diff --git a/jspwiki-main/src/main/java/org/apache/wiki/plugin/DefaultPluginManager.java b/jspwiki-main/src/main/java/org/apache/wiki/plugin/DefaultPluginManager.java
index e7dfdb5a80..2d609e0633 100644
--- a/jspwiki-main/src/main/java/org/apache/wiki/plugin/DefaultPluginManager.java
+++ b/jspwiki-main/src/main/java/org/apache/wiki/plugin/DefaultPluginManager.java
@@ -238,7 +238,24 @@ public Pattern getPluginPattern() {
      *  @throws ClassNotFoundException if no such class exists.
      */
     private Class< ? > findPluginClass( final String classname ) throws ClassNotFoundException {
-        return ClassUtil.findClass( m_searchPath, m_externalJars, classname );
+        final Class< ? > clazz = ClassUtil.findClass( m_searchPath, m_externalJars, classname );
+        if( !Plugin.class.isAssignableFrom( clazz ) || !isInSearchPath( clazz ) ) {
+            // Reject with the same error as an unknown class, so that page markup cannot be used to
+            // load or instantiate arbitrary classpath classes, nor to probe for their presence.
+            throw new ClassNotFoundException( "Class '" + classname + "' not found in search path!" );
+        }
+        return clazz;
+    }
+
+    /**
+     *  Checks whether the package of the given class is one of the packages on the plugin search path.
+     *
+     *  @param clazz The class to check.
+     *  @return true, if the class' package is on the plugin search path.
+     */
+    private boolean isInSearchPath( final Class< ? > clazz ) {
+        final Package pkg = clazz.getPackage();
+        return pkg != null && m_searchPath.contains( pkg.getName() );
     }
 
     /** Outputs an HTML-formatted version of a stack trace. */
diff --git a/jspwiki-main/src/test/java/org/apache/wiki/plugin/DefaultPluginManagerTest.java b/jspwiki-main/src/test/java/org/apache/wiki/plugin/DefaultPluginManagerTest.java
index fd6879048d..86b78b55ec 100644
--- a/jspwiki-main/src/test/java/org/apache/wiki/plugin/DefaultPluginManagerTest.java
+++ b/jspwiki-main/src/test/java/org/apache/wiki/plugin/DefaultPluginManagerTest.java
@@ -22,6 +22,7 @@ Licensed to the Apache Software Foundation (ASF) under one
 import org.apache.wiki.TestEngine;
 import org.apache.wiki.api.core.Context;
 import org.apache.wiki.api.core.Engine;
+import org.apache.wiki.api.exceptions.PluginException;
 import org.apache.wiki.api.exceptions.ProviderException;
 import org.apache.wiki.api.spi.Wiki;
 import org.apache.wiki.pages.PageManager;
@@ -153,4 +154,16 @@ public void testParserPlugin() throws Exception {
         Assertions.assertTrue( SamplePlugin.c_rendered );
     }
 
+    /** Page markup must not load or instantiate arbitrary classpath classes; only plugins on the search path. */
+    @Test
+    public void testInsertArbitraryClass() {
+        final PluginException timer = Assertions.assertThrows( PluginException.class,
+                                                               () -> manager.execute( context, "{INSERT java.util.Timer}" ) );
+        final PluginException missing = Assertions.assertThrows( PluginException.class,
+                                                                 () -> manager.execute( context, "{INSERT java.util.NoSuchClass}" ) );
+        // Rejected with the same "could not find" error as a nonexistent class, so markup
+        // cannot be used as a classpath-probing oracle either.
+        Assertions.assertEquals( missing.getMessage().replace( "NoSuchClass", "Timer" ), timer.getMessage() );
+    }
+
 }