Bypassing Serialization Binders with BinaryFormatter Mutation

Author: Adam Sitnik

src/libraries/System.Runtime.Serialization.Formatters/src/System/Runtime/Serialization/Formatters/Binary/BinaryObjectWriter.cs

@@ -375,7 +375,7 @@ private void WriteMembers(NameInfo memberNameInfo,
                 return;
             }
 
-            if (!WriteKnownValueClass(memberNameInfo, memberTypeNameInfo, memberData!))
+            if (!WriteKnownValueClass(memberNameInfo, memberTypeNameInfo, memberData!, ref assignUniqueIdToValueType))
             {
                 if (outType == null)
                 {
@@ -624,10 +624,10 @@ private void WriteArrayMember(WriteObjectInfo objectInfo, NameInfo arrayElemType
                 actualTypeInfo._isArrayItem = true;
             }
 
-            if (!WriteKnownValueClass(arrayElemTypeNameInfo, actualTypeInfo, data!))
+            bool assignUniqueIdForValueTypes = false;
+            if (!WriteKnownValueClass(arrayElemTypeNameInfo, actualTypeInfo, data!, ref assignUniqueIdForValueTypes))
             {
                 object obj = data!;
-                bool assignUniqueIdForValueTypes = false;
                 if (ReferenceEquals(arrayElemTypeNameInfo._type, Converter.s_typeofObject))
                 {
                     assignUniqueIdForValueTypes = true;
@@ -807,11 +807,12 @@ private long Schedule(object obj, bool assignUniqueIdToValueType, Type? type, Wr
         }
 
         // Determines if a type is a primitive type, if it is it is written
-        private bool WriteKnownValueClass(NameInfo memberNameInfo, NameInfo typeNameInfo, object data)
+        private bool WriteKnownValueClass(NameInfo memberNameInfo, NameInfo typeNameInfo, object data, ref bool assignUniqueIdToValueType)
         {
             if (ReferenceEquals(typeNameInfo._type, Converter.s_typeofString))
             {
                 WriteString(memberNameInfo, typeNameInfo, data);
+                return true;
             }
             else
             {
@@ -825,18 +826,22 @@ private bool WriteKnownValueClass(NameInfo memberNameInfo, NameInfo typeNameInfo
                     if (typeNameInfo._isArray) // null if an array
                     {
                         _serWriter.WriteItem(memberNameInfo, typeNameInfo, data);
+                        return true;
                     }
-                    else
+                    else if (memberNameInfo._type == typeNameInfo._type
+                        || memberNameInfo._type == typeof(object)
+                        || (memberNameInfo._type != null && Nullable.GetUnderlyingType(memberNameInfo._type) != null))
                     {
                         _serWriter.WriteMember(memberNameInfo, typeNameInfo, data);
+                        return true;
                     }
                 }
             }
 
-            return true;
+            assignUniqueIdToValueType = true;
+            return false;
         }
 
-
         // Writes an object reference to the stream.
         private void WriteObjectRef(NameInfo nameInfo, long objectId) =>
             _serWriter!.WriteMemberObjectRef(nameInfo, (int)objectId);

src/libraries/System.Runtime.Serialization.Formatters/tests/BinaryFormatterTests.cs

@@ -535,6 +535,25 @@ public void Roundtrip_ArrayContainingArrayAtNonZeroLowerBound()
             BinaryFormatterHelpers.Clone(Array.CreateInstance(typeof(uint[]), new[] { 5 }, new[] { 1 }));
         }
 
+        [Fact]
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, ".NET Framework has not been patched yet.")]
+        public void BypassingSerializationBinders()
+        {
+            Tuple<IComparable, object> tuple = new Tuple<IComparable, object>(42, new byte[] { 1, 2, 3, 4 });
+            BinaryFormatter formatter = new BinaryFormatter();
+
+            using (MemoryStream stream = new MemoryStream())
+            {
+                formatter.Serialize(stream, tuple);
+
+                stream.Position = 0;
+
+                Tuple<IComparable, object> deserialized = (Tuple<IComparable, object>)formatter.Deserialize(stream);
+                Assert.Equal(tuple.Item1, deserialized.Item1);
+                Assert.Equal(tuple.Item2, deserialized.Item2);
+            }
+        }
+
         private static void ValidateEqualityComparer(object obj)
         {
             Type objType = obj.GetType();