WW-5621 Harden XML parsers against Entity Expansion (Billion Laughs) attacks (#1642)

Modern JDKs (7u45+) already protect against this attack with a built-in
64K entity expansion limit. These changes add defense-in-depth hardening
and remove unnecessary attack surface.

- Remove unused parseStringAsXML feature from StringAdapter to eliminate
  a theoretical XML Entity Expansion vector
- Deprecate setParseStringAsXML() and getParseStringAsXML() for removal
- Enable SECURE_PROCESSING feature in DigesterDefinitionsReader
- Add unit test verifying JDK's entity expansion limit rejects
  Billion Laughs payloads
- Add research document with vulnerability analysis

Co-authored-by: Claude <noreply@anthropic.com>

plugins/tiles/src/main/java/org/apache/tiles/core/definition/digester/DigesterDefinitionsReader.java

@@ -35,6 +35,7 @@ import org.xml.sax.SAXNotRecognizedException;
 import org.xml.sax.SAXNotSupportedException;
 import org.xml.sax.SAXParseException;
 
+import javax.xml.XMLConstants;
 import javax.xml.parsers.ParserConfigurationException;
 import java.io.IOException;
 import java.io.InputStream;
@@ -164,6 +165,8 @@ public class DigesterDefinitionsReader implements DefinitionsReader {
             // Disable external DTDs as well
             digester.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);
             digester.setXIncludeAware(false);
+            // Enable secure processing to limit entity expansion (prevents Billion Laughs attack)
+            digester.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
         } catch (ParserConfigurationException | SAXNotRecognizedException | SAXNotSupportedException e) {
             throw new StrutsException("Unable to disable external XML entity parsing", e);
         }

plugins/tiles/src/test/java/org/apache/tiles/core/definition/digester/TestDigesterDefinitionsReader.java

@@ -26,9 +26,11 @@ import org.apache.tiles.core.definition.DefinitionsFactoryException;
 import org.junit.Before;
 import org.junit.Test;
 
+import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.net.URL;
+import java.nio.charset.StandardCharsets;
 import java.util.List;
 import java.util.Map;
 
@@ -268,6 +270,27 @@ public class TestDigesterDefinitionsReader {
         assertNull(reader.read(null));
     }
 
+    /**
+     * Tests that the Digester parser is protected against Billion Laughs (XML Entity Expansion) attack.
+     * FEATURE_SECURE_PROCESSING is enabled in DigesterDefinitionsReader to limit entity expansion.
+     */
+    @Test(expected = DefinitionsFactoryException.class)
+    public void testBillionLaughsProtection() {
+        String xml = "<?xml version=\"1.0\"?>" +
+            "<!DOCTYPE root [" +
+            "<!ENTITY lol0 \"lol\">" +
+            "<!ENTITY lol1 \"&lol0;&lol0;&lol0;&lol0;&lol0;&lol0;&lol0;&lol0;&lol0;&lol0;\">" +
+            "<!ENTITY lol2 \"&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;\">" +
+            "<!ENTITY lol3 \"&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;\">" +
+            "<!ENTITY lol4 \"&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;\">" +
+            "<!ENTITY lol5 \"&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;\">" +
+            "]>" +
+            "<root>&lol5;</root>";
+
+        InputStream source = new ByteArrayInputStream(xml.getBytes(StandardCharsets.UTF_8));
+        reader.read(source);
+    }
+
     /**
      * Tests {@link DigesterDefinitionsReader#addDefinition(Definition)}.
      */

plugins/xslt/src/main/java/org/apache/struts2/result/xslt/StringAdapter.java

@@ -18,13 +18,10 @@
  */
 package org.apache.struts2.result.xslt;
 
-import org.apache.struts2.util.DomHelper;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.w3c.dom.Node;
-import org.xml.sax.InputSource;
 
-import java.io.StringReader;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -39,17 +36,13 @@ import java.util.List;
  *
  * <p>
  * Subclasses may override the getStringValue() method in order to use StringAdapter
- * as a simplified custom XML adapter for Java types.  A subclass can enable XML
- * parsing of the value string via the setParseStringAsXML() method and then
- * override getStringValue() to return a String containing the custom formatted XML.
+ * as a simplified custom XML adapter for Java types.
  * </p>
  */
 public class StringAdapter extends AbstractAdapterElement {
 
     private static final Logger LOG = LogManager.getLogger(StringAdapter.class);
 
-    boolean parseStringAsXML;
-
     public StringAdapter() {
     }
 
@@ -58,16 +51,11 @@ public class StringAdapter extends AbstractAdapterElement {
     }
 
     /**
-     * <p>
      * Get the object to be adapted as a String value.
-     * </p>
      *
      * <p>
      * This method can be overridden by subclasses that wish to use StringAdapter
-     * as a simplified customizable XML adapter for Java types. A subclass can
-     * enable parsing of the value string as containing XML text via the
-     * setParseStringAsXML() method and then override getStringValue() to return a
-     * String containing the custom formatted XML.
+     * as a simplified customizable XML adapter for Java types.
      * </p>
      *
      * @return the string value
@@ -78,17 +66,8 @@ public class StringAdapter extends AbstractAdapterElement {
 
     @Override
     protected List<Node> buildChildAdapters() {
-        Node node;
-        if (getParseStringAsXML()) {
-            LOG.debug("parsing string as xml: {}", getStringValue());
-            // Parse the String to a DOM, then proxy that as our child
-            node = DomHelper.parse(new InputSource(new StringReader(getStringValue())));
-            node = getAdapterFactory().proxyNode(this, node);
-        } else {
-            LOG.debug("using string as is: {}", getStringValue());
-            // Create a Text node as our child
-            node = new SimpleTextNode(getAdapterFactory(), this, "text", getStringValue());
-        }
+        LOG.debug("using string as is: {}", getStringValue());
+        Node node = new SimpleTextNode(getAdapterFactory(), this, "text", getStringValue());
 
         List<Node> children = new ArrayList<>();
         children.add(node);
@@ -96,26 +75,23 @@ public class StringAdapter extends AbstractAdapterElement {
     }
 
     /**
-     * @return is this StringAdapter to interpret its string values as containing
-     * XML Text?
-     *
-     * @see #setParseStringAsXML(boolean)
+     * @return always returns false
+     * @deprecated This feature has been removed for security reasons (potential XML Entity Expansion attacks).
+     *             This method now always returns false and will be removed in a future version.
      */
+    @Deprecated(forRemoval = true, since = "7.2.0")
     public boolean getParseStringAsXML() {
-        return parseStringAsXML;
+        return false;
     }
 
     /**
-     * When set to true the StringAdapter will interpret its String value
-     * as containing XML text and parse it to a DOM Element.  The new DOM
-     * Element will be a child of this String element. (i.e. wrapped in an
-     * element of the property name specified for this StringAdapter).
-     *
-     * @param parseStringAsXML when set to true the StringAdapter will interpret its String value as containing XML text
-     * @see #getParseStringAsXML()
+     * @param parseStringAsXML ignored
+     * @deprecated This feature has been removed for security reasons (potential XML Entity Expansion attacks).
+     *             This method is now a no-op and will be removed in a future version.
      */
+    @Deprecated(forRemoval = true, since = "7.2.0")
     public void setParseStringAsXML(boolean parseStringAsXML) {
-        this.parseStringAsXML = parseStringAsXML;
+        // no-op - feature removed for security reasons
     }
 
 }