diff --git a/config/defaults.inc.php b/config/defaults.inc.php
index 35901ab698e..760765c5522 100644
--- a/config/defaults.inc.php
+++ b/config/defaults.inc.php
@@ -1563,3 +1563,14 @@
 // 0 - Reply-All always
 // 1 - Reply-List if mailing list is detected
 $config['reply_all_mode'] = 0;
+
+// The Content-Security-Policy to use if no remote objects are allowed to
+// be loaded. If you use plugins you might need to extend this.
+// Only change this if you know what you're doing! You can break the whole
+// application with changes to this setting!
+// To disable completely set the value to `false`;
+$config['content_security_policy'] = "default-src 'self' data: blob:; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline'";
+
+// Additions to the Content-Security-Policy to use if remote objects *are*
+// allowed to be loaded.
+$config['content_security_policy_add_allow_remote'] = 'img-src *; media-src *; font-src: *; frame-src: *';
diff --git a/program/include/rcmail_output_html.php b/program/include/rcmail_output_html.php
index 75e9fbf0e17..1de462b33f8 100644
--- a/program/include/rcmail_output_html.php
+++ b/program/include/rcmail_output_html.php
@@ -728,6 +728,8 @@ public function page_headers()
                 $this->header('X-Frame-Options: sameorigin', true);
             }
         }
+
+        $this->add_csp_header();
     }
 
     /**
@@ -2717,4 +2719,41 @@ protected function get_template_logo($type = null, $match = null)
 
         return $template_logo;
     }
+
+    /**
+     * Add the Content-Security-Policy to the HTTP response headers (unless it
+     * is disabled).
+     */
+    protected function add_csp_header(): void
+    {
+        $csp = $this->get_csp_value('content_security_policy');
+        if ($csp !== false) {
+            $csp_parts = [$csp];
+            if (isset($this->env['safemode']) && $this->env['safemode'] === true) {
+                $csp_allow_remote = $this->get_csp_value('content_security_policy_add_allow_remote');
+                if ($csp_allow_remote !== false) {
+                    $csp_parts[] = $csp_allow_remote;
+                }
+            }
+            $this->header('Content-Security-Policy: ' . implode('; ', $csp_parts));
+        }
+    }
+
+    /**
+     * Get a CSP-related value from the config, stripped by surrounding
+     * whitespace and semicolons (and NUL byte, because it's included in the
+     * default second argument to trim(), too).
+     *
+     * @param $name string The key of the wanted config value
+     *
+     * @return string|false
+     */
+    protected function get_csp_value($name)
+    {
+        $value = $this->app->config->get($name);
+        if (is_string($value)) {
+            return trim($value, "; \n\r\t\v\x00");
+        }
+        return false;
+    }
 }
diff --git a/program/lib/Roundcube/rcube_config.php b/program/lib/Roundcube/rcube_config.php
index 60833754254..2b635212187 100644
--- a/program/lib/Roundcube/rcube_config.php
+++ b/program/lib/Roundcube/rcube_config.php
@@ -24,6 +24,11 @@ class rcube_config
 {
     public const DEFAULT_SKIN = 'elastic';
 
+    public const DEFAULTS = [
+        'content_security_policy' => "default-src 'self' data: blob:; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline'",
+        'content_security_policy_add_allow_remote' => 'img-src *; media-src *; font-src: *; frame-src: *',
+    ];
+
     /** @var string A skin configured in the config file (before being replaced by a user preference) */
     public $system_skin = 'elastic';
 
@@ -376,8 +381,12 @@ public function get($name, $def = null)
     {
         if (isset($this->prop[$name])) {
             $result = $this->prop[$name];
-        } else {
+        } elseif (isset($def)) {
             $result = $def;
+        } elseif (isset(self::DEFAULTS[$name])) {
+            $result = self::DEFAULTS[$name];
+        } else {
+            $result = null;
         }
 
         $result = $this->getenv_default('ROUNDCUBE_' . strtoupper($name), $result);
diff --git a/tests/Framework/ConfigTest.php b/tests/Framework/ConfigTest.php
index d3da1e8b8fa..ca551507709 100644
--- a/tests/Framework/ConfigTest.php
+++ b/tests/Framework/ConfigTest.php
@@ -96,4 +96,22 @@ public function test_parse_env()
         $this->assertSame([1], invokeMethod($object, 'parse_env', ['[1]', 'array']));
         $this->assertSame(['test' => 1], (array) invokeMethod($object, 'parse_env', ['{"test":1}', 'object']));
     }
+
+    // Test if values in defaults.inc.php and values in rcube_config::DEFAULTS match
+    public function test_defaults_values(): void
+    {
+        // Initialize the variable to avoid warnings.
+        $config = null;
+        // Load the values from defaults.inc.php manually (we don't want to
+        // test the whole loading mechanics of `rcube_config` here).
+        ob_start();
+        require realpath(RCUBE_INSTALL_PATH . 'config/defaults.inc.php');
+        ob_end_clean();
+
+        $this->assertIsArray($config); // @phpstan-ignore-line
+
+        foreach (\rcube_config::DEFAULTS as $name => $hardcoded_value) {
+            $this->assertSame($hardcoded_value, $config[$name], "The value for '{$name}' in defaults.inc.php does not match the hardcoded default in rcube_config!");
+        }
+    }
 }
diff --git a/tests/Rcmail/OutputHtmlTest.php b/tests/Rcmail/OutputHtmlTest.php
index 45c359b288b..8fc9b524789 100644
--- a/tests/Rcmail/OutputHtmlTest.php
+++ b/tests/Rcmail/OutputHtmlTest.php
@@ -5,6 +5,8 @@
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 
+use function Roundcube\Tests\invokeMethod;
+
 /**
  * Test class to test rcmail_output_html class
  */
@@ -429,4 +431,66 @@ public function test_charset_selector()
         $this->assertTrue(strpos($result, '<select name="_charset">') === 0);
         $this->assertTrue(strpos($result, '<option value="UTF-8" selected="selected">UTF-8 (Unicode)</option>') !== false);
     }
+
+    /**
+     * Test adding the Content-Security-Policy with "safemode" off.
+     */
+    public function test_add_csp_header_default_without_safemode()
+    {
+        $headers = $this->run_add_csp_header_and_get_headers(false);
+
+        $this->assertCount(1, $headers);
+        $this->assertSame("Content-Security-Policy: default-src 'self' data: blob:; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline'", $headers[0]);
+    }
+
+    /**
+     * Test adding the Content-Security-Policy with "safemode" on.
+     */
+    public function test_add_csp_header_default_with_safemode()
+    {
+        $headers = $this->run_add_csp_header_and_get_headers(true);
+
+        $this->assertCount(1, $headers);
+        $this->assertSame("Content-Security-Policy: default-src 'self' data: blob:; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline'; img-src *; media-src *; font-src: *; frame-src: *", $headers[0]);
+    }
+
+    /**
+     * Test adding a custom configured Content-Security-Policy.
+     */
+    public function test_add_csp_header_custom()
+    {
+        $rcmail = \rcube::get_instance();
+        // Some a little strange strings.
+        $rcmail->config->set('content_security_policy', 'default-src * ; ');
+        $rcmail->config->set('content_security_policy_add_allow_remote', "; script-src 'unsafe-inline'");
+
+        $headers = $this->run_add_csp_header_and_get_headers(true);
+
+        $this->assertCount(1, $headers);
+        $this->assertSame("Content-Security-Policy: default-src *; script-src 'unsafe-inline'", $headers[0]);
+    }
+
+    protected function run_add_csp_header_and_get_headers(bool $safemode): array
+    {
+        // Use a subclass to get hands on the headers to be sent. There's not
+        // other way to do this, unfortunately, since the PHP CLI SAPI silently
+        // swallows all calls to `header()` (as done in
+        // `rcube_output::header()`).
+        // The class `OutputHtmlMock` is mocking a lot more than we need, so we
+        // rather roll our own.
+        $output = new class extends \rcmail_output_html {
+            public $http_headers = [];
+
+            #[\Override]
+            public function header($header, $replace = true)
+            {
+                $this->http_headers[] = $header;
+            }
+        };
+
+        $output->set_env('safemode', $safemode);
+        invokeMethod($output, 'add_csp_header');
+
+        return $output->http_headers;
+    }
 }