diff --git a/x/auth/tx/decoder.go b/x/auth/tx/decoder.go
index a32a118604f9..d93c7446db60 100644
--- a/x/auth/tx/decoder.go
+++ b/x/auth/tx/decoder.go
@@ -1,6 +1,10 @@
 package tx
 
 import (
+	"fmt"
+
+	"google.golang.org/protobuf/encoding/protowire"
+
 	"github.com/cosmos/cosmos-sdk/codec"
 	"github.com/cosmos/cosmos-sdk/codec/unknownproto"
 	sdk "github.com/cosmos/cosmos-sdk/types"
@@ -11,10 +15,16 @@ import (
 // DefaultTxDecoder returns a default protobuf TxDecoder using the provided Marshaler.
 func DefaultTxDecoder(cdc codec.ProtoCodecMarshaler) sdk.TxDecoder {
 	return func(txBytes []byte) (sdk.Tx, error) {
+		// Make sure txBytes follow ADR-027.
+		err := rejectNonADR027(txBytes)
+		if err != nil {
+			return nil, sdkerrors.Wrap(sdkerrors.ErrTxDecode, err.Error())
+		}
+
 		var raw tx.TxRaw
 
 		// reject all unknown proto fields in the root TxRaw
-		err := unknownproto.RejectUnknownFieldsStrict(txBytes, &raw, cdc.InterfaceRegistry())
+		err = unknownproto.RejectUnknownFieldsStrict(txBytes, &raw, cdc.InterfaceRegistry())
 		if err != nil {
 			return nil, sdkerrors.Wrap(sdkerrors.ErrTxDecode, err.Error())
 		}
@@ -79,3 +89,77 @@ func DefaultJSONTxDecoder(cdc codec.ProtoCodecMarshaler) sdk.TxDecoder {
 		}, nil
 	}
 }
+
+// rejectNonADR027 rejects txBytes that do not follow ADR-027. This function
+// only checks that:
+// - field numbers are in ascending order (1, 2, and potentially multiple 3s),
+// - and varints as as short as possible.
+// All other ADR-027 edge cases (e.g. TxRaw fields having default values) will
+// not happen with TxRaw.
+func rejectNonADR027(txBytes []byte) error {
+	// Make sure all fields are ordered in ascending order with this variable.
+	prevTagNum := protowire.Number(0)
+
+	for len(txBytes) > 0 {
+		tagNum, wireType, m := protowire.ConsumeTag(txBytes)
+		if m < 0 {
+			return fmt.Errorf("invalid length; %w", protowire.ParseError(m))
+		}
+		if wireType != protowire.BytesType {
+			return fmt.Errorf("expected %d wire type, got %d", protowire.VarintType, wireType)
+		}
+		if tagNum < prevTagNum {
+			return fmt.Errorf("txRaw must follow ADR-027, got tagNum %d after tagNum %d", tagNum, prevTagNum)
+		}
+		prevTagNum = tagNum
+
+		// All 3 fields of TxRaw have wireType == 2, so their next component
+		// is a varint.
+		// We make sure that the varint is as short as possible.
+		lengthPrefix, m := protowire.ConsumeVarint(txBytes[m:])
+		if m < 0 {
+			return fmt.Errorf("invalid length; %w", protowire.ParseError(m))
+		}
+		n := varintMinLength(lengthPrefix)
+		if n != m {
+			return fmt.Errorf("length prefix varint for tagNum %d is not as short as possible, read %d, only need %d", tagNum, m, n)
+		}
+
+		// Skip over the bytes that store fieldNumber and wireType bytes.
+		_, _, m = protowire.ConsumeField(txBytes)
+		if m < 0 {
+			return fmt.Errorf("invalid length; %w", protowire.ParseError(m))
+		}
+		txBytes = txBytes[m:]
+	}
+
+	return nil
+}
+
+// varintMinLength returns the minimum number of bytes necessary to encode an
+// uint using varint encoding.
+func varintMinLength(n uint64) int {
+	switch {
+	// Note: 1<<N == 2**N.
+	case n < 1<<7:
+		return 1
+	case n < 1<<14:
+		return 2
+	case n < 1<<21:
+		return 3
+	case n < 1<<28:
+		return 4
+	case n < 1<<35:
+		return 5
+	case n < 1<<42:
+		return 6
+	case n < 1<<49:
+		return 7
+	case n < 1<<56:
+		return 8
+	case n < 1<<63:
+		return 9
+	default:
+		return 10
+	}
+}
diff --git a/x/auth/tx/encode_decode_test.go b/x/auth/tx/encode_decode_test.go
index 55fff38c366f..d4d5ad7554d0 100644
--- a/x/auth/tx/encode_decode_test.go
+++ b/x/auth/tx/encode_decode_test.go
@@ -1,20 +1,21 @@
 package tx
 
 import (
+	"encoding/binary"
 	"fmt"
+	"math"
 	"testing"
 
-	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
-
-	"github.com/cosmos/cosmos-sdk/types/tx"
-	signingtypes "github.com/cosmos/cosmos-sdk/types/tx/signing"
-	"github.com/cosmos/cosmos-sdk/x/auth/signing"
-
 	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/encoding/protowire"
 
 	"github.com/cosmos/cosmos-sdk/codec"
 	codectypes "github.com/cosmos/cosmos-sdk/codec/types"
 	"github.com/cosmos/cosmos-sdk/testutil/testdata"
+	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
+	"github.com/cosmos/cosmos-sdk/types/tx"
+	signingtypes "github.com/cosmos/cosmos-sdk/types/tx/signing"
+	"github.com/cosmos/cosmos-sdk/x/auth/signing"
 )
 
 func TestDefaultTxDecoderError(t *testing.T) {
@@ -159,3 +160,124 @@ func TestUnknownFields(t *testing.T) {
 	_, err = decoder(txBz)
 	require.Error(t, err)
 }
+
+func TestRejectNonADR027(t *testing.T) {
+	registry := codectypes.NewInterfaceRegistry()
+	cdc := codec.NewProtoCodec(registry)
+	decoder := DefaultTxDecoder(cdc)
+
+	body := &testdata.TestUpdatedTxBody{Memo: "AAA"} // Look for "65 65 65" when debugging the bytes stream.
+	bodyBz, err := body.Marshal()
+	require.NoError(t, err)
+	authInfo := &testdata.TestUpdatedAuthInfo{Fee: &tx.Fee{GasLimit: 127}} // Look for "127" when debugging the bytes stream.
+	authInfoBz, err := authInfo.Marshal()
+	txRaw := &tx.TxRaw{
+		BodyBytes:     bodyBz,
+		AuthInfoBytes: authInfoBz,
+		Signatures:    [][]byte{{41}, {42}, {43}}, // Look for "42" when debugging the bytes stream.
+	}
+
+	// We know these bytes are ADR-027-compliant.
+	txBz, err := txRaw.Marshal()
+
+	// From the `txBz`, we extract the 3 components:
+	// bodyBz, authInfoBz, sigsBz.
+	// In our tests, we will try to decode txs with those 3 components in all
+	// possible orders.
+	//
+	// Consume "BodyBytes" field.
+	_, _, m := protowire.ConsumeField(txBz)
+	bodyBz = append([]byte{}, txBz[:m]...)
+	txBz = txBz[m:] // Skip over "BodyBytes" bytes.
+	// Consume "AuthInfoBytes" field.
+	_, _, m = protowire.ConsumeField(txBz)
+	authInfoBz = append([]byte{}, txBz[:m]...)
+	txBz = txBz[m:] // Skip over "AuthInfoBytes" bytes.
+	// Consume "Signature" field, it's the remaining bytes.
+	sigsBz := append([]byte{}, txBz...)
+
+	// bodyBz's length prefix is 5, with `5` as varint encoding. We also try a
+	// longer varint encoding for 5: `133 00`.
+	longVarintBodyBz := append(append([]byte{bodyBz[0]}, byte(133), byte(00)), bodyBz[2:]...)
+
+	tests := []struct {
+		name      string
+		txBz      []byte
+		shouldErr bool
+	}{
+		{
+			"authInfo, body, sigs",
+			append(append(authInfoBz, bodyBz...), sigsBz...),
+			true,
+		},
+		{
+			"authInfo, sigs, body",
+			append(append(authInfoBz, sigsBz...), bodyBz...),
+			true,
+		},
+		{
+			"sigs, body, authInfo",
+			append(append(sigsBz, bodyBz...), authInfoBz...),
+			true,
+		},
+		{
+			"sigs, authInfo, body",
+			append(append(sigsBz, authInfoBz...), bodyBz...),
+			true,
+		},
+		{
+			"body, sigs, authInfo",
+			append(append(bodyBz, sigsBz...), authInfoBz...),
+			true,
+		},
+		{
+			"body, authInfo, sigs (valid txRaw)",
+			append(append(bodyBz, authInfoBz...), sigsBz...),
+			false,
+		},
+		{
+			"longer varint than needed",
+			append(append(longVarintBodyBz, authInfoBz...), sigsBz...),
+			true,
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			_, err = decoder(tt.txBz)
+			if tt.shouldErr {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+			}
+		})
+	}
+}
+
+func TestVarintMinLength(t *testing.T) {
+	tests := []struct {
+		n uint64
+	}{
+		{1<<7 - 1}, {1 << 7},
+		{1<<14 - 1}, {1 << 14},
+		{1<<21 - 1}, {1 << 21},
+		{1<<28 - 1}, {1 << 28},
+		{1<<35 - 1}, {1 << 35},
+		{1<<42 - 1}, {1 << 42},
+		{1<<49 - 1}, {1 << 49},
+		{1<<56 - 1}, {1 << 56},
+		{1<<63 - 1}, {1 << 63},
+		{math.MaxUint64},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(fmt.Sprintf("test %d", tt.n), func(t *testing.T) {
+			l1 := varintMinLength(tt.n)
+			buf := make([]byte, binary.MaxVarintLen64)
+			l2 := binary.PutUvarint(buf, tt.n)
+			require.Equal(t, l2, l1)
+		})
+	}
+}