req.VoteExtensionSignBytes)
diff --git a/signer/serialization.go b/signer/serialization.go
deleted file mode 100644
index ee6b5ab4..00000000
--- a/signer/serialization.go
+++ /dev/null
@@ -1,44 +0,0 @@
-package signer
-
-import (
-	"errors"
-	"io"
-
-	"github.com/cometbft/cometbft/libs/protoio"
-	cometprotoprivval "github.com/cometbft/cometbft/proto/tendermint/privval"
-	cometproto "github.com/cometbft/cometbft/proto/tendermint/types"
-)
-
-// ReadMsg reads a message from an io.Reader
-func ReadMsg(reader io.Reader) (msg cometprotoprivval.Message, err error) {
-	const maxRemoteSignerMsgSize = 1024 * 10
-	protoReader := protoio.NewDelimitedReader(reader, maxRemoteSignerMsgSize)
-	_, err = protoReader.ReadMsg(&msg)
-	return msg, err
-}
-
-// WriteMsg writes a message to an io.Writer
-func WriteMsg(writer io.Writer, msg cometprotoprivval.Message) (err error) {
-	protoWriter := protoio.NewDelimitedWriter(writer)
-	_, err = protoWriter.WriteMsg(&msg)
-	return err
-}
-
-// UnpackHRS deserializes sign bytes and gets the height, round, and step
-func UnpackHRST(signBytes []byte) (HRSTKey, error) {
-	{
-		var proposal cometproto.CanonicalProposal
-		if err := protoio.UnmarshalDelimited(signBytes, &proposal); err == nil {
-			return HRSTKey{proposal.Height, proposal.Round, stepPropose, proposal.Timestamp.UnixNano()}, nil
-		}
-	}
-
-	{
-		var vote cometproto.CanonicalVote
-		if err := protoio.UnmarshalDelimited(signBytes, &vote); err == nil {
-			return HRSTKey{vote.Height, vote.Round, CanonicalVoteToStep(&vote), vote.Timestamp.UnixNano()}, nil
-		}
-	}
-
-	return HRSTKey{0, 0, 0, 0}, errors.New("could not UnpackHRS from sign bytes")
-}
diff --git a/signer/serialization_test.go b/signer/serialization_test.go
deleted file mode 100644
index d51a678c..00000000
--- a/signer/serialization_test.go
+++ /dev/null
@@ -1,57 +0,0 @@
-package signer
-
-import (
-	"testing"
-
-	cometproto "github.com/cometbft/cometbft/proto/tendermint/types"
-	comet "github.com/cometbft/cometbft/types"
-	"github.com/stretchr/testify/require"
-)
-
-func TestUnpackHRSPrevote(t *testing.T) {
-	vote := cometproto.Vote{
-		Height: 1,
-		Round:  2,
-		Type:   cometproto.PrevoteType,
-	}
-
-	signBytes := comet.VoteSignBytes("chain-id", &vote)
-
-	hrs, err := UnpackHRST(signBytes)
-	require.NoError(t, err)
-	require.Equal(t, int64(1), hrs.Height)
-	require.Equal(t, int64(2), hrs.Round)
-	require.Equal(t, int8(2), hrs.Step)
-}
-
-func TestUnpackHRSPrecommit(t *testing.T) {
-	vote := cometproto.Vote{
-		Height: 3,
-		Round:  2,
-		Type:   cometproto.PrecommitType,
-	}
-
-	signBytes := comet.VoteSignBytes("chain-id", &vote)
-
-	hrs, err := UnpackHRST(signBytes)
-	require.NoError(t, err)
-	require.Equal(t, int64(3), hrs.Height)
-	require.Equal(t, int64(2), hrs.Round)
-	require.Equal(t, int8(3), hrs.Step)
-}
-
-func TestUnpackHRSProposal(t *testing.T) {
-	proposal := cometproto.Proposal{
-		Height: 1,
-		Round:  2,
-		Type:   cometproto.ProposalType,
-	}
-
-	signBytes := comet.ProposalSignBytes("chain-id", &proposal)
-
-	hrs, err := UnpackHRST(signBytes)
-	require.NoError(t, err)
-	require.Equal(t, int64(1), hrs.Height)
-	require.Equal(t, int64(2), hrs.Round)
-	require.Equal(t, int8(1), hrs.Step)
-}
diff --git a/signer/sign_state.go b/signer/sign_state.go
index 819a36b0..f6c55eb0 100644
--- a/signer/sign_state.go
+++ b/signer/sign_state.go
@@ -100,12 +100,13 @@ func StepToType(step int8) cometproto.SignedMsgType {
 
 // SignState stores signing information for high level watermark management.
 type SignState struct {
-	Height      int64               `json:"height"`
-	Round       int64               `json:"round"`
-	Step        int8                `json:"step"`
-	NoncePublic []byte              `json:"nonce_public"`
-	Signature   []byte              `json:"signature,omitempty"`
-	SignBytes   cometbytes.HexBytes `json:"signbytes,omitempty"`
+	Height                 int64               `json:"height"`
+	Round                  int64               `json:"round"`
+	Step                   int8                `json:"step"`
+	NoncePublic            []byte              `json:"nonce_public"`
+	Signature              []byte              `json:"signature,omitempty"`
+	SignBytes              cometbytes.HexBytes `json:"signbytes,omitempty"`
+	VoteExtensionSignature []byte              `json:"vote_ext_signature,omitempty"`
 
 	filePath string
 
@@ -232,6 +233,7 @@ func (signState *SignState) cacheAndMarshal(ssc SignStateConsensus) []byte {
 	signState.Step = ssc.Step
 	signState.Signature = ssc.Signature
 	signState.SignBytes = ssc.SignBytes
+	signState.VoteExtensionSignature = ssc.VoteExtensionSignature
 
 	jsonBytes, err := cometjson.MarshalIndent(signState, "", "  ")
 	if err != nil {
@@ -416,13 +418,14 @@ func (signState *SignState) GetErrorIfLessOrEqual(height int64, round int64, ste
 // including the most recent sign state.
 func (signState *SignState) FreshCache() *SignState {
 	newSignState := &SignState{
-		Height:      signState.Height,
-		Round:       signState.Round,
-		Step:        signState.Step,
-		NoncePublic: signState.NoncePublic,
-		Signature:   signState.Signature,
-		SignBytes:   signState.SignBytes,
-		cache:       make(map[HRSKey]SignStateConsensus),
+		Height:                 signState.Height,
+		Round:                  signState.Round,
+		Step:                   signState.Step,
+		NoncePublic:            signState.NoncePublic,
+		Signature:              signState.Signature,
+		SignBytes:              signState.SignBytes,
+		VoteExtensionSignature: signState.VoteExtensionSignature,
+		cache:                  make(map[HRSKey]SignStateConsensus),
 
 		filePath: signState.filePath,
 	}
@@ -434,11 +437,12 @@ func (signState *SignState) FreshCache() *SignState {
 		Round:  signState.Round,
 		Step:   signState.Step,
 	}] = SignStateConsensus{
-		Height:    signState.Height,
-		Round:     signState.Round,
-		Step:      signState.Step,
-		Signature: signState.Signature,
-		SignBytes: signState.SignBytes,
+		Height:                 signState.Height,
+		Round:                  signState.Round,
+		Step:                   signState.Step,
+		Signature:              signState.Signature,
+		SignBytes:              signState.SignBytes,
+		VoteExtensionSignature: signState.VoteExtensionSignature,
 	}
 
 	return newSignState
diff --git a/signer/single_signer_validator.go b/signer/single_signer_validator.go
index 33d78de0..846e283d 100644
--- a/signer/single_signer_validator.go
+++ b/signer/single_signer_validator.go
@@ -65,7 +65,7 @@ func (pv *SingleSignerValidator) Sign(
 	chainState.pvMutex.Lock()
 	defer chainState.pvMutex.Unlock()
 
-	return chainState.filePV.Sign(block)
+	return chainState.filePV.Sign(chainID, block)
 }
 
 func (pv *SingleSignerValidator) loadChainStateIfNecessary(chainID string) (*SingleSignerChainState, error) {
diff --git a/signer/threshold_validator.go b/signer/threshold_validator.go
index ba33dbb1..7ca5f87b 100644
--- a/signer/threshold_validator.go
+++ b/signer/threshold_validator.go
@@ -699,7 +699,14 @@ func (pv *ThresholdValidator) Sign(
 	}
 
 	var voteExtNonces *CosignerUUIDNonces
-	if step == stepPrecommit && len(voteExtensionSignBytes) > 0 {
+
+	_, hasVoteExtensions, err := verifySignPayload(chainID, signBytes, voteExtensionSignBytes)
+	if err != nil {
+		pv.notifyBlockSignError(chainID, block.HRSKey(), signBytes)
+		return nil, nil, stamp, fmt.Errorf("failed to verify payload: %w", err)
+	}
+
+	if hasVoteExtensions {
 		voteExtNonces, err = pv.nonceCache.GetNonces(cosignersForThisBlock)
 		if err != nil {
 			// TODO how to handle fallback for vote extensions?
@@ -853,7 +860,7 @@ func (pv *ThresholdValidator) Sign(
 
 	var voteExtSig []byte
 
-	if step == stepPrecommit && len(voteExtensionSignBytes) > 0 {
+	if hasVoteExtensions {
 		// collect all valid responses into array of partial signatures
 		voteExtShareSigs := make([]PartialSignature, 0, pv.threshold)
 		for idx, shareSig := range voteExtShareSignatures {
diff --git a/signer/threshold_validator_test.go b/signer/threshold_validator_test.go
index 05d499d5..ca016cdc 100644
--- a/signer/threshold_validator_test.go
+++ b/signer/threshold_validator_test.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"crypto/rand"
+	"crypto/sha256"
 	"fmt"
 	mrand "math/rand"
 	"path/filepath"
@@ -254,9 +255,13 @@ func testThresholdValidator(t *testing.T, threshold, total uint8) {
 		err = eg.Wait()
 		require.NoError(t, err)
 
+		blockIDHash := sha256.New()
+		blockIDHash.Write([]byte("something"))
+
 		precommit := cometproto.Vote{
 			Height:    int64(i),
 			Round:     0,
+			BlockID:   cometproto.BlockID{Hash: blockIDHash.Sum(nil)},
 			Type:      cometproto.PrecommitType,
 			Timestamp: time.Now(),
 			Extension: []byte("test"),