diff --git a/lib/defaults.js b/lib/defaults.js
index f43253ccb..83ed582af 100644
--- a/lib/defaults.js
+++ b/lib/defaults.js
@@ -16,15 +16,15 @@ Default.commonCSP = function (domain, sandbox) {
          *  it is recommended that you configure these fields to match the
          *  domain which will serve your CryptPad instance.
          */
-        "child-src 'self' blob: " + domain + sandbox,
+        "child-src " + domain,
         // IE/Edge
-        "frame-src 'self' blob: " + domain + sandbox,
+        "frame-src 'self' blob: " + sandbox,
 
         /*  this allows connections over secure or insecure websockets
             if you are deploying to production, you'll probably want to remove
-            the ws://* directive, and change '*' to your domain
+            the ws://* directive
          */
-        "connect-src 'self' ws: wss: blob: " + domain + sandbox,
+        "connect-src 'self' ws: blob: " + domain + sandbox,
 
         // data: is used by codemirror
         "img-src 'self' data: blob:" + domain,
@@ -32,6 +32,7 @@ Default.commonCSP = function (domain, sandbox) {
 
         // for accounts.cryptpad.fr authentication and cross-domain iframe sandbox
         "frame-ancestors *",
+        "worker-src 'self'",
         ""
     ];
 };
diff --git a/server.js b/server.js
index ddd2fcb88..73f63a3e1 100644
--- a/server.js
+++ b/server.js
@@ -84,12 +84,6 @@ var setHeaders = (function () {
     }
     if (Object.keys(headers).length) {
         return function (req, res) {
-            // apply a bunch of cross-origin headers for XLSX export in FF and printing elsewhere
-            /*
-            applyHeaderMap(res, {
-                "Cross-Origin-Opener-Policy": /^\/(sheet|presentation|doc|convert)\//.test(req.url)? 'same-origin': '',
-            });*/
-
             if (Env.NO_SANDBOX) { // handles correct configuration for local development
             // https://stackoverflow.com/questions/11531121/add-duplicate-http-response-headers-in-nodejs
                 applyHeaderMap(res, {
diff --git a/www/checkup/app-checkup.less b/www/checkup/app-checkup.less
index d5cab9c63..064958272 100644
--- a/www/checkup/app-checkup.less
+++ b/www/checkup/app-checkup.less
@@ -14,7 +14,7 @@ html, body {
 
     .report {
         font-size: 30px;
-        max-width: 26em;
+        max-width: 30em;
         margin: auto;
         padding-top: 15px;
     }
diff --git a/www/checkup/checkup-tools.js b/www/checkup/checkup-tools.js
index f9259ff28..5e0eef42d 100644
--- a/www/checkup/checkup-tools.js
+++ b/www/checkup/checkup-tools.js
@@ -1,5 +1,7 @@
 define([
-], function () {
+    'jquery',
+    '/common/common-util.js',
+], function ($, Util) {
     var Tools = {};
     Tools.supportsSharedArrayBuffers = function () {
         try {
@@ -51,5 +53,25 @@ define([
         return navigator.userAgent + "\n" + navigator.vendor;
     };
 
+    Tools.cacheBuster = function (url) {
+        if (/\?/.test(url)) { return url; }
+        return url + '?test=' + (+new Date());
+    };
+
+    var common_map = {};
+    Tools.common_xhr = function (url, _cb) {
+        var cb = Util.once(Util.once(Util.mkAsync(_cb)));
+        var ready = common_map[url];
+        if (ready) { return void ready.reg(cb); }
+        ready = common_map[url] = Util.mkEvent(true);
+        ready.reg(cb);
+        return void $.ajax(Tools.cacheBuster(url), {
+            dataType: 'text',
+            complete: function (xhr) {
+                ready.fire(xhr);
+            },
+        });
+    };
+
     return Tools;
 });
diff --git a/www/checkup/main.js b/www/checkup/main.js
index 8d8452726..e54b0b98c 100644
--- a/www/checkup/main.js
+++ b/www/checkup/main.js
@@ -67,9 +67,7 @@ define([
         $(msg).removeClass('cp-danger').addClass('cp-warning');
     };
 
-    var cacheBuster = function (url) {
-        return url + '?test=' + (+new Date());
-    };
+    var cacheBuster = Tools.cacheBuster;
 
     var trimmedSafe = trimSlashes(ApiConfig.httpSafeOrigin);
     var trimmedUnsafe = trimSlashes(ApiConfig.httpUnsafeOrigin);
@@ -371,7 +369,6 @@ define([
         var expect = {
             'cross-origin-resource-policy': 'cross-origin',
             'cross-origin-embedder-policy': 'require-corp',
-            //'cross-origin-opener-policy': 'same-origin', // FIXME this is in our nginx config but not server.js
         };
 
         $.ajax(url, {
@@ -656,7 +653,7 @@ define([
         ]);
     };
 
-    assert(function (_cb, msg) {
+    assert(function (_cb, msg) { // FIXME possibly superseded by more advanced CSP tests?
         var url = '/sheet/inner.html';
         var cb = Util.once(Util.mkAsync(_cb));
         msg.appendChild(CSP_WARNING(url));
@@ -672,7 +669,7 @@ define([
         });
     });
 
-    assert(function (cb, msg) {
+    assert(function (cb, msg) { // FIXME possibly superseded by more advanced CSP tests?
         var url = '/common/onlyoffice/v5/web-apps/apps/spreadsheeteditor/main/index.html';
         msg.appendChild(CSP_WARNING(url));
         deferredPostMessage({
@@ -805,36 +802,7 @@ define([
         cb(isHTTPS(trimmedUnsafe) && isHTTPS(trimmedSafe));
     });
 
-    [
-        //'sheet',
-        //'presentation',
-        //'doc',
-        //'convert',
-    ].forEach(function (url) {
-        assert(function (cb, msg) {
-            var header = 'cross-origin-opener-policy';
-            var expected = 'same-origin';
-            deferredPostMessage({
-                command: 'GET_HEADER',
-                content: {
-                    url: '/' + url + '/',
-                    header: header,
-                }
-            }, function (content) {
-                msg.appendChild(h('span', [
-                    code(url),
-                    ' was served without the correct ',
-                    code(header),
-                    ' HTTP header value (',
-                    code(expected),
-                    '). This will interfere with your ability to convert between office file formats.'
-                ]));
-                cb(content === expected);
-            });
-        });
-    });
-
-    assert(function (cb, msg) {
+    assert(function (cb, msg) { // FIXME this test has been superceded, but the descriptive text is still useful
         // check that the sandbox domain is included in connect-src
         msg.appendChild(h('span', [
             "This instance's ",
@@ -848,65 +816,254 @@ define([
             " See the provided NGINX configuration file for an example of how to set this header correctly.",
         ]));
 
-        $.ajax(cacheBuster('/'), {
-            dataType: 'text',
-            complete: function (xhr) {
-                var CSP = parseCSP(xhr.getResponseHeader('content-security-policy'));
-                var connect = (CSP && CSP['connect-src']) || "";
-                if (connect.includes(trimmedSafe)) {
-                    return void cb(true);
-                }
-                cb(CSP);
-            },
+        Tools.common_xhr('/', function (xhr) {
+            var CSP = parseCSP(xhr.getResponseHeader('content-security-policy'));
+            var connect = (CSP && CSP['connect-src']) || "";
+            if (connect.includes(trimmedSafe)) {
+                return void cb(true);
+            }
+            cb(CSP);
         });
     });
 
     assert(function (cb, msg) {
-        var directives = [
-            'img-src',
-            'media-src',
-            'child-src',
-            'frame-src'
-        ];
-
         msg.appendChild(h('span', [
-            "This instance's ",
-            code("Content-Security-Policy"),
-            " headers are unnecessarily permissive.",
-            h('br'),
-            h('br'),
-            " Review the recommended settings for ",
-            code('img-src'), ', ',
-            code('media-src'), ', ',
-            code('child-src'), ', and ',
-            code('frame-src'),
-            " in the provided NGINX configuration file for an example of how to set these headers correctly.",
+            code('/api/config'),
+            " returned an HTTP status code other than ",
+            code('200'),
+            ' when accessed from the sandbox domain.',
         ]));
-        $.ajax(cacheBuster('/'), {
-            dataType: 'text',
-            complete: function (xhr) {
-                var CSP = parseCSP(xhr.getResponseHeader('content-security-policy'));
-                // check that the relevant CSP directives are defined
-                // and that none of them permit general remote content via '*'
-                if (directives.every(function (k) {
-                    return typeof(CSP[k]) === 'string' && !/ \* /.test(CSP[k]);
-                })) {
-                    return void cb(true);
-                }
-                cb(CSP);
+        deferredPostMessage({
+            command: 'CHECK_HTTP_STATUS',
+            content: {
+                url: cacheBuster('/api/config'),
             },
+        }, function (content) {
+            cb(content === 200 || content);
         });
     });
 
+/*
     assert(function (cb, msg) {
-        msg.appendChild(h('span', 'pewpew'));
+        msg.appendChild(h('span', [
+            'all headers',
+        ]));
+
+        Tools.common_xhr('/', function (xhr) {
+            var all_headers = xhr.getAllResponseHeaders().split(/\r|\n/).filter(Boolean);
+            cb(all_headers);
+        });
+    });
+*/
+
+    var validateCSP = function (raw, msg, expected) {
+        var CSP = parseCSP(raw);
+        var checkRule = function (attr, rules) {
+            var v = CSP[attr];
+            // return `true` if you fail this test...
+            if (typeof(v) !== 'string' || !v) { return true; }
+            var l = rules.length;
+            for (var i = 0;i < l;i++) {
+                if (typeof(rules[i]) !== 'undefined' && !v.includes(rules[i])) { return true; }
+                v = v.replace(rules[i], '');
+            }
+            return v.trim();
+        };
+        if (Object.keys(expected).some(function (dir) {
+            var result = checkRule(dir, expected[dir]);
+            if (result) {
+                msg.appendChild(h('p', [
+                    'A value of ',
+                    code('"' + expected[dir].filter(Boolean).join(' ') + '"'),
+                    ' was expected for the ',
+                    code(dir),
+                    ' directive.',
+                ]));
+
+                console.log('BAD_HEADER:', {
+                    rule: dir,
+                    expected: expected[dir],
+                    result: result,
+                });
+            }
+
+            return result;
+        })) {
+            return parseCSP(raw);
+        }
+        return true;
+    };
+
+    assert(function (_cb, msg) {
+        var url = '/sheet/inner.html';
+        var cb = Util.once(Util.mkAsync(_cb));
+        msg.appendChild(h('span', [
+            code(trimmedUnsafe + url),
+            ' was served with incorrect ',
+            code('Content-Security-Policy'),
+            ' headers.',
+        ]));
+
+        //msg.appendChild(CSP_WARNING(url));
         deferredPostMessage({
-            command: 'CHECK_HTTP_STATUS',
+            command: 'GET_HEADER',
             content: {
-                url: cacheBuster('/api/config'),
+                url: url,
+                header: 'content-security-policy',
             },
-        }, function (content) {
-            cb(content === 200 || content);
+        }, function (raw) {
+            var $outer = trimmedUnsafe;
+            var $sandbox = trimmedSafe;
+            var result = validateCSP(raw, msg, {
+                'default-src': ["'none'"],
+                'style-src': ["'unsafe-inline'", "'self'", $outer],
+                'font-src': ["'self'", 'data:', $outer],
+                'child-src': [$outer], //["'self'", 'blob:', $outer, $sandbox],
+                'frame-src': ["'self'", 'blob:', /*$outer, */$sandbox],
+                'script-src': ["'self'", 'resource:', $outer,
+                    "'unsafe-eval'",
+                    "'unsafe-inline'",
+                ],
+                'connect-src': [
+                    "'self'",
+                    'blob:',
+                    $outer,
+                    $sandbox,
+                    /https:\/\//.test($outer)? $outer.replace('https://', 'wss://') : 'ws:',
+                ],
+
+                'img-src': ["'self'", 'data:', 'blob:', $outer],
+                'media-src': ['blob:'],
+                //'frame-ancestors': ['*'], // XXX IFF you want to support remote embedding
+                'worker-src': ["'self'"], // , $outer, $sandbox],
+            });
+            cb(result);
+        });
+    });
+
+    assert(function (cb, msg) {
+        var header = 'content-security-policy';
+        msg.appendChild(h('span', [
+            code(trimmedUnsafe + '/'),
+            ' was served with incorrect ',
+            code('Content-Security-Policy'),
+            ' headers.',
+        ]));
+        Tools.common_xhr('/', function (xhr) {
+            var raw = xhr.getResponseHeader(header);
+            var $outer = trimmedUnsafe;
+            var $sandbox = trimmedSafe;
+            var result = validateCSP(raw, msg, {
+                'default-src': ["'none'"],
+                'style-src': ["'unsafe-inline'", "'self'", $outer],
+                'font-src': ["'self'", 'data:', $outer],
+                'child-src': [$outer], //["'self'", 'blob:', $outer, $sandbox],
+                'frame-src': ["'self'", 'blob:', /*$outer,*/ $sandbox],
+                'script-src': ["'self'", 'resource:', $outer],
+                'connect-src': [
+                    "'self'",
+                    'blob:',
+                    $outer,
+                    $sandbox,
+                    /https:\/\//.test($outer)? $outer.replace('https://', 'wss://') : 'ws:',
+                ],
+                'img-src': ["'self'", 'data:', 'blob:', $outer],
+                'media-src': ['blob:'],
+                //'frame-ancestors': ['*'], // XXX IFF you want to support remote embedding
+                'worker-src': ["'self'"],//, $outer, $sandbox],
+            });
+
+            cb(result);
+        });
+    });
+
+    assert(function (cb, msg) {
+        var header = 'Access-Control-Allow-Origin';
+        msg.appendChild(h('span', [
+            'Assets must be served with an ',
+            code(header),
+            ' header with a value of ',
+            code("'*'"),
+            ' if you wish to support embedding of encrypted media on third party websites.',
+        ]));
+        Tools.common_xhr('/', function (xhr) {
+            var raw = xhr.getResponseHeader(header);
+            cb(raw === "*" || raw);
+        });
+    });
+
+    assert(function (cb, msg) {
+        var header = 'Cross-Origin-Embedder-Policy';
+        msg.appendChild(h('span', [
+            "Assets must be served with a ",
+            code(header),
+            ' value of ',
+            code('require-corp'),
+            " to enable browser features required for client-side document conversion.",
+        ]));
+        Tools.common_xhr('/', function (xhr) {
+            var raw = xhr.getResponseHeader(header);
+            cb(raw === 'require-corp' || raw);
+        });
+    });
+
+    assert(function (cb, msg) {
+        var header = 'Cross-Origin-Resource-Policy';
+        msg.appendChild(h('span', [
+            "Assets must be served with a ",
+            code(header),
+            ' value of ',
+            code('cross-origin'),
+            " to enable browser features required for client-side document conversion.",
+        ]));
+        Tools.common_xhr('/', function (xhr) {
+            var raw = xhr.getResponseHeader(header);
+            cb(raw === 'cross-origin' || raw);
+        });
+    });
+
+    assert(function (cb, msg) {
+        var header = 'X-Content-Type-Options';
+        msg.appendChild(h('span', [
+            "Assets should be served with an ",
+            code(header),
+            ' header with a value of ',
+            code('nosniff'),
+            '.',
+        ]));
+        Tools.common_xhr('/', function (xhr) {
+            var raw = xhr.getResponseHeader(header);
+            cb(raw === 'nosniff' || raw);
+        });
+    });
+
+    assert(function (cb, msg) {
+        var header = 'Cache-Control';
+        msg.appendChild(h('span', [
+            'Assets requested without a version parameter should be served with a ',
+            code('no-cache'),
+            ' value for the ',
+            code("Cache-Control"),
+            ' header.',
+        ]));
+        // Cache-Control should be 'no-cache' unless the URL includes ver=
+        Tools.common_xhr('/', function (xhr) {
+            var raw = xhr.getResponseHeader(header);
+            cb(raw === 'no-cache' || raw);
+        });
+    });
+
+    assert(function (cb, msg) {
+        var header = 'Cache-Control';
+        msg.appendChild(h('span', [
+            'Assets requested with a version parameter should be served with a long-lived ',
+            code('Cache-Control'),
+            ' header.',
+        ]));
+        // Cache-Control should be 'max-age=<number>' if the URL includes 'ver='
+        Tools.common_xhr('/customize/messages.js?ver=' +(+new Date()), function (xhr) {
+            var raw = xhr.getResponseHeader(header);
+            cb(/max\-age=\d+$/.test(raw) || raw);
         });
     });
 
@@ -959,13 +1116,6 @@ define([
     });
 */
 
-    if (false) {
-        assert(function (cb, msg) {
-            msg.innerText = 'fake test to simulate failure';
-            cb(false);
-        });
-    }
-
     var row = function (cells) {
         return h('tr', cells.map(function (cell) {
             return h('td', cell);
diff --git a/www/checkup/sandbox/main.js b/www/checkup/sandbox/main.js
index 47a7be13b..22130b3cf 100644
--- a/www/checkup/sandbox/main.js
+++ b/www/checkup/sandbox/main.js
@@ -11,13 +11,10 @@ define([
         window.parent.postMessage(JSON.stringify(content), '*');
     };
     postMessage({ command: "READY", });
-    var getHeaders = function (url, cb) {
-        $.ajax(url + "?test=" + (+new Date()), {
-            dataType: 'text',
-            complete: function (xhr) {
-                var allHeaders = xhr.getAllResponseHeaders();
-                return void cb(void 0, allHeaders, xhr);
-            },
+    var getHeaders = function (url, cb) { // XXX reuse XHR objects?
+        Tools.common_xhr(url, function (xhr) {
+            var allHeaders = xhr.getAllResponseHeaders();
+            return void cb(void 0, allHeaders, xhr);
         });
     };
     var COMMANDS = {};
@@ -49,11 +46,8 @@ define([
     };
 
     COMMANDS.CHECK_HTTP_STATUS = function (content, cb) {
-        $.ajax(content.url, {
-            dataType: 'text',
-            complete: function (xhr) {
-                cb(xhr.status);
-            },
+        Tools.common_xhr(content.url, function (xhr) {
+            cb(xhr.status);
         });
     };