referer: work towards the greatest level of referrer security

Pull request with discussion: ether#3636

What's already there:
* `meta name=referrer`: already done in 1.6.1:
  ether#3044

  https://caniuse.com/#feat=referrer-policy
  https://w3c.github.io/webappsec-referrer-policy/#referrer-policy-delivery-meta
  (Chrome>=78, Firefox>=70, Safari>=13, Opera>=64, ~IE[1], ~Edge[1])

The previous two commits (by @joelpurra) I backported in this batch:
* `<a rel=noreferrer>`: a pull request denied before:
  ether#2498

  https://html.spec.whatwg.org/multipage/links.html#link-type-noreferrer
  https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types
  (Firefox>=37, I can't find more info about support)

This commit adds the following:
* `<a rel="noopener">`: fixing a not-so-well-known way to extract referer
  https://html.spec.whatwg.org/multipage/links.html#link-type-noopener
  (Chrome>=49, Firefox>=52, Safari>=10.1, Opera>=36, !IE, !Edge)

* `Referrer-Policy: same-origin`: the last bastion of referrer security
  https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy
  (Chrome>=61, Firefox>=52, Safari>=11.1, Opera>=48, !IE, !Edge)

meta name=referrer wasn't enough. I happened to leak a few referrers with my
Firefox browser, though for some browsers it could have been enough.

[1] IE>=11, Edge>=18 use a different syntax for meta name=referrer, making it
    most probably incompatible (but I may be wrong on that, they may support
    both, but I have no way to test it currently). The next Edge release will be
    based on Chromium, so for that the Chrome version applies.

CHANGELOG.md

@@ -1,3 +1,6 @@
+# 1.8
+* SECURITY: improve referrer policy so that Etherpad addresses aren't leaked when links are clicked (see https://github.com/ether/etherpad-lite/pull/3636)
+
 # 1.8-beta.1
 * FEATURE: code was migrated to `async`/`await`, getting rid of a lot of callbacks (see https://github.com/ether/etherpad-lite/issues/3540)
 * FEATURE: support configuration via environment variables

src/node/hooks/express.js

@@ -76,6 +76,15 @@ exports.restartServer = function () {
     // https://github.com/ether/etherpad-lite/issues/2547
     res.header("X-UA-Compatible", "IE=Edge,chrome=1");
 
+    // Enable a strong referrer policy. Same-origin won't drop Referers when
+    // loading local resources, but it will drop them when loading foreign resources.
+    // It's still a last bastion of referrer security. External URLs should be
+    // already marked with rel="noreferer" and user-generated content pages are already
+    // marked with <meta name="referrer" content="no-referrer">
+    // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy
+    // https://github.com/ether/etherpad-lite/pull/3636
+    res.header("Referrer-Policy", "same-origin");
+
     // send git version in the Server response header if exposeVersion is true.
     if (settings.exposeVersion) {
       res.header("Server", serverName);

src/node/utils/ExportHtml.js

@@ -287,8 +287,12 @@ function getHTMLFromAtext(pad, atext, authorColors)
         processNextChars(startIndex - idx);
         // Using rel="noreferrer" stops leaking the URL/location of the exported HTML when clicking links in the document.
         // Not all browsers understand this attribute, but it's part of the HTML5 standard.
-        // http://www.w3.org/TR/html5/links.html#link-type-noreferrer
-        assem.append('<a href="' + Security.escapeHTMLAttribute(url) + '" rel="noreferrer">');
+        // https://html.spec.whatwg.org/multipage/links.html#link-type-noreferrer
+        // Additionally, we do rel="noopener" to ensure a higher level of referrer security.
+        // https://html.spec.whatwg.org/multipage/links.html#link-type-noopener
+        // https://mathiasbynens.github.io/rel-noopener/
+        // https://github.com/ether/etherpad-lite/pull/3636
+        assem.append('<a href="' + Security.escapeHTMLAttribute(url) + '" rel="noreferrer noopener">');
         processNextChars(urlLength);
         assem.append('</a>');
       });

src/static/js/domline.js

@@ -200,8 +200,12 @@ domline.createDomLine = function(nonEmpty, doesWrap, optBrowser, optDocument)
         }
         // Using rel="noreferrer" stops leaking the URL/location of the pad when clicking links in the document.
         // Not all browsers understand this attribute, but it's part of the HTML5 standard.
-        // http://www.w3.org/TR/html5/links.html#link-type-noreferrer
-        extraOpenTags = extraOpenTags + '<a href="' + Security.escapeHTMLAttribute(href) + '" rel="noreferrer">';
+        // https://html.spec.whatwg.org/multipage/links.html#link-type-noreferrer
+        // Additionally, we do rel="noopener" to ensure a higher level of referrer security.
+        // https://html.spec.whatwg.org/multipage/links.html#link-type-noopener
+        // https://mathiasbynens.github.io/rel-noopener/
+        // https://github.com/ether/etherpad-lite/pull/3636
+        extraOpenTags = extraOpenTags + '<a href="' + Security.escapeHTMLAttribute(href) + '" rel="noreferrer noopener">';
         extraCloseTags = '</a>' + extraCloseTags;
       }
       if (simpleTags)

src/static/js/pad_utils.js

@@ -225,8 +225,12 @@ var padutils = {
         advanceTo(startIndex);
         // Using rel="noreferrer" stops leaking the URL/location of the pad when clicking links in the document.
         // Not all browsers understand this attribute, but it's part of the HTML5 standard.
-        // http://www.w3.org/TR/html5/links.html#link-type-noreferrer
-        pieces.push('<a ', (target ? 'target="' + Security.escapeHTMLAttribute(target) + '" ' : ''), 'href="', Security.escapeHTMLAttribute(href), '" rel="noreferrer">');
+        // https://html.spec.whatwg.org/multipage/links.html#link-type-noreferrer
+        // Additionally, we do rel="noopener" to ensure a higher level of referrer security.
+        // https://html.spec.whatwg.org/multipage/links.html#link-type-noopener
+        // https://mathiasbynens.github.io/rel-noopener/
+        // https://github.com/ether/etherpad-lite/pull/3636
+        pieces.push('<a ', (target ? 'target="' + Security.escapeHTMLAttribute(target) + '" ' : ''), 'href="', Security.escapeHTMLAttribute(href), '" rel="noreferrer noopener">');
         advanceTo(startIndex + href.length);
         pieces.push('</a>');
       }