diff -u b/core/modules/dblog/src/Controller/DbLogController.php b/core/modules/dblog/src/Controller/DbLogController.php
--- b/core/modules/dblog/src/Controller/DbLogController.php
+++ b/core/modules/dblog/src/Controller/DbLogController.php
@@ -267,7 +267,7 @@
         ),
         array(
           array('data' => $this->t('Message'), 'header' => TRUE),
-          $message,
+          array('data' => array('#markup' => $message)),
         ),
         array(
           array('data' => $this->t('Severity'), 'header' => TRUE),
@@ -337,7 +337,7 @@
    *   The record from the watchdog table. The object properties are: wid, uid,
    *   severity, type, timestamp, message, variables, link, name.
    *
-   * @return string|\Drupal\Core\StringTranslation\TranslatableMarkup|false
+   * @return string|false
    *   The formatted log message or FALSE if the message or variables properties
    *   are not set.
    */
only in patch2:
unchanged:
--- a/core/modules/dblog/src/Tests/DbLogTest.php
+++ b/core/modules/dblog/src/Tests/DbLogTest.php
@@ -809,11 +809,19 @@ public function testOverviewLinks() {
     // Make sure HTML tags are filtered out.
     $this->assertRaw('title="alert(&#039;foo&#039;);Lorem ipsum dolor sit amet, consectetur adipiscing &amp; elit. Entry #0">&lt;script&gt;alert(&#039;foo&#039;);&lt;/script&gt;Lorem ipsum dolor sit…</a>');
     $this->assertNoRaw("<script>alert('foo');</script>");
+  }
+
+  /**
+   * Make sure the log messages in detailed pages are properly escaped.
+   */
+  public function testLogEventPageMessageEscaped() {
+    $this->drupalLogin($this->adminUser);
 
     // Make sure HTML tags are filtered out in admin/reports/dblog/event/ too.
     $this->generateLogEntries(1, ['message' => "<script>alert('foo');</script> <strong>Lorem ipsum</strong>"]);
     $wid = db_query('SELECT MAX(wid) FROM {watchdog}')->fetchField();
     $this->drupalGet('admin/reports/dblog/event/' . $wid);
+    $this->assertResponse(200);
     $this->assertNoRaw("<script>alert('foo');</script>");
     $this->assertRaw("alert('foo'); <strong>Lorem ipsum</strong>");
   }