diff --git a/common/types.go b/common/types.go
index b1666d733..fec986164 100644
--- a/common/types.go
+++ b/common/types.go
@@ -19,10 +19,12 @@ package common
 import (
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"math/big"
 	"math/rand"
 	"reflect"
+	"strings"
 )
 
 const (
@@ -30,6 +32,8 @@ const (
 	AddressLength = 20
 )
 
+var hashJsonLengthErr = errors.New("common: unmarshalJSON failed: hash must be exactly 32 bytes")
+
 type (
 	Hash    [HashLength]byte
 	Address [AddressLength]byte
@@ -58,6 +62,15 @@ func (h *Hash) UnmarshalJSON(input []byte) error {
 	if length >= 2 && input[0] == '"' && input[length-1] == '"' {
 		input = input[1 : length-1]
 	}
+	// strip "0x" for length check
+	if len(input) > 1 && strings.ToLower(string(input[:2])) == "0x" {
+		input = input[2:]
+	}
+
+	// validate the length of the input hash
+	if len(input) != HashLength*2 {
+		return hashJsonLengthErr
+	}
 	h.SetBytes(FromHex(string(input)))
 	return nil
 }
diff --git a/common/types_test.go b/common/types_test.go
index edf8d4d14..f2dfbf0c9 100644
--- a/common/types_test.go
+++ b/common/types_test.go
@@ -29,3 +29,25 @@ func TestBytesConversion(t *testing.T) {
 		t.Errorf("expected %x got %x", exp, hash)
 	}
 }
+
+func TestHashJsonValidation(t *testing.T) {
+	var h Hash
+	var tests = []struct {
+		Prefix string
+		Size   int
+		Error  error
+	}{
+		{"", 2, hashJsonLengthErr},
+		{"", 62, hashJsonLengthErr},
+		{"", 66, hashJsonLengthErr},
+		{"", 65, hashJsonLengthErr},
+		{"0X", 64, nil},
+		{"0x", 64, nil},
+		{"0x", 62, hashJsonLengthErr},
+	}
+	for i, test := range tests {
+		if err := h.UnmarshalJSON(append([]byte(test.Prefix), make([]byte, test.Size)...)); err != test.Error {
+			t.Error(i, "expected", test.Error, "got", err)
+		}
+	}
+}