diff --git a/CHANGES.rst b/CHANGES.rst
index b47f417e9a1..2bd6aa3e471 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -15,6 +15,11 @@ Deprecated
 Features added
 --------------
 
+* #13098: HTML Search: the contents of the search index are sanitised
+  (reassigned to frozen null-prototype JavaScript objects), to reduce
+  the risk of unintended access or modification.
+  Patch by James Addison
+
 Bugs fixed
 ----------
 
diff --git a/sphinx/themes/basic/static/searchtools.js b/sphinx/themes/basic/static/searchtools.js
index aaf078d2b91..b2f75e6ab8b 100644
--- a/sphinx/themes/basic/static/searchtools.js
+++ b/sphinx/themes/basic/static/searchtools.js
@@ -219,6 +219,18 @@ const Search = {
     (document.body.appendChild(document.createElement("script")).src = url),
 
   setIndex: (index) => {
+    const stack = [index];
+    const isObject = x => x instanceof Object;
+    let obj;
+    while (obj = stack.pop()) {
+      if (Array.isArray(obj)) {
+        stack.push(...obj.filter(isObject));
+      } else {
+        stack.push(...Object.values(obj).filter(isObject));
+        Object.setPrototypeOf(obj, null);
+      }
+      Object.freeze(obj);
+    }
     Search._index = index;
     if (Search._queued_query !== null) {
       const query = Search._queued_query;
@@ -474,7 +486,7 @@ const Search = {
       const descr = objName + _(", in ") + title;
 
       // add custom score for some objects according to scorer
-      if (Scorer.objPrio.hasOwnProperty(match[2]))
+      if (match[2] in Scorer.objPrio)
         score += Scorer.objPrio[match[2]];
       else score += Scorer.objPrioDefault;
 
@@ -520,13 +532,13 @@ const Search = {
       // add support for partial matches
       if (word.length > 2) {
         const escapedWord = _escapeRegExp(word);
-        if (!terms.hasOwnProperty(word)) {
+        if (!(word in terms)) {
           Object.keys(terms).forEach((term) => {
             if (term.match(escapedWord))
               arr.push({ files: terms[term], score: Scorer.partialTerm });
           });
         }
-        if (!titleTerms.hasOwnProperty(word)) {
+        if (!(word in titleTerms)) {
           Object.keys(titleTerms).forEach((term) => {
             if (term.match(escapedWord))
               arr.push({ files: titleTerms[term], score: Scorer.partialTitle });
diff --git a/tests/js/searchtools.spec.js b/tests/js/searchtools.spec.js
index cfe5fdcf7ed..13da533f6c0 100644
--- a/tests/js/searchtools.spec.js
+++ b/tests/js/searchtools.spec.js
@@ -25,6 +25,39 @@ describe('Basic html theme search', function() {
     expect(nextExpected).toEqual(undefined);
   }
 
+  describe('index immutability', function() {
+
+    it('should not be possible to modify index contents', function() {
+      eval(loadFixture("cpp/searchindex.js"));
+
+      // record some initial state
+      const initialTitle = Search._index.titles[0];
+      const initialTitlesProto = Search._index.titles.__proto__;
+      const initialTerms = Search._index.terms;
+      const initialDocNames = [...Search._index.docnames];
+      const initialObject = [...Search._index.objects[''][0]];
+
+      // attempt to mutate the index state
+      try { Search._index.objects[''][0].pop(); } catch {};
+      try { Search._index.objects[''][0].push('extra'); } catch {};
+      try { Search._index.docnames.pop(); } catch {};
+      try { Search._index.docnames.push('extra'); } catch {};
+      Search._index.titles[0] += 'modified';
+      Search._index.titles.__proto__ = 'anotherProto';
+      Search._index.terms = {'fake': [1, 2, 3]};
+      Search._index.__proto__ = 'otherProto';
+
+      // ensure that none of the modifications were applied
+      expect(Search._index.__proto__).toBe(undefined);
+      expect(Search._index.terms).toEqual(initialTerms);
+      expect(Search._index.titles.__proto__).toEqual(initialTitlesProto);
+      expect(Search._index.titles[0]).toEqual(initialTitle);
+      expect(Search._index.docnames).toEqual(initialDocNames);
+      expect(Search._index.objects[''][0]).toEqual(initialObject);
+    });
+
+  });
+
   describe('terms search', function() {
 
     it('should find "C++" when in index', function() {