diff --git a/CHANGELOG_PENDING.md b/CHANGELOG_PENDING.md
index ef2586f6a..10b297be8 100644
--- a/CHANGELOG_PENDING.md
+++ b/CHANGELOG_PENDING.md
@@ -18,6 +18,7 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 - Go API
   - [abci/client, proxy] \#5673 `Async` funcs return an error, `Sync` and `Async` funcs accept `context.Context` (@melekes)
   - [p2p] Removed unused function `MakePoWTarget`. (@erikgrinaker)
+  - [libs/bits] \#5720 Validate `BitArray` in `FromProto`, which now returns an error (@melekes)
 
 - [libs/os] Kill() and {Must,}{Read,Write}File() functions have been removed. (@alessio)
 
diff --git a/consensus/msgs.go b/consensus/msgs.go
index 4de96b5f4..91c091ba1 100644
--- a/consensus/msgs.go
+++ b/consensus/msgs.go
@@ -167,11 +167,14 @@ func MsgFromProto(msg *tmcons.Message) (Message, error) {
 	case *tmcons.Message_NewValidBlock:
 		pbPartSetHeader, err := types.PartSetHeaderFromProto(&msg.NewValidBlock.BlockPartSetHeader)
 		if err != nil {
-			return nil, fmt.Errorf("parts to proto error: %w", err)
+			return nil, fmt.Errorf("parts header to proto error: %w", err)
 		}
 
 		pbBits := new(bits.BitArray)
-		pbBits.FromProto(msg.NewValidBlock.BlockParts)
+		err = pbBits.FromProto(msg.NewValidBlock.BlockParts)
+		if err != nil {
+			return nil, fmt.Errorf("parts to proto error: %w", err)
+		}
 
 		pb = &NewValidBlockMessage{
 			Height:             msg.NewValidBlock.Height,
@@ -191,7 +194,10 @@ func MsgFromProto(msg *tmcons.Message) (Message, error) {
 		}
 	case *tmcons.Message_ProposalPol:
 		pbBits := new(bits.BitArray)
-		pbBits.FromProto(&msg.ProposalPol.ProposalPol)
+		err := pbBits.FromProto(&msg.ProposalPol.ProposalPol)
+		if err != nil {
+			return nil, fmt.Errorf("proposal PoL to proto error: %w", err)
+		}
 		pb = &ProposalPOLMessage{
 			Height:           msg.ProposalPol.Height,
 			ProposalPOLRound: msg.ProposalPol.ProposalPolRound,
@@ -237,10 +243,13 @@ func MsgFromProto(msg *tmcons.Message) (Message, error) {
 	case *tmcons.Message_VoteSetBits:
 		bi, err := types.BlockIDFromProto(&msg.VoteSetBits.BlockID)
 		if err != nil {
-			return nil, fmt.Errorf("voteSetBits msg to proto error: %w", err)
+			return nil, fmt.Errorf("block ID to proto error: %w", err)
 		}
 		bits := new(bits.BitArray)
-		bits.FromProto(&msg.VoteSetBits.Votes)
+		err = bits.FromProto(&msg.VoteSetBits.Votes)
+		if err != nil {
+			return nil, fmt.Errorf("votes to proto error: %w", err)
+		}
 
 		pb = &VoteSetBitsMessage{
 			Height:  msg.VoteSetBits.Height,
diff --git a/libs/bits/bit_array.go b/libs/bits/bit_array.go
index 9d6901460..1a41d87f9 100644
--- a/libs/bits/bit_array.go
+++ b/libs/bits/bit_array.go
@@ -2,7 +2,9 @@ package bits
 
 import (
 	"encoding/binary"
+	"errors"
 	"fmt"
+	"math"
 	"regexp"
 	"strings"
 	"sync"
@@ -27,7 +29,7 @@ func NewBitArray(bits int) *BitArray {
 	}
 	return &BitArray{
 		Bits:  bits,
-		Elems: make([]uint64, (bits+63)/64),
+		Elems: make([]uint64, numElems(bits)),
 	}
 }
 
@@ -100,7 +102,7 @@ func (bA *BitArray) copy() *BitArray {
 }
 
 func (bA *BitArray) copyBits(bits int) *BitArray {
-	c := make([]uint64, (bits+63)/64)
+	c := make([]uint64, numElems(bits))
 	copy(c, bA.Elems)
 	return &BitArray{
 		Bits:  bits,
@@ -418,27 +420,45 @@ func (bA *BitArray) UnmarshalJSON(bz []byte) error {
 	return nil
 }
 
-// ToProto converts BitArray to protobuf
+// ToProto converts BitArray to protobuf. It returns nil if BitArray is
+// nil/empty.
+//
+// XXX: It does not copy the array.
 func (bA *BitArray) ToProto() *tmprotobits.BitArray {
-	if bA == nil || len(bA.Elems) == 0 {
+	if bA == nil ||
+		(len(bA.Elems) == 0 && bA.Bits == 0) { // empty
 		return nil
 	}
 
-	return &tmprotobits.BitArray{
-		Bits:  int64(bA.Bits),
-		Elems: bA.Elems,
-	}
+	return &tmprotobits.BitArray{Bits: int64(bA.Bits), Elems: bA.Elems}
 }
 
-// FromProto sets a protobuf BitArray to the given pointer.
-func (bA *BitArray) FromProto(protoBitArray *tmprotobits.BitArray) {
+// FromProto sets BitArray to the given protoBitArray. It returns an error if
+// protoBitArray is invalid.
+//
+// XXX: It does not copy the array.
+func (bA *BitArray) FromProto(protoBitArray *tmprotobits.BitArray) error {
 	if protoBitArray == nil {
-		bA = nil
-		return
+		return nil
+	}
+
+	// Validate protoBitArray.
+	if protoBitArray.Bits < 0 {
+		return errors.New("negative Bits")
+	}
+	// #[32bit]
+	if protoBitArray.Bits > math.MaxInt32 { // prevent overflow on 32bit systems
+		return errors.New("too many Bits")
+	}
+	if got, exp := len(protoBitArray.Elems), numElems(int(protoBitArray.Bits)); got != exp {
+		return fmt.Errorf("invalid number of Elems: got %d, but exp %d", got, exp)
 	}
 
 	bA.Bits = int(protoBitArray.Bits)
-	if len(protoBitArray.Elems) > 0 {
-		bA.Elems = protoBitArray.Elems
-	}
+	bA.Elems = protoBitArray.Elems
+	return nil
+}
+
+func numElems(bits int) int {
+	return (bits + 63) / 64
 }
diff --git a/libs/bits/bit_array_test.go b/libs/bits/bit_array_test.go
index e4306ecf2..10d607ef2 100644
--- a/libs/bits/bit_array_test.go
+++ b/libs/bits/bit_array_test.go
@@ -4,12 +4,14 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"math"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	tmrand "github.com/tendermint/tendermint/libs/rand"
+	tmprotobits "github.com/tendermint/tendermint/proto/tendermint/libs/bits"
 )
 
 func randBitArray(bits int) (*BitArray, []byte) {
@@ -266,7 +268,7 @@ func TestJSONMarshalUnmarshal(t *testing.T) {
 	}
 }
 
-func TestBitArrayProtoBuf(t *testing.T) {
+func TestBitArrayToFromProto(t *testing.T) {
 	testCases := []struct {
 		msg     string
 		bA1     *BitArray
@@ -280,11 +282,41 @@ func TestBitArrayProtoBuf(t *testing.T) {
 	for _, tc := range testCases {
 		protoBA := tc.bA1.ToProto()
 		ba := new(BitArray)
-		ba.FromProto(protoBA)
+		err := ba.FromProto(protoBA)
 		if tc.expPass {
+			assert.NoError(t, err)
 			require.Equal(t, tc.bA1, ba, tc.msg)
 		} else {
 			require.NotEqual(t, tc.bA1, ba, tc.msg)
 		}
 	}
 }
+
+func TestBitArrayFromProto(t *testing.T) {
+	testCases := []struct {
+		pbA    *tmprotobits.BitArray
+		resA   *BitArray
+		expErr bool
+	}{
+		0: {nil, &BitArray{}, false},
+		1: {&tmprotobits.BitArray{}, &BitArray{}, false},
+
+		2: {&tmprotobits.BitArray{Bits: 1, Elems: make([]uint64, 1)}, &BitArray{Bits: 1, Elems: make([]uint64, 1)}, false},
+
+		3: {&tmprotobits.BitArray{Bits: -1, Elems: make([]uint64, 1)}, &BitArray{}, true},
+		4: {&tmprotobits.BitArray{Bits: math.MaxInt32 + 1, Elems: make([]uint64, 1)}, &BitArray{}, true},
+		5: {&tmprotobits.BitArray{Bits: 1, Elems: make([]uint64, 2)}, &BitArray{}, true},
+	}
+
+	for i, tc := range testCases {
+		bA := new(BitArray)
+		err := bA.FromProto(tc.pbA)
+		if tc.expErr {
+			assert.Error(t, err, "#%d", i)
+			assert.Equal(t, tc.resA, bA, "#%d", i)
+		} else {
+			assert.NoError(t, err, "#%d", i)
+			assert.Equal(t, tc.resA, bA, "#%d", i)
+		}
+	}
+}
diff --git a/test/maverick/consensus/msgs.go b/test/maverick/consensus/msgs.go
index 4de96b5f4..a1ac7c1a4 100644
--- a/test/maverick/consensus/msgs.go
+++ b/test/maverick/consensus/msgs.go
@@ -167,11 +167,14 @@ func MsgFromProto(msg *tmcons.Message) (Message, error) {
 	case *tmcons.Message_NewValidBlock:
 		pbPartSetHeader, err := types.PartSetHeaderFromProto(&msg.NewValidBlock.BlockPartSetHeader)
 		if err != nil {
-			return nil, fmt.Errorf("parts to proto error: %w", err)
+			return nil, fmt.Errorf("parts header to proto error: %w", err)
 		}
 
 		pbBits := new(bits.BitArray)
-		pbBits.FromProto(msg.NewValidBlock.BlockParts)
+		err = pbBits.FromProto(msg.NewValidBlock.BlockParts)
+		if err != nil {
+			return nil, fmt.Errorf("parts to proto error: %w", err)
+		}
 
 		pb = &NewValidBlockMessage{
 			Height:             msg.NewValidBlock.Height,
@@ -191,7 +194,11 @@ func MsgFromProto(msg *tmcons.Message) (Message, error) {
 		}
 	case *tmcons.Message_ProposalPol:
 		pbBits := new(bits.BitArray)
-		pbBits.FromProto(&msg.ProposalPol.ProposalPol)
+		err := pbBits.FromProto(&msg.ProposalPol.ProposalPol)
+		if err != nil {
+			return nil, fmt.Errorf("proposal PoL to proto error: %w", err)
+		}
+
 		pb = &ProposalPOLMessage{
 			Height:           msg.ProposalPol.Height,
 			ProposalPOLRound: msg.ProposalPol.ProposalPolRound,
@@ -237,10 +244,13 @@ func MsgFromProto(msg *tmcons.Message) (Message, error) {
 	case *tmcons.Message_VoteSetBits:
 		bi, err := types.BlockIDFromProto(&msg.VoteSetBits.BlockID)
 		if err != nil {
-			return nil, fmt.Errorf("voteSetBits msg to proto error: %w", err)
+			return nil, fmt.Errorf("block ID msg to proto error: %w", err)
 		}
 		bits := new(bits.BitArray)
-		bits.FromProto(&msg.VoteSetBits.Votes)
+		err = bits.FromProto(&msg.VoteSetBits.Votes)
+		if err != nil {
+			return nil, fmt.Errorf("votes to proto error: %w", err)
+		}
 
 		pb = &VoteSetBitsMessage{
 			Height:  msg.VoteSetBits.Height,