diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1e0eb43..edc57fa 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+### Upcoming 
+
+- Unserialize bug/security fixes: (by @souvlakias)
+  - Ensures object keys are either `number` or `string`, and not equal to `__proto__`.
+  - Ensures serializable classes are not unserialized as `O:notserializable-class`.
+
 ### 5.1.3
 
 - Maintenance release with updated homepage in manifest
diff --git a/src/helpers.ts b/src/helpers.ts
index dfac809..8fb7d22 100644
--- a/src/helpers.ts
+++ b/src/helpers.ts
@@ -19,6 +19,17 @@ export function isInteger(value: any): boolean {
   return typeof value === 'number' && Number.parseInt(value.toString(), 10) === value
 }
 
+export function isValidKey(key: any): [boolean, string] {
+  if (!(typeof key === 'string' || typeof key === 'number')) {
+    return [false, `Invalid key type '${typeof key}' encountered while unserializing`]
+  }
+  if (key === '__proto__') {
+    // Prevent prototype pollution
+    return [false, 'Key "__proto__" is not allowed']
+  }
+  return [true, '']
+}
+
 export function getIncompleteClass(name: string) {
   return new __PHP_Incomplete_Class(name)
 }
diff --git a/src/unserialize.ts b/src/unserialize.ts
index a76dacf..6a4d4a5 100644
--- a/src/unserialize.ts
+++ b/src/unserialize.ts
@@ -1,6 +1,6 @@
 // eslint-disable-next-line import/no-cycle
 import Parser from './parser'
-import { isInteger, getClass, getIncompleteClass, __PHP_Incomplete_Class, invariant } from './helpers'
+import { isInteger, getClass, getIncompleteClass, __PHP_Incomplete_Class, invariant, isValidKey } from './helpers'
 
 export type Options = {
   strict: boolean
@@ -71,6 +71,7 @@ function unserializeItem(parser: Parser, scope: Record<string, any>, options: Op
     const isArray = pairs.every((item, idx) => isInteger(item.key) && idx === item.key)
     const result = isArray ? [] : {}
     pairs.forEach(({ key, value }) => {
+      invariant(...isValidKey(key))
       result[key] = value
     })
     return result
@@ -80,10 +81,12 @@ function unserializeItem(parser: Parser, scope: Record<string, any>, options: Op
     parser.seekExpected(':')
     const pairs = parser.getByLength('{', '}', length => unserializePairs(parser, length, scope, options))
     const result = getClassReference(name, scope, options.strict)
+    invariant(!result.unserialize, `Found unserialize method on class ${name} but expected notserializable-class`)
 
     const PREFIX_PRIVATE = `\u0000${name}\u0000`
     const PREFIX_PROTECTED = `\u0000*\u0000`
     pairs.forEach(({ key, value }) => {
+      invariant(... isValidKey(key));
       if (key.startsWith(PREFIX_PRIVATE)) {
         // Private field
         result[key.slice(PREFIX_PRIVATE.length)] = value
diff --git a/test/unserialize-test.ts b/test/unserialize-test.ts
index b1f7c69..1a9d040 100644
Binary files a/test/unserialize-test.ts and b/test/unserialize-test.ts differ