diff --git a/src/main/java/net/sourceforge/plantuml/servlet/DiagramResponse.java b/src/main/java/net/sourceforge/plantuml/servlet/DiagramResponse.java
index c0ef9b7..b3af58a 100644
--- a/src/main/java/net/sourceforge/plantuml/servlet/DiagramResponse.java
+++ b/src/main/java/net/sourceforge/plantuml/servlet/DiagramResponse.java
@@ -69,6 +69,12 @@ class DiagramResponse {
         map.put(FileFormat.BASE64, "text/plain; charset=x-user-defined");
         CONTENT_TYPE = Collections.unmodifiableMap(map);
     }
+    static {
+        OptionFlags.ALLOW_INCLUDE = false;
+        if ("true".equalsIgnoreCase(System.getenv("ALLOW_PLANTUML_INCLUDE"))) {
+            OptionFlags.ALLOW_INCLUDE = true;
+        }
+    }
 
     DiagramResponse(HttpServletResponse r, FileFormat f, HttpServletRequest rq) {
         response = r;
diff --git a/src/main/java/net/sourceforge/plantuml/servlet/ProxyServlet.java b/src/main/java/net/sourceforge/plantuml/servlet/ProxyServlet.java
index 979abde..43fd8f7 100644
--- a/src/main/java/net/sourceforge/plantuml/servlet/ProxyServlet.java
+++ b/src/main/java/net/sourceforge/plantuml/servlet/ProxyServlet.java
@@ -56,6 +56,13 @@ import javax.net.ssl.SSLPeerUnverifiedException;
 @SuppressWarnings("serial")
 public class ProxyServlet extends HttpServlet {
 
+    static {
+        OptionFlags.ALLOW_INCLUDE = false;
+        if ("true".equalsIgnoreCase(System.getenv("ALLOW_PLANTUML_INCLUDE"))) {
+            OptionFlags.ALLOW_INCLUDE = true;
+        }
+    }
+
     @Override
     public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
 
diff --git a/src/main/java/net/sourceforge/plantuml/servlet/UmlDiagramService.java b/src/main/java/net/sourceforge/plantuml/servlet/UmlDiagramService.java
index a90f178..fd996c3 100644
--- a/src/main/java/net/sourceforge/plantuml/servlet/UmlDiagramService.java
+++ b/src/main/java/net/sourceforge/plantuml/servlet/UmlDiagramService.java
@@ -42,6 +42,13 @@ import java.util.regex.Pattern;
 @SuppressWarnings("serial")
 public abstract class UmlDiagramService extends HttpServlet {
 
+    static {
+        OptionFlags.ALLOW_INCLUDE = false;
+        if ("true".equalsIgnoreCase(System.getenv("ALLOW_PLANTUML_INCLUDE"))) {
+            OptionFlags.ALLOW_INCLUDE = true;
+        }
+    }
+
     @Override
     public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
 
diff --git a/src/main/java/net/sourceforge/plantuml/servlet/utility/UmlExtractor.java b/src/main/java/net/sourceforge/plantuml/servlet/utility/UmlExtractor.java
index 6b2b5e8..fd6b789 100644
--- a/src/main/java/net/sourceforge/plantuml/servlet/utility/UmlExtractor.java
+++ b/src/main/java/net/sourceforge/plantuml/servlet/utility/UmlExtractor.java
@@ -36,6 +36,13 @@ import net.sourceforge.plantuml.code.TranscoderUtil;
  */
 public class UmlExtractor {
 
+    static {
+        OptionFlags.ALLOW_INCLUDE = false;
+        if ("true".equalsIgnoreCase(System.getenv("ALLOW_PLANTUML_INCLUDE"))) {
+            OptionFlags.ALLOW_INCLUDE = true;
+        }
+    }
+
     /**
      * Build the complete UML source from the compressed source extracted from the HTTP URI.
      *