diff --git a/lib/schema.js b/lib/schema.js
index 781ccdbbcf8..730bddeac49 100644
--- a/lib/schema.js
+++ b/lib/schema.js
@@ -554,6 +554,10 @@ Schema.prototype.add = function add(obj, prefix) {
   const keys = Object.keys(obj);
   const typeKey = this.options.typeKey;
   for (const key of keys) {
+    if (utils.specialProperties.has(key)) {
+      continue;
+    }
+
     const fullPath = prefix + key;
     const val = obj[key];
 
@@ -854,6 +858,9 @@ Schema.prototype.path = function(path, obj) {
   let fullPath = '';
 
   for (const sub of subpaths) {
+    if (utils.specialProperties.has(sub)) {
+      throw new Error('Cannot set special property `' + sub + '` on a schema');
+    }
     fullPath = fullPath += (fullPath.length > 0 ? '.' : '') + sub;
     if (!branch[sub]) {
       this.nested[fullPath] = true;
diff --git a/test/schema.test.js b/test/schema.test.js
index 7bc6299e23e..6ef548b0059 100644
--- a/test/schema.test.js
+++ b/test/schema.test.js
@@ -2792,4 +2792,14 @@ describe('schema', function() {
       });
     }, /Cannot use schema-level projections.*subdocument_mapping.not_selected/);
   });
+
+  it('disallows setting special properties with `add()` or constructor (gh-12085)', async function() {
+    const maliciousPayload = '{"__proto__.toString": "Number"}';
+
+    assert.throws(() => {
+      mongoose.Schema(JSON.parse(maliciousPayload));
+    }, /__proto__/);
+
+    assert.ok({}.toString());
+  });
 });