diff --git a/app/cypress/e2e/injection.spec.js b/app/cypress/e2e/injection.spec.js
new file mode 100644
index 0000000..85782f8
--- /dev/null
+++ b/app/cypress/e2e/injection.spec.js
@@ -0,0 +1,197 @@
+// cypress/e2e/injection.spec.js
+
+describe('Untrusted content in the modal', () => {
+  const base = {
+    name: 'Test Item',
+    identifier: 'ABC123',
+    icon: { class: 'fa fa-test' },
+  };
+
+  beforeEach(() => {
+    cy.visit('/');
+    cy.window().then(win => {
+      cy.stub(win.navigator.clipboard, 'writeText').resolves();
+      cy.stub(win, 'alert');
+    });
+  });
+
+  function open(item = {}) {
+    cy.window().invoke('openDynamicPopup', { ...base, ...item });
+  }
+
+  describe('markdown rendered into innerHTML', () => {
+    it('strips a plain script URL', () => {
+      open({
+        warning: '[click me](javascript:window.__xss = true)',
+        info: '![x](data:text/html;base64,PHNjcmlwdD4=)',
+      });
+
+      cy.get('#dynamicModalWarningText').find('a').should('not.exist');
+      cy.get('#dynamicModalWarningText').should('contain.text', 'click me');
+      cy.get('#dynamicModalInfoText').find('img').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('strips a script URL hidden behind character references', () => {
+      open({
+        warning:
+          '[a](javascript:window.__xss=1) [b](javascript:window.__xss=1)',
+        info: '[c](java	script:window.__xss=1) [d](java
script:window.__xss=1)',
+      });
+
+      cy.get('#dynamicModalWarningText').find('a').should('not.exist');
+      cy.get('#dynamicModalInfoText').find('a').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('strips a script URL written as a reference-style link', () => {
+      open({
+        warning: '[click me][ref]\n\n[ref]: javascript:window.__xss=1',
+      });
+
+      cy.get('#dynamicModalWarningText').find('a').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('neutralises raw HTML', () => {
+      open({
+        warning: '<img src=x onerror="window.__xss = true">',
+        info: '<a href="javascript:window.__xss = true">x</a>',
+      });
+
+      cy.get('#dynamicModalWarningText').find('img').should('not.exist');
+      cy.get('#dynamicModalWarningText').should('contain.text', 'onerror');
+      cy.get('#dynamicModalInfoText').find('a').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('neutralises raw HTML used as the text of a stripped link', () => {
+      open({ warning: '[<img src=x onerror="window.__xss = true">](javascript:bad)' });
+
+      cy.get('#dynamicModalWarningText').find('img').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('keeps ordinary markdown', () => {
+      open({ warning: 'See [Matrix](https://matrix.org/) and **mind** this' });
+
+      cy.get('#dynamicModalWarningText')
+        .find('a')
+        .should('have.attr', 'href', 'https://matrix.org/');
+      cy.get('#dynamicModalWarningText').find('strong').should('have.text', 'mind');
+    });
+  });
+
+  describe('values interpolated outside markdown', () => {
+    it('does not treat the name or the icon class as markup', () => {
+      open({
+        name: '<img src=x onerror="window.__xss = true">',
+        icon: { class: 'fa" onmouseover="window.__xss = true' },
+        alternatives: [
+          {
+            name: '<img src=y onerror="window.__xss = true">',
+            identifier: 'ALT1',
+            icon: { class: 'fa-alt' },
+          },
+        ],
+      });
+
+      cy.get('#dynamicModalLabel').find('img').should('not.exist');
+      cy.get('#dynamicModalLabel').should('contain.text', 'onerror');
+      cy.get('#dynamicAlternativesList').find('img').should('not.exist');
+      cy.get('#dynamicAlternativesList').should('contain.text', 'onerror');
+      cy.window().should('not.have.property', '__xss');
+    });
+  });
+
+  describe('the link the modal offers', () => {
+    it('drops a URL that uses an unsafe scheme', () => {
+      open({ url: 'javascript:window.__xss = true', description: 'Bad' });
+
+      cy.get('#dynamicModalLinkHref').should('not.have.attr', 'href');
+      cy.get('#dynamicModalLinkHref').should('have.text', 'Bad');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('keeps an ordinary URL', () => {
+      open({ url: 'https://example.com', description: 'Good' });
+
+      cy.get('#dynamicModalLinkHref').should(
+        'have.attr',
+        'href',
+        'https://example.com',
+      );
+    });
+
+    it('keeps a mailto URL', () => {
+      open({ url: 'mailto:kevin@veen.world', description: 'Write' });
+
+      cy.get('#dynamicModalLinkHref').should(
+        'have.attr',
+        'href',
+        'mailto:kevin@veen.world',
+      );
+    });
+
+    it('restores the link after a popup whose URL was dropped', () => {
+      open({ url: 'javascript:window.__xss = true', description: 'Bad' });
+      cy.get('#dynamicModalLinkHref').should('not.have.attr', 'href');
+
+      open({ url: 'https://example.com', description: 'Good' });
+      cy.get('#dynamicModalLinkHref').should(
+        'have.attr',
+        'href',
+        'https://example.com',
+      );
+    });
+
+    it('does not let one popup iframe handler outlive it', () => {
+      open({ url: 'https://a.test/', description: 'A', iframe: true });
+      cy.get('#dynamicModalLinkHref').should('have.class', 'iframe');
+
+      open({ url: 'https://b.test/', description: 'B' });
+
+      cy.get('#dynamicModalLinkHref').should('not.have.class', 'iframe');
+      cy.get('#dynamicModalLinkHref').should($anchor => {
+        expect($anchor[0].onclick, 'stale click handler').to.equal(null);
+      });
+    });
+  });
+});
+
+describe('Untrusted content reaching the iframe', () => {
+  const AFTER_THE_FADE = 3000;
+
+  it('refuses to open a script URL handed over by the modal', () => {
+    cy.visit('/');
+    cy.window().invoke('openDynamicPopup', {
+      name: 'Bad',
+      icon: { class: 'fa fa-test' },
+      url: 'javascript:window.__xss = true',
+      description: 'Watch',
+      iframe: true,
+    });
+
+    cy.get('#dynamicModalLinkHref').click({ force: true });
+
+    cy.wait(AFTER_THE_FADE);
+    cy.get('#main').find('iframe').should('not.exist');
+    cy.window().should('not.have.property', '__xss');
+  });
+
+  it('refuses a script URL supplied through the query string', () => {
+    cy.visit('/?iframe=javascript:window.__xss%20%3D%20true');
+
+    cy.wait(AFTER_THE_FADE);
+    cy.get('#main').find('iframe').should('not.exist');
+    cy.window().should('not.have.property', '__xss');
+  });
+
+  it('still opens an ordinary URL from the query string', () => {
+    cy.visit('/?iframe=https://example.com/');
+
+    cy.get('#main')
+      .find('iframe', { timeout: AFTER_THE_FADE })
+      .should('have.attr', 'src', 'https://example.com/');
+  });
+});
diff --git a/app/static/js/iframe.js b/app/static/js/iframe.js
index aa19bae..1895ee5 100644
--- a/app/static/js/iframe.js
+++ b/app/static/js/iframe.js
@@ -34,6 +34,10 @@ function syncIframeHeight() {
 
 // Function to open a URL in an iframe (jQuery version mit 1500 ms Fade)
 function openIframe(url) {
+    if (!isSafeUrl(url)) {
+        return;
+    }
+
     var $container    = scrollbarContainer    ? $(scrollbarContainer)    : null;
     var $customScroll = customScrollbar       ? $(customScrollbar)       : null;
     var $main         = $(mainElement);
diff --git a/app/static/js/modal.js b/app/static/js/modal.js
index 9109f45..ed77827 100644
--- a/app/static/js/modal.js
+++ b/app/static/js/modal.js
@@ -1,11 +1,48 @@
+function t(source) {
+  return (window.I18N || {})[source] || source;
+}
+
+const SAFE_URL_SCHEMES = ['http:', 'https:', 'mailto:'];
+
+function isSafeUrl(url) {
+  const probe = document.createElement('a');
+  probe.href = String(url == null ? '' : url);
+  return SAFE_URL_SCHEMES.includes(probe.protocol);
+}
+
+function iconAndName(item) {
+  const nodes = [];
+  if (item.icon && item.icon.class) {
+    const icon = document.createElement('i');
+    icon.className = item.icon.class;
+    nodes.push(icon, document.createTextNode(' '));
+  }
+  nodes.push(document.createTextNode(item.name == null ? '' : item.name));
+  return nodes;
+}
+
+function renderMarkdown(content) {
+  const escaped = String(content).replace(/</g, '&lt;').replace(/>/g, '&gt;');
+  const parsed = new DOMParser().parseFromString(marked.parse(escaped), 'text/html');
+
+  parsed.querySelectorAll('a[href]').forEach((anchor) => {
+    if (!SAFE_URL_SCHEMES.includes(anchor.protocol)) {
+      anchor.replaceWith(...anchor.childNodes);
+    }
+  });
+  parsed.querySelectorAll('img[src]').forEach((image) => {
+    if (!SAFE_URL_SCHEMES.includes(image.protocol)) {
+      image.replaceWith(image.alt || '');
+    }
+  });
+
+  return parsed.body.innerHTML;
+}
+
 function openDynamicPopup(subitem) {
   closeAllModals();
   const modalTitle = document.getElementById('dynamicModalLabel');
-  if (subitem.icon && subitem.icon.class) {
-    modalTitle.innerHTML = `<i class="${subitem.icon.class}"></i> ${subitem.name}`;
-  } else {
-    modalTitle.innerText = subitem.name;
-  }
+  modalTitle.replaceChildren(...iconAndName(subitem));
 
   const identifierBox = document.getElementById('dynamicIdentifierBox');
   const modalContent = document.getElementById('dynamicModalContent');
@@ -21,7 +58,7 @@ function openDynamicPopup(subitem) {
     const box = document.getElementById(boxId);
     if (content) {
       box.classList.remove('d-none');
-      document.getElementById(textId).innerHTML = marked.parse(content);
+      document.getElementById(textId).innerHTML = renderMarkdown(content);
     } else {
       box.classList.add('d-none');
     }
@@ -44,16 +81,19 @@ function openDynamicPopup(subitem) {
   if (subitem.url) {
     linkBox.classList.remove('d-none');
     linkHref.href = subitem.url;
-    linkHref.innerText = subitem.description || "Open Link";
+    if (!isSafeUrl(subitem.url)) {
+      linkHref.removeAttribute('href');
+    }
+    linkHref.innerText = subitem.description || t("Open Link");
+    linkHref.classList.remove('iframe');
+    linkHref.onclick = null;
     if (subitem.iframe) {
       linkHref.classList.add('iframe');
-      // Attach an event listener that prevents the default behavior and
-      // opens the URL in an iframe when clicked.
-      linkHref.addEventListener('click', function(event) {
+      linkHref.onclick = function(event) {
         event.preventDefault();
         openIframe(subitem.url);
-        closeAllModals()
-      });
+        closeAllModals();
+      };
     }
   } else {
     linkBox.classList.add('d-none');
@@ -69,13 +109,13 @@ function openDynamicPopup(subitem) {
       items.forEach(item => {
         const listItem = document.createElement('li');
         listItem.classList.add('list-group-item', 'd-flex', 'justify-content-between', 'align-items-center');
-        listItem.innerHTML = `
-          <span>
-            <i class="${item.icon.class}"></i> ${item.name}
-          </span>
-          <button class="btn btn-outline-secondary btn-sm">Open</button>
-        `;
-        listItem.querySelector('button').addEventListener('click', () => onClickHandler(item));
+        const label = document.createElement('span');
+        label.replaceChildren(...iconAndName(item));
+        const button = document.createElement('button');
+        button.className = 'btn btn-outline-secondary btn-sm';
+        button.textContent = t('Open');
+        listItem.replaceChildren(label, button);
+        button.addEventListener('click', () => onClickHandler(item));
         list.appendChild(listItem);
       });
     } else {
@@ -90,7 +130,7 @@ function openDynamicPopup(subitem) {
   copyButton.onclick = () => {
     modalContent.select();
     navigator.clipboard.writeText(modalContent.value).then(() => {
-      alert('Identifier copied to clipboard!');
+      alert(t('Identifier copied to clipboard!'));
     });
   };