Prevent an inappropriate use of BinaryUnmarshaler

Don't allow CBOR byte string (major type 2) as input to Go's
Time.UnmarshalBinary.

Time values should only be encoded/decoded using these
CBOR data types: pos or neg integer, float, and text string.

For more info, see RFC 7049 section 2.4.1.

Closes Issue #8.

README.md

@@ -7,12 +7,12 @@
 
 # fxamacker/cbor - CBOR library in Go    
 
-CBOR is a concise binary alternative to JSON, and is specified in RFC 7049.
+CBOR is a concise binary alternative to JSON, and is specified in [RFC 7049](https://tools.ietf.org/html/rfc7049).
 
 fxamacker/cbor is designed to be:
 * __Easy__ -- idiomatic API like `encoding/json`.
 * __Safe and reliable__ -- no `unsafe` pkg, coverage at 96%, and 10+ hrs of [fuzzing](https://github.com/fxamacker/cbor-fuzz) before each release.
-* __Standards-compliant__ -- supports [CBOR](https://tools.ietf.org/html/rfc7049), including canonical CBOR encodings ([RFC 7049](https://tools.ietf.org/html/rfc7049#section-3.9) and [CTAP2](https://fidoalliance.org/specs/fido-v2.0-id-20180227/fido-client-to-authenticator-protocol-v2.0-id-20180227.html#ctap2-canonical-cbor-encoding-form)).
+* __Standards-compliant__ -- supports [CBOR](https://tools.ietf.org/html/rfc7049), including [canonical CBOR encodings](https://tools.ietf.org/html/rfc7049#section-3.9) (RFC 7049 and [CTAP2](https://fidoalliance.org/specs/fido-v2.0-id-20180227/fido-client-to-authenticator-protocol-v2.0-id-20180227.html#ctap2-canonical-cbor-encoding-form)).
 * __Small and self-contained__ -- compiles to under 0.5 MB and has no external dependencies.
 
 fxamacker/cbor balances speed, safety, and compiled size.  To keep size small, it avoids code generation.  For safety, it avoids Go's `unsafe`. For speed, it uses safe optimizations: cache struct metadata, bypass `reflect` when appropriate, use `sync.Pool` to reuse transient objects, and etc.  
@@ -21,7 +21,7 @@ fxamacker/cbor balances speed, safety, and compiled size.  To keep size small, i
 
 Version 1.x has:
 * __Stable API__ -- won't make breaking API changes.  
-* __Stable requirements__ -- won't require newer than Go 1.12.  
+* __Stable requirements__ -- will always support Go v1.12.  
 * __Passed 30+ hrs of fuzzing__ -- v1.1.1 on linux_amd64 using prior corpus and [RFC 7049 tests](https://tools.ietf.org/html/rfc7049#appendix-A) as seed.
 
 Oct 18, 2019: Released v1.1.1 to improve encoding speed: slice 50%, struct 30%, and map 14%.  
@@ -57,7 +57,7 @@ Library sizes:
 
 This library implements CBOR as specified in [RFC 7049](https://tools.ietf.org/html/rfc7049), with minor [limitations](#limitations).
 
-It also supports canonical CBOR encodings (both [RFC 7049](https://tools.ietf.org/html/rfc7049#section-3.9) and [CTAP2](https://fidoalliance.org/specs/fido-v2.0-id-20180227/fido-client-to-authenticator-protocol-v2.0-id-20180227.html#ctap2-canonical-cbor-encoding-form)).  CTAP2 canonical CBOR encoding is used by [CTAP](https://fidoalliance.org/specs/fido-v2.0-id-20180227/fido-client-to-authenticator-protocol-v2.0-id-20180227.html) and [WebAuthn](https://www.w3.org/TR/webauthn/) in [FIDO2](https://fidoalliance.org/fido2/) framework.
+It also supports [canonical CBOR encodings](https://tools.ietf.org/html/rfc7049#section-3.9) (both RFC 7049 and [CTAP2](https://fidoalliance.org/specs/fido-v2.0-id-20180227/fido-client-to-authenticator-protocol-v2.0-id-20180227.html#ctap2-canonical-cbor-encoding-form)).  CTAP2 canonical CBOR encoding is used by [CTAP](https://fidoalliance.org/specs/fido-v2.0-id-20180227/fido-client-to-authenticator-protocol-v2.0-id-20180227.html) and [WebAuthn](https://www.w3.org/TR/webauthn/) in [FIDO2](https://fidoalliance.org/fido2/) framework.
 
 ## Limitations
 

decode.go

@@ -740,7 +740,7 @@ func fillFloat(t cborType, val float64, v reflect.Value) error {
 }
 
 func fillByteString(t cborType, val []byte, v reflect.Value) error {
-	if reflect.PtrTo(v.Type()).Implements(typeBinaryUnmarshaler) {
+	if v.Type() != typeTime && reflect.PtrTo(v.Type()).Implements(typeBinaryUnmarshaler) {
 		pv := reflect.New(v.Type())
 		pv.Elem().Set(v)
 		u := pv.Interface().(encoding.BinaryUnmarshaler)

decode_test.go

@@ -1276,19 +1276,19 @@ func TestDecodeTime(t *testing.T) {
 			wantTime:        time.Time{},
 		},
 		{
-			name:            "time without fractional seconds",
+			name:            "time without fractional seconds", // positive integer
 			cborRFC3339Time: hexDecode("74323031332d30332d32315432303a30343a30305a"),
 			cborUnixTime:    hexDecode("1a514b67b0"),
 			wantTime:        parseTime(time.RFC3339Nano, "2013-03-21T20:04:00Z"),
 		},
 		{
-			name:            "time with fractional seconds",
+			name:            "time with fractional seconds", // float
 			cborRFC3339Time: hexDecode("76323031332d30332d32315432303a30343a30302e355a"),
 			cborUnixTime:    hexDecode("fb41d452d9ec200000"),
 			wantTime:        parseTime(time.RFC3339Nano, "2013-03-21T20:04:00.5Z"),
 		},
 		{
-			name:            "time before January 1, 1970 UTC without fractional seconds",
+			name:            "time before January 1, 1970 UTC without fractional seconds", // negative integer
 			cborRFC3339Time: hexDecode("74313936392d30332d32315432303a30343a30305a"),
 			cborUnixTime:    hexDecode("3a0177f2cf"),
 			wantTime:        parseTime(time.RFC3339Nano, "1969-03-21T20:04:00Z"),
@@ -1323,6 +1323,11 @@ func TestDecodeTimeError(t *testing.T) {
 			cborData:     hexDecode("7f657374726561646d696e67ff"),
 			wantErrorMsg: "cbor: cannot set streaming for time.Time",
 		},
+		{
+			name:         "byte string data cannot be decoded into time.Time",
+			cborData:     hexDecode("4f013030303030303030e03031ed3030"),
+			wantErrorMsg: "cbor: cannot unmarshal byte string into Go value of type time.Time",
+		},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {