Repository: fxamacker/cbor (go)

Issue:
bug: Marshal can produce malformed CBOR when marshaling a cbor.Marshaler that produces malformed CBOR
### What version of fxamacker/cbor are you using?

v2.5.0

### Does this issue reproduce with the latest release?

Yes.

### What OS and CPU architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env

</pre></details>

### What did you do?

Marshaled `RawMessage([]byte{0xa6})` without error, then called `Wellformed` on the output.

<!--
If possible, provide steps and/or code to reproduce the problem. 
-->

https://go.dev/play/p/n_OxskfI3oA

### What did you expect to see?

I expected the call to Marshal to return a non-nil error refusing to output malformed CBOR.

### What did you see instead?

The Marshaler (in this case a `RawMessage`) is trusted to produce well-formed CBOR, and as a result the output from Marshal is not well-formed.

Pull request:
Check well-formedness of data from MarshalCBOR
Closes #482

`MarshalerError` is returned if CBOR data item returned from `MarshalCBOR()` fails either:
- well-formedness check, or
- tag validation for builtin tags 0-3

Many thanks to @benluddy for reporting this bug!  :+1: 

Patch:
diff --git a/decode.go b/decode.go
index 0b44124..4981794 100644
--- a/decode.go
+++ b/decode.go
@@ -601,69 +601,96 @@ const (
 	defaultMaxMapPairs = 131072
 	minMaxMapPairs     = 16
 	maxMaxMapPairs     = 2147483647
+
+	defaultMaxNestedLevels = 32
+	minMaxNestedLevels     = 4
+	maxMaxNestedLevels     = 65535
 )
 
 func (opts DecOptions) decMode() (*decMode, error) {
 	if !opts.DupMapKey.valid() {
 		return nil, errors.New("cbor: invalid DupMapKey " + strconv.Itoa(int(opts.DupMapKey)))
 	}
+
 	if !opts.TimeTag.valid() {
 		return nil, errors.New("cbor: invalid TimeTag " + strconv.Itoa(int(opts.TimeTag)))
 	}
+
 	if !opts.IndefLength.valid() {
 		return nil, errors.New("cbor: invalid IndefLength " + strconv.Itoa(int(opts.IndefLength)))
 	}
+
 	if !opts.TagsMd.valid() {
 		return nil, errors.New("cbor: invalid TagsMd " + strconv.Itoa(int(opts.TagsMd)))
 	}
+
 	if !opts.IntDec.valid() {
 		return nil, errors.New("cbor: invalid IntDec " + strconv.Itoa(int(opts.IntDec)))
 	}
+
 	if !opts.MapKeyByteString.valid() {
 		return nil, errors.New("cbor: invalid MapKeyByteString " + strconv.Itoa(int(opts.MapKeyByteString)))
 	}
+
 	if opts.MaxNestedLevels == 0 {
-		opts.MaxNestedLevels = 32
-	} else if opts.MaxNestedLevels < 4 || opts.MaxNestedLevels > 65535 {
-		return nil, errors.New("cbor: invalid MaxNestedLevels " + strconv.Itoa(opts.MaxNestedLevels) + " (range is [4, 65535])")
+		opts.MaxNestedLevels = defaultMaxNestedLevels
+	} else if opts.MaxNestedLevels < minMaxNestedLevels || opts.MaxNestedLevels > maxMaxNestedLevels {
+		return nil, errors.New("cbor: invalid MaxNestedLevels " + strconv.Itoa(opts.MaxNestedLevels) +
+			" (range is [" + strconv.Itoa(minMaxNestedLevels) + ", " + strconv.Itoa(maxMaxNestedLevels) + "])")
 	}
+
 	if opts.MaxArrayElements == 0 {
 		opts.MaxArrayElements = defaultMaxArrayElements
 	} else if opts.MaxArrayElements < minMaxArrayElements || opts.MaxArrayElements > maxMaxArrayElements {
-		return nil, errors.New("cbor: invalid MaxArrayElements " + strconv.Itoa(opts.MaxArrayElements) + " (range is [" + strconv.Itoa(minMaxArrayElements) + ", " + strconv.Itoa(maxMaxArrayElements) + "])")
+		return nil, errors.New("cbor: invalid MaxArrayElements " + strconv.Itoa(opts.MaxArrayElements) +
+			" (range is [" + strconv.Itoa(minMaxArrayElements) + ", " + strconv.Itoa(maxMaxArrayElements) + "])")
 	}
+
 	if opts.MaxMapPairs == 0 {
 		opts.MaxMapPairs = defaultMaxMapPairs
 	} else if opts.MaxMapPairs < minMaxMapPairs || opts.MaxMapPairs > maxMaxMapPairs {
-		return nil, errors.New("cbor: invalid MaxMapPairs " + strconv.Itoa(opts.MaxMapPairs) + " (range is [" + strconv.Itoa(minMaxMapPairs) + ", " + strconv.Itoa(maxMaxMapPairs) + "])")
+		return nil, errors.New("cbor: invalid MaxMapPairs " + strconv.Itoa(opts.MaxMapPairs) +
+			" (range is [" + strconv.Itoa(minMaxMapPairs) + ", " + strconv.Itoa(maxMaxMapPairs) + "])")
 	}
+
 	if !opts.ExtraReturnErrors.valid() {
 		return nil, errors.New("cbor: invalid ExtraReturnErrors " + strconv.Itoa(int(opts.ExtraReturnErrors)))
 	}
+
 	if opts.DefaultMapType != nil && opts.DefaultMapType.Kind() != reflect.Map {
 		return nil, fmt.Errorf("cbor: invalid DefaultMapType %s", opts.DefaultMapType)
 	}
+
 	if !opts.UTF8.valid() {
 		return nil, errors.New("cbor: invalid UTF8 " + strconv.Itoa(int(opts.UTF8)))
 	}
+
 	if !opts.FieldNameMatching.valid() {
 		return nil, errors.New("cbor: invalid FieldNameMatching " + strconv.Itoa(int(opts.FieldNameMatching)))
 	}
+
 	if !opts.BigIntDec.valid() {
 		return nil, errors.New("cbor: invalid BigIntDec " + strconv.Itoa(int(opts.BigIntDec)))
 	}
-	if opts.DefaultByteStringType != nil && opts.DefaultByteStringType.Kind() != reflect.String && (opts.DefaultByteStringType.Kind() != reflect.Slice || opts.DefaultByteStringType.Elem().Kind() != reflect.Uint8) {
+
+	if opts.DefaultByteStringType != nil &&
+		opts.DefaultByteStringType.Kind() != reflect.String &&
+		(opts.DefaultByteStringType.Kind() != reflect.Slice || opts.DefaultByteStringType.Elem().Kind() != reflect.Uint8) {
 		return nil, fmt.Errorf("cbor: invalid DefaultByteStringType: %s is not of kind string or []uint8", opts.DefaultByteStringType)
 	}
+
 	if !opts.ByteStringToString.valid() {
 		return nil, errors.New("cbor: invalid ByteStringToString " + strconv.Itoa(int(opts.ByteStringToString)))
 	}
+
 	if !opts.FieldNameByteString.valid() {
 		return nil, errors.New("cbor: invalid FieldNameByteString " + strconv.Itoa(int(opts.FieldNameByteString)))
 	}
+
 	if !opts.UnrecognizedTagToAny.valid() {
 		return nil, errors.New("cbor: invalid UnrecognizedTagToAnyMode " + strconv.Itoa(int(opts.UnrecognizedTagToAny)))
 	}
+
 	dm := decMode{
 		dupMapKey:             opts.DupMapKey,
 		timeTag:               opts.TimeTag,
@@ -684,6 +711,7 @@ func (opts DecOptions) decMode() (*decMode, error) {
 		fieldNameByteString:   opts.FieldNameByteString,
 		unrecognizedTagToAny:  opts.UnrecognizedTagToAny,
 	}
+
 	return &dm, nil
 }
 
@@ -790,9 +818,9 @@ func (dm *decMode) Unmarshal(data []byte, v interface{}) error {
 	d := decoder{data: data, dm: dm}
 
 	// Check well-formedness.
-	off := d.off               // Save offset before data validation
-	err := d.wellformed(false) // don't allow any extra data after valid data item.
-	d.off = off                // Restore offset
+	off := d.off                      // Save offset before data validation
+	err := d.wellformed(false, false) // don't allow any extra data after valid data item.
+	d.off = off                       // Restore offset
 	if err != nil {
 		return err
 	}
@@ -810,9 +838,9 @@ func (dm *decMode) UnmarshalFirst(data []byte, v interface{}) (rest []byte, err
 	d := decoder{data: data, dm: dm}
 
 	// check well-formedness.
-	off := d.off             // Save offset before data validation
-	err = d.wellformed(true) // allow extra data after well-formed data item
-	d.off = off              // Restore offset
+	off := d.off                    // Save offset before data validation
+	err = d.wellformed(true, false) // allow extra data after well-formed data item
+	d.off = off                     // Restore offset
 
 	// If it is well-formed, parse the value. This is structured like this to allow
 	// better test coverage
@@ -853,7 +881,7 @@ func (dm *decMode) Valid(data []byte) error {
 // an ExtraneousDataError is returned.
 func (dm *decMode) Wellformed(data []byte) error {
 	d := decoder{data: data, dm: dm}
-	return d.wellformed(false)
+	return d.wellformed(false, false)
 }
 
 // NewDecoder returns a new decoder that reads from r using dm DecMode.
diff --git a/diagnose.go b/diagnose.go
index 43e6a14..7b4a467 100644
--- a/diagnose.go
+++ b/diagnose.go
@@ -235,7 +235,7 @@ func (di *diagnose) diagFirst() (string, []byte, error) {
 
 func (di *diagnose) wellformed(allowExtraData bool) error {
 	off := di.d.off
-	err := di.d.wellformed(allowExtraData)
+	err := di.d.wellformed(allowExtraData, false)
 	di.d.off = off
 	return err
 }
diff --git a/encode.go b/encode.go
index 86cc47a..f04559f 100644
--- a/encode.go
+++ b/encode.go
@@ -100,6 +100,23 @@ type Marshaler interface {
 	MarshalCBOR() ([]byte, error)
 }
 
+// MarshalerError represents error from checking encoded CBOR data item
+// returned from MarshalCBOR for well-formedness and some very limited tag validation.
+type MarshalerError struct {
+	typ reflect.Type
+	err error
+}
+
+func (e *MarshalerError) Error() string {
+	return "cbor: error calling MarshalCBOR for type " +
+		e.typ.String() +
+		": " + e.err.Error()
+}
+
+func (e *MarshalerError) Unwrap() error {
+	return e.err
+}
+
 // UnsupportedTypeError is returned by Marshal when attempting to encode value
 // of an unsupported type.
 type UnsupportedTypeError struct {
@@ -632,6 +649,75 @@ type encMode struct {
 
 var defaultEncMode, _ = EncOptions{}.encMode()
 
+// These four decoding modes are used by getMarshalerDecMode.
+// maxNestedLevels, maxArrayElements, and maxMapPairs are
+// set to max allowed limits to avoid rejecting Marshaler
+// output that would have been the allowable output of a
+// non-Marshaler object that exceeds default limits.
+var (
+	marshalerForbidIndefLengthForbidTagsDecMode = decMode{
+		maxNestedLevels:  maxMaxNestedLevels,
+		maxArrayElements: maxMaxArrayElements,
+		maxMapPairs:      maxMaxMapPairs,
+		indefLength:      IndefLengthForbidden,
+		tagsMd:           TagsForbidden,
+	}
+
+	marshalerAllowIndefLengthForbidTagsDecMode = decMode{
+		maxNestedLevels:  maxMaxNestedLevels,
+		maxArrayElements: maxMaxArrayElements,
+		maxMapPairs:      maxMaxMapPairs,
+		indefLength:      IndefLengthAllowed,
+		tagsMd:           TagsForbidden,
+	}
+
+	marshalerForbidIndefLengthAllowTagsDecMode = decMode{
+		maxNestedLevels:  maxMaxNestedLevels,
+		maxArrayElements: maxMaxArrayElements,
+		maxMapPairs:      maxMaxMapPairs,
+		indefLength:      IndefLengthForbidden,
+		tagsMd:           TagsAllowed,
+	}
+
+	marshalerAllowIndefLengthAllowTagsDecMode = decMode{
+		maxNestedLevels:  maxMaxNestedLevels,
+		maxArrayElements: maxMaxArrayElements,
+		maxMapPairs:      maxMaxMapPairs,
+		indefLength:      IndefLengthAllowed,
+		tagsMd:           TagsAllowed,
+	}
+)
+
+// getMarshalerDecMode returns one of four existing decoding modes
+// which can be reused (safe for parallel use) for the purpose of
+// checking if data returned by Marshaler is well-formed.
+func getMarshalerDecMode(indefLength IndefLengthMode, tagsMd TagsMode) *decMode {
+	switch {
+	case indefLength == IndefLengthAllowed && tagsMd == TagsAllowed:
+		return &marshalerAllowIndefLengthAllowTagsDecMode
+
+	case indefLength == IndefLengthAllowed && tagsMd == TagsForbidden:
+		return &marshalerAllowIndefLengthForbidTagsDecMode
+
+	case indefLength == IndefLengthForbidden && tagsMd == TagsAllowed:
+		return &marshalerForbidIndefLengthAllowTagsDecMode
+
+	case indefLength == IndefLengthForbidden && tagsMd == TagsForbidden:
+		return &marshalerForbidIndefLengthForbidTagsDecMode
+
+	default:
+		// This should never happen, unless we add new options to
+		// IndefLengthMode or TagsMode without updating this function.
+		return &decMode{
+			maxNestedLevels:  maxMaxNestedLevels,
+			maxArrayElements: maxMaxArrayElements,
+			maxMapPairs:      maxMaxMapPairs,
+			indefLength:      indefLength,
+			tagsMd:           tagsMd,
+		}
+	}
+}
+
 // EncOptions returns user specified options used to create this EncMode.
 func (em *encMode) EncOptions() EncOptions {
 	return EncOptions{
@@ -1345,6 +1431,14 @@ func encodeMarshalerType(e *encoderBuffer, em *encMode, v reflect.Value) error {
 	if err != nil {
 		return err
 	}
+
+	// Verify returned CBOR data item from MarshalCBOR() is well-formed and passes tag validity for builtin tags 0-3.
+	d := decoder{data: data, dm: getMarshalerDecMode(em.indefLength, em.tagsMd)}
+	err = d.wellformed(false, true)
+	if err != nil {
+		return &MarshalerError{typ: v.Type(), err: err}
+	}
+
 	e.Write(data)
 	return nil
 }
diff --git a/stream.go b/stream.go
index 02fea43..7ce0c43 100644
--- a/stream.go
+++ b/stream.go
@@ -84,7 +84,7 @@ func (dec *Decoder) readNext() (int, error) {
 		if dec.off < len(dec.buf) {
 			dec.d.reset(dec.buf[dec.off:])
 			off := dec.off // Save offset before data validation
-			validErr = dec.d.wellformed(true)
+			validErr = dec.d.wellformed(true, false)
 			dec.off = off // Restore offset
 
 			if validErr == nil {
diff --git a/valid.go b/valid.go
index a5213d0..11013fa 100644
--- a/valid.go
+++ b/valid.go
@@ -82,11 +82,11 @@ func (e *ExtraneousDataError) Error() string {
 // allowExtraData indicates if extraneous data is allowed after the CBOR data item.
 // - use allowExtraData = true when using Decoder.Decode()
 // - use allowExtraData = false when using Unmarshal()
-func (d *decoder) wellformed(allowExtraData bool) error {
+func (d *decoder) wellformed(allowExtraData bool, checkBuiltinTags bool) error {
 	if len(d.data) == d.off {
 		return io.EOF
 	}
-	_, err := d.wellformedInternal(0)
+	_, err := d.wellformedInternal(0, checkBuiltinTags)
 	if err == nil {
 		if !allowExtraData && d.off != len(d.data) {
 			err = &ExtraneousDataError{len(d.data) - d.off, d.off}
@@ -96,7 +96,7 @@ func (d *decoder) wellformed(allowExtraData bool) error {
 }
 
 // wellformedInternal checks data's well-formedness and returns max depth and error.
-func (d *decoder) wellformedInternal(depth int) (int, error) {
+func (d *decoder) wellformedInternal(depth int, checkBuiltinTags bool) (int, error) {
 	t, ai, val, err := d.wellformedHead()
 	if err != nil {
 		return 0, err
@@ -108,7 +108,7 @@ func (d *decoder) wellformedInternal(depth int) (int, error) {
 			if d.dm.indefLength == IndefLengthForbidden {
 				return 0, &IndefiniteLengthError{t}
 			}
-			return d.wellformedIndefiniteString(t, depth)
+			return d.wellformedIndefiniteString(t, depth, checkBuiltinTags)
 		}
 		valInt := int(val)
 		if valInt < 0 {
@@ -119,6 +119,7 @@ func (d *decoder) wellformedInternal(depth int) (int, error) {
 			return 0, io.ErrUnexpectedEOF
 		}
 		d.off += valInt
+
 	case cborTypeArray, cborTypeMap:
 		depth++
 		if depth > d.dm.maxNestedLevels {
@@ -129,7 +130,7 @@ func (d *decoder) wellformedInternal(depth int) (int, error) {
 			if d.dm.indefLength == IndefLengthForbidden {
 				return 0, &IndefiniteLengthError{t}
 			}
-			return d.wellformedIndefiniteArrayOrMap(t, depth)
+			return d.wellformedIndefiniteArrayOrMap(t, depth, checkBuiltinTags)
 		}
 
 		valInt := int(val)
@@ -156,7 +157,7 @@ func (d *decoder) wellformedInternal(depth int) (int, error) {
 		for j := 0; j < count; j++ {
 			for i := 0; i < valInt; i++ {
 				var dpt int
-				if dpt, err = d.wellformedInternal(depth); err != nil {
+				if dpt, err = d.wellformedInternal(depth, checkBuiltinTags); err != nil {
 					return 0, err
 				}
 				if dpt > maxDepth {
@@ -165,20 +166,29 @@ func (d *decoder) wellformedInternal(depth int) (int, error) {
 			}
 		}
 		depth = maxDepth
+
 	case cborTypeTag:
 		if d.dm.tagsMd == TagsForbidden {
 			return 0, &TagsMdError{}
 		}
 
+		tagNum := val
+
 		// Scan nested tag numbers to avoid recursion.
 		for {
 			if len(d.data) == d.off { // Tag number must be followed by tag content.
 				return 0, io.ErrUnexpectedEOF
 			}
+			if checkBuiltinTags {
+				err = validBuiltinTag(tagNum, d.data[d.off])
+				if err != nil {
+					return 0, err
+				}
+			}
 			if cborType(d.data[d.off]&0xe0) != cborTypeTag {
 				break
 			}
-			if _, _, _, err = d.wellformedHead(); err != nil {
+			if _, _, tagNum, err = d.wellformedHead(); err != nil {
 				return 0, err
 			}
 			depth++
@@ -187,13 +197,14 @@ func (d *decoder) wellformedInternal(depth int) (int, error) {
 			}
 		}
 		// Check tag content.
-		return d.wellformedInternal(depth)
+		return d.wellformedInternal(depth, checkBuiltinTags)
 	}
+
 	return depth, nil
 }
 
 // wellformedIndefiniteString checks indefinite length byte/text string's well-formedness and returns max depth and error.
-func (d *decoder) wellformedIndefiniteString(t cborType, depth int) (int, error) {
+func (d *decoder) wellformedIndefiniteString(t cborType, depth int, checkBuiltinTags bool) (int, error) {
 	var err error
 	for {
 		if len(d.data) == d.off {
@@ -211,7 +222,7 @@ func (d *decoder) wellformedIndefiniteString(t cborType, depth int) (int, error)
 		if (d.data[d.off] & 0x1f) == 31 {
 			return 0, &SyntaxError{"cbor: indefinite-length " + t.String() + " chunk is not definite-length"}
 		}
-		if depth, err = d.wellformedInternal(depth); err != nil {
+		if depth, err = d.wellformedInternal(depth, checkBuiltinTags); err != nil {
 			return 0, err
 		}
 	}
@@ -219,7 +230,7 @@ func (d *decoder) wellformedIndefiniteString(t cborType, depth int) (int, error)
 }
 
 // wellformedIndefiniteArrayOrMap checks indefinite length array/map's well-formedness and returns max depth and error.
-func (d *decoder) wellformedIndefiniteArrayOrMap(t cborType, depth int) (int, error) {
+func (d *decoder) wellformedIndefiniteArrayOrMap(t cborType, depth int, checkBuiltinTags bool) (int, error) {
 	var err error
 	maxDepth := depth
 	i := 0
@@ -232,7 +243,7 @@ func (d *decoder) wellformedIndefiniteArrayOrMap(t cborType, depth int) (int, er
 			break
 		}
 		var dpt int
-		if dpt, err = d.wellformedInternal(depth); err != nil {
+		if dpt, err = d.wellformedInternal(depth, checkBuiltinTags); err != nil {
 			return 0, err
 		}
 		if dpt > maxDepth {


Test patch:
diff --git a/encode_test.go b/encode_test.go
index bb4bdad..2ed416b 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -4183,3 +4183,163 @@ func TestMarshalFieldNameType(t *testing.T) {
 		})
 	}
 }
+
+func TestMarshalRawMessageContainingMalformedCBORData(t *testing.T) {
+	testCases := []struct {
+		name         string
+		value        interface{}
+		wantErrorMsg string
+	}{
+		// Nil RawMessage and empty RawMessage are encoded as CBOR nil.
+		{
+			name:         "truncated data",
+			value:        RawMessage{0xa6},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.RawMessage: unexpected EOF",
+		},
+		{
+			name:         "malformed data",
+			value:        RawMessage{0x1f},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.RawMessage: cbor: invalid additional information 31 for type positive integer",
+		},
+		{
+			name:         "extraneous data",
+			value:        RawMessage{0x01, 0x01},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.RawMessage: cbor: 1 bytes of extraneous data starting at index 1",
+		},
+		{
+			name:         "invalid builtin tag",
+			value:        RawMessage{0xc0, 0x01},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.RawMessage: cbor: tag number 0 must be followed by text string, got positive integer",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			b, err := Marshal(tc.value)
+			if err == nil {
+				t.Errorf("Marshal(%v) didn't return an error, want error %q", tc.value, tc.wantErrorMsg)
+			} else if _, ok := err.(*MarshalerError); !ok {
+				t.Errorf("Marshal(%v) error type %T, want *MarshalerError", tc.value, err)
+			} else if err.Error() != tc.wantErrorMsg {
+				t.Errorf("Marshal(%v) error %q, want %q", tc.value, err.Error(), tc.wantErrorMsg)
+			}
+			if b != nil {
+				t.Errorf("Marshal(%v) = 0x%x, want nil", tc.value, b)
+			}
+		})
+	}
+}
+
+type marshaler struct {
+	data []byte
+}
+
+func (m marshaler) MarshalCBOR() (data []byte, err error) {
+	return m.data, nil
+}
+
+func TestMarshalerReturnsMalformedCBORData(t *testing.T) {
+
+	testCases := []struct {
+		name         string
+		value        interface{}
+		wantErrorMsg string
+	}{
+		{
+			name:         "truncated data",
+			value:        marshaler{data: []byte{0xa6}},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.marshaler: unexpected EOF",
+		},
+		{
+			name:         "malformed data",
+			value:        marshaler{data: []byte{0x1f}},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.marshaler: cbor: invalid additional information 31 for type positive integer",
+		},
+		{
+			name:         "extraneous data",
+			value:        marshaler{data: []byte{0x01, 0x01}},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.marshaler: cbor: 1 bytes of extraneous data starting at index 1",
+		},
+		{
+			name:         "invalid builtin tag",
+			value:        marshaler{data: []byte{0xc0, 0x01}},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.marshaler: cbor: tag number 0 must be followed by text string, got positive integer",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			b, err := Marshal(tc.value)
+			if err == nil {
+				t.Errorf("Marshal(%v) didn't return an error, want error %q", tc.value, tc.wantErrorMsg)
+			} else if _, ok := err.(*MarshalerError); !ok {
+				t.Errorf("Marshal(%v) error type %T, want *MarshalerError", tc.value, err)
+			} else if err.Error() != tc.wantErrorMsg {
+				t.Errorf("Marshal(%v) error %q, want %q", tc.value, err.Error(), tc.wantErrorMsg)
+			}
+			if b != nil {
+				t.Errorf("Marshal(%v) = 0x%x, want nil", tc.value, b)
+			}
+		})
+	}
+}
+
+func TestMarshalerReturnsDisallowedCBORData(t *testing.T) {
+
+	testCases := []struct {
+		name         string
+		encOpts      EncOptions
+		value        interface{}
+		wantErrorMsg string
+	}{
+		{
+			name:         "enc mode forbids indefinite length, data has indefinite length",
+			encOpts:      EncOptions{IndefLength: IndefLengthForbidden},
+			value:        marshaler{data: hexDecode("5f42010243030405ff")},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.marshaler: cbor: indefinite-length byte string isn't allowed",
+		},
+		{
+			name:    "enc mode allows indefinite length, data has indefinite length",
+			encOpts: EncOptions{IndefLength: IndefLengthAllowed},
+			value:   marshaler{data: hexDecode("5f42010243030405ff")},
+		},
+		{
+			name:         "enc mode forbids tags, data has tags",
+			encOpts:      EncOptions{TagsMd: TagsForbidden},
+			value:        marshaler{data: hexDecode("c074323031332d30332d32315432303a30343a30305a")},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.marshaler: cbor: CBOR tag isn't allowed",
+		},
+		{
+			name:    "enc mode allows tags, data has tags",
+			encOpts: EncOptions{TagsMd: TagsAllowed},
+			value:   marshaler{data: hexDecode("c074323031332d30332d32315432303a30343a30305a")},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			em, err := tc.encOpts.EncMode()
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			b, err := em.Marshal(tc.value)
+			if tc.wantErrorMsg == "" {
+				if err != nil {
+					t.Errorf("Marshal(%v) returned error %q", tc.value, err)
+				}
+			} else {
+				if err == nil {
+					t.Errorf("Marshal(%v) didn't return an error, want error %q", tc.value, tc.wantErrorMsg)
+				} else if _, ok := err.(*MarshalerError); !ok {
+					t.Errorf("Marshal(%v) error type %T, want *MarshalerError", tc.value, err)
+				} else if err.Error() != tc.wantErrorMsg {
+					t.Errorf("Marshal(%v) error %q, want %q", tc.value, err.Error(), tc.wantErrorMsg)
+				}
+				if b != nil {
+					t.Errorf("Marshal(%v) = 0x%x, want nil", tc.value, b)
+				}
+			}
+		})
+	}
+}
diff --git a/tag_test.go b/tag_test.go
index 4f24331..dec3181 100644
--- a/tag_test.go
+++ b/tag_test.go
@@ -1444,3 +1444,49 @@ func TestDecodeRegisterTagForUnmarshaler(t *testing.T) {
 		t.Errorf("Marshal(%v) returned %v, want %v", v2, b, data)
 	}
 }
+
+func TestMarshalRawTagContainingMalformedCBORData(t *testing.T) {
+	testCases := []struct {
+		name         string
+		value        interface{}
+		wantErrorMsg string
+	}{
+		// Nil RawMessage and empty RawMessage are encoded as CBOR nil.
+		{
+			name:         "truncated data",
+			value:        RawTag{Number: 100, Content: RawMessage{0xa6}},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.RawTag: unexpected EOF",
+		},
+		{
+			name:         "malformed data",
+			value:        RawTag{Number: 100, Content: RawMessage{0x1f}},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.RawTag: cbor: invalid additional information 31 for type positive integer",
+		},
+		{
+			name:         "extraneous data",
+			value:        RawTag{Number: 100, Content: RawMessage{0x01, 0x01}},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.RawTag: cbor: 1 bytes of extraneous data starting at index 3",
+		},
+		{
+			name:         "invalid builtin tag",
+			value:        RawTag{Number: 0, Content: RawMessage{0x01}},
+			wantErrorMsg: "cbor: error calling MarshalCBOR for type cbor.RawTag: cbor: tag number 0 must be followed by text string, got positive integer",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			b, err := Marshal(tc.value)
+			if err == nil {
+				t.Errorf("Marshal(%v) didn't return an error, want error %q", tc.value, tc.wantErrorMsg)
+			} else if _, ok := err.(*MarshalerError); !ok {
+				t.Errorf("Marshal(%v) error type %T, want *MarshalerError", tc.value, err)
+			} else if err.Error() != tc.wantErrorMsg {
+				t.Errorf("Marshal(%v) error %q, want %q", tc.value, err.Error(), tc.wantErrorMsg)
+			}
+			if b != nil {
+				t.Errorf("Marshal(%v) = 0x%x, want nil", tc.value, b)
+			}
+		})
+	}
+}
diff --git a/valid_test.go b/valid_test.go
index 64a7e58..f4637b8 100644
--- a/valid_test.go
+++ b/valid_test.go
@@ -63,7 +63,7 @@ func TestValidOnStreamingData(t *testing.T) {
 	}
 	d := decoder{data: buf.Bytes(), dm: defaultDecMode}
 	for i := 0; i < len(marshalTests); i++ {
-		if err := d.wellformed(true); err != nil {
+		if err := d.wellformed(true, false); err != nil {
 			t.Errorf("wellformed() returned error %v", err)
 		}
 	}
@@ -111,7 +111,7 @@ func TestDepth(t *testing.T) {
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 			d := decoder{data: tc.data, dm: defaultDecMode}
-			depth, err := d.wellformedInternal(0)
+			depth, err := d.wellformedInternal(0, false)
 			if err != nil {
 				t.Errorf("wellformed(0x%x) returned error %v", tc.data, err)
 			}
@@ -176,7 +176,7 @@ func TestDepthError(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			dm, _ := tc.opts.decMode()
 			d := decoder{data: tc.data, dm: dm}
-			if _, err := d.wellformedInternal(0); err == nil {
+			if _, err := d.wellformedInternal(0, false); err == nil {
 				t.Errorf("wellformed(0x%x) didn't return an error", tc.data)
 			} else if _, ok := err.(*MaxNestedLevelError); !ok {
 				t.Errorf("wellformed(0x%x) returned wrong error type %T, want (*MaxNestedLevelError)", tc.data, err)
@@ -186,3 +186,113 @@ func TestDepthError(t *testing.T) {
 		})
 	}
 }
+
+func TestValidBuiltinTagTest(t *testing.T) {
+	testCases := []struct {
+		name string
+		data []byte
+	}{
+		{
+			name: "tag 0",
+			data: hexDecode("c074323031332d30332d32315432303a30343a30305a"),
+		},
+		{
+			name: "tag 1",
+			data: hexDecode("c11a514b67b0"),
+		},
+		{
+			name: "tag 2",
+			data: hexDecode("c249010000000000000000"),
+		},
+		{
+			name: "tag 3",
+			data: hexDecode("c349010000000000000000"),
+		},
+		{
+			name: "nested tag 0",
+			data: hexDecode("d9d9f7c074323031332d30332d32315432303a30343a30305a"),
+		},
+		{
+			name: "nested tag 1",
+			data: hexDecode("d9d9f7c11a514b67b0"),
+		},
+		{
+			name: "nested tag 2",
+			data: hexDecode("d9d9f7c249010000000000000000"),
+		},
+		{
+			name: "nested tag 3",
+			data: hexDecode("d9d9f7c349010000000000000000"),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			d := decoder{data: tc.data, dm: defaultDecMode}
+			if err := d.wellformed(true, true); err != nil {
+				t.Errorf("wellformed(0x%x) returned error %v", tc.data, err)
+			}
+		})
+	}
+}
+
+func TestInvalidBuiltinTagTest(t *testing.T) {
+	testCases := []struct {
+		name         string
+		data         []byte
+		wantErrorMsg string
+	}{
+		{
+			name:         "tag 0",
+			data:         hexDecode("c01a514b67b0"),
+			wantErrorMsg: "cbor: tag number 0 must be followed by text string, got positive integer",
+		},
+		{
+			name:         "tag 1",
+			data:         hexDecode("c174323031332d30332d32315432303a30343a30305a"),
+			wantErrorMsg: "cbor: tag number 1 must be followed by integer or floating-point number, got UTF-8 text string",
+		},
+		{
+			name:         "tag 2",
+			data:         hexDecode("c269010000000000000000"),
+			wantErrorMsg: "cbor: tag number 2 or 3 must be followed by byte string, got UTF-8 text string",
+		},
+		{
+			name:         "tag 3",
+			data:         hexDecode("c300"),
+			wantErrorMsg: "cbor: tag number 2 or 3 must be followed by byte string, got positive integer",
+		},
+		{
+			name:         "nested tag 0",
+			data:         hexDecode("d9d9f7c01a514b67b0"),
+			wantErrorMsg: "cbor: tag number 0 must be followed by text string, got positive integer",
+		},
+		{
+			name:         "nested tag 1",
+			data:         hexDecode("d9d9f7c174323031332d30332d32315432303a30343a30305a"),
+			wantErrorMsg: "cbor: tag number 1 must be followed by integer or floating-point number, got UTF-8 text string",
+		},
+		{
+			name:         "nested tag 2",
+			data:         hexDecode("d9d9f7c269010000000000000000"),
+			wantErrorMsg: "cbor: tag number 2 or 3 must be followed by byte string, got UTF-8 text string",
+		},
+		{
+			name:         "nested tag 3",
+			data:         hexDecode("d9d9f7c300"),
+			wantErrorMsg: "cbor: tag number 2 or 3 must be followed by byte string, got positive integer",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			d := decoder{data: tc.data, dm: defaultDecMode}
+			err := d.wellformed(true, true)
+			if err == nil {
+				t.Errorf("wellformed(0x%x) didn't return an error", tc.data)
+			} else if err.Error() != tc.wantErrorMsg {
+				t.Errorf("wellformed(0x%x) error %q, want %q", tc.data, err.Error(), tc.wantErrorMsg)
+			}
+		})
+	}
+}