diff --git a/browser-extensions/extension/src/js/content-scripts/content-script-parkrunner.js b/browser-extensions/extension/src/js/content-scripts/content-script-parkrunner.js
index 8b444206..591ea206 100644
--- a/browser-extensions/extension/src/js/content-scripts/content-script-parkrunner.js
+++ b/browser-extensions/extension/src/js/content-scripts/content-script-parkrunner.js
@@ -21,7 +21,9 @@ function get_table(id, caption) {
       .replace("–", "")
       .trim()
       .replace(/\s+/g, " ");
-    const normalisedExpectedCaption = (caption || "").trim().replace(/\s+/g, " ");
+    const normalisedExpectedCaption = (caption || "")
+      .trim()
+      .replace(/\s+/g, " ");
     return normalisedPageCaption === normalisedExpectedCaption;
   });
 }
@@ -63,22 +65,27 @@ function parse_volunteer_table(result) {
 }
 
 function set_complete_progress_message(errors) {
-  var messages = [
-    'Additional badges provided by <a href="https://running-challenges.co.uk" target="_blank">Running Challenges</a>',
-  ];
+  const safe_link =
+    'Additional badges provided by <a href="https://running-challenges.co.uk" target="_blank">Running Challenges</a>';
+  const messages = [safe_link];
   $.each(errors, function (index, error_message) {
-    messages.push(error_message);
+    messages.push(escape_html(error_message));
   });
   if (errors.length > 0) {
     messages.push("Refresh the page to try again");
   }
-  set_progress_message(messages.join("<br/><br/>"));
+  set_progress_message_html(messages.join("<br/><br/>"));
 }
 
 function set_progress_message(progress_message) {
   console.log("Progress: " + progress_message);
-  // $("div[id=running_challenges_messages_div]").html($("div[id=running_challenges_messages_div]").html() + "<br/>" + progress_message)
-  $("div[id=running_challenges_messages_div]").html(progress_message);
+  $("div[id=running_challenges_messages_div]").html(
+    escape_html(progress_message),
+  );
+}
+
+function set_progress_message_html(safe_html) {
+  $("div[id=running_challenges_messages_div]").html(safe_html);
 }
 
 function parsePageAthleteInfo() {
@@ -261,8 +268,8 @@ function create_skeleton_elements(id_map) {
     id_map["messages"],
   );
   running_challenges_message_spacer.after(running_challenges_messages_div);
-  // Use the progress message function to se interval
-  set_progress_message(
+  // Use the progress message function to set initial message
+  set_progress_message_html(
     'Loading <a href="https://running-challenges.co.uk" target="_blank">Running Challenges</a> Badges',
   );
 
diff --git a/browser-extensions/extension/src/js/lib/escape-html.js b/browser-extensions/extension/src/js/lib/escape-html.js
new file mode 100644
index 00000000..f2efd92a
--- /dev/null
+++ b/browser-extensions/extension/src/js/lib/escape-html.js
@@ -0,0 +1,17 @@
+function escape_html(unsafe) {
+  if (unsafe == null) return "";
+  const s = String(unsafe);
+  return s
+    .replace(/&/g, "&amp;")
+    .replace(/</g, "&lt;")
+    .replace(/>/g, "&gt;")
+    .replace(/"/g, "&quot;")
+    .replace(/'/g, "&#039;");
+}
+
+if (typeof window !== "undefined") {
+  window.escape_html = escape_html;
+}
+if (typeof module !== "undefined" && module.exports) {
+  module.exports = { escape_html };
+}
diff --git a/browser-extensions/extension/src/js/tests/test/test_escape_html.js b/browser-extensions/extension/src/js/tests/test/test_escape_html.js
new file mode 100644
index 00000000..fa8ff589
--- /dev/null
+++ b/browser-extensions/extension/src/js/tests/test/test_escape_html.js
@@ -0,0 +1,38 @@
+const assert = require("assert");
+const { escape_html } = require("../../lib/escape-html.js");
+
+describe("escape_html", function () {
+  it("escapes angle brackets", function () {
+    assert.strictEqual(
+      escape_html("<script>alert(1)</script>"),
+      "&lt;script&gt;alert(1)&lt;/script&gt;",
+    );
+  });
+
+  it("escapes ampersand and quotes", function () {
+    assert.strictEqual(
+      escape_html("\"foo\" & 'bar'"),
+      "&quot;foo&quot; &amp; &#039;bar&#039;",
+    );
+  });
+
+  it("returns empty string for null", function () {
+    assert.strictEqual(escape_html(null), "");
+  });
+
+  it("returns empty string for undefined", function () {
+    assert.strictEqual(escape_html(undefined), "");
+  });
+
+  it("converts non-strings to string then escapes", function () {
+    assert.strictEqual(escape_html(1), "1");
+    assert.strictEqual(escape_html("<"), "&lt;");
+  });
+
+  it("leaves safe text unchanged", function () {
+    assert.strictEqual(
+      escape_html("Parsing Athlete Info"),
+      "Parsing Athlete Info",
+    );
+  });
+});
diff --git a/browser-extensions/extension/src/manifest/base.json b/browser-extensions/extension/src/manifest/base.json
index bda5138b..bb1a085a 100644
--- a/browser-extensions/extension/src/manifest/base.json
+++ b/browser-extensions/extension/src/manifest/base.json
@@ -54,6 +54,7 @@
         "js/lib/i18n.js",
         "js/lib/challenges.js",
         "js/lib/challenges_ui.js",
+        "js/lib/escape-html.js",
         "js/content-scripts/content-script-parkrunner.js"
       ],
       "css": [
diff --git a/browser-extensions/extension/userscript/entry.js b/browser-extensions/extension/userscript/entry.js
index ccb46d58..7429d3ee 100644
--- a/browser-extensions/extension/userscript/entry.js
+++ b/browser-extensions/extension/userscript/entry.js
@@ -15,6 +15,7 @@ import "../src/js/lib/cache.js";
 import "../src/js/lib/i18n.js";
 import "../src/js/lib/challenges.js";
 import "../src/js/lib/challenges_ui.js";
+import "../src/js/lib/escape-html.js";
 import "../src/js/content-scripts/content-script-parkrunner.js";
 
 // Ensure the main page initialiser runs once the DOM is ready.