diff --git a/guard/jsqltranspiler/src/main/java/org/eclipse/daanse/sql/guard/jsqltranspiler/ReadOnlyStatementValidator.java b/guard/jsqltranspiler/src/main/java/org/eclipse/daanse/sql/guard/jsqltranspiler/ReadOnlyStatementValidator.java
new file mode 100644
index 0000000..9259e26
--- /dev/null
+++ b/guard/jsqltranspiler/src/main/java/org/eclipse/daanse/sql/guard/jsqltranspiler/ReadOnlyStatementValidator.java
@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2024 Contributors to the Eclipse Foundation.
+ *
+ * This program and the accompanying materials are made
+ * available under the terms of the Eclipse Public License 2.0
+ * which is available at https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *   SmartCity Jena - initial
+ *   Stefan Bischof (bipolis.org) - initial
+ */
+
+package org.eclipse.daanse.sql.guard.jsqltranspiler;
+
+import java.util.Optional;
+
+import net.sf.jsqlparser.statement.ParenthesedStatement;
+import net.sf.jsqlparser.statement.Statement;
+import net.sf.jsqlparser.statement.select.ParenthesedSelect;
+import net.sf.jsqlparser.statement.select.Select;
+import net.sf.jsqlparser.statement.select.WithItem;
+import net.sf.jsqlparser.statement.delete.ParenthesedDelete;
+import net.sf.jsqlparser.statement.insert.ParenthesedInsert;
+import net.sf.jsqlparser.statement.update.ParenthesedUpdate;
+import net.sf.jsqlparser.util.TablesNamesFinder;
+
+/**
+ * Walks a parsed {@link Select} tree and detects data-modifying statements that
+ * are wrapped in a {@code WITH} item (a data-modifying CTE), such as
+ * {@code WITH x AS (UPDATE foo ...) SELECT ...}.
+ * <p>
+ * Such statements parse as a {@code Select} and therefore pass the top-level
+ * {@code instanceof Select} gate, yet on PostgreSQL/MariaDB they execute the
+ * wrapped {@code UPDATE}/{@code INSERT}/{@code DELETE}. The upstream resolver
+ * does not populate its update/insert column lists for a {@code WithItem}, so it
+ * cannot be relied on to detect this case.
+ * <p>
+ * This validator reuses {@link TablesNamesFinder}'s full recursive traversal
+ * (subqueries, joins, set operations, nested {@code WITH} clauses) and inspects
+ * every {@code WithItem}. A {@code WithItem} is only permitted to wrap a
+ * {@link ParenthesedSelect}; anything else is a violation.
+ */
+class ReadOnlyStatementValidator extends TablesNamesFinder<Void> {
+
+    static final String DELETE_IS_NOT_PERMITTED = "DELETE is not permitted.";
+    static final String UPDATE_IS_NOT_PERMITTED = "UPDATE is not permitted.";
+    static final String INSERT_IS_NOT_PERMITTED = "INSERT is not permitted.";
+    static final String STATEMENT_IS_NOT_PERMITTED = "Statement is not permitted.";
+
+    private String violation;
+
+    /**
+     * Runs the read-only traversal over the given select and returns the first
+     * violation message, or an empty optional when the statement is read-only.
+     */
+    Optional<String> firstViolation(Select select) {
+        violation = null;
+        // drives the recursive traversal; the returned table set is irrelevant
+        getTables((Statement) select);
+        return Optional.ofNullable(violation);
+    }
+
+    @Override
+    public <S> Void visit(WithItem<?> withItem, S context) {
+        ParenthesedStatement statement = withItem.getParenthesedStatement();
+        if (statement instanceof ParenthesedSelect) {
+            // read-only CTE body: keep descending so nested WITH clauses are inspected too.
+            // The base implementation resolves getSelect(), which is only valid here.
+            return super.visit(withItem, context);
+        }
+        // Anything other than a ParenthesedSelect is a data-modifying (or otherwise
+        // unexpected) CTE. Record it and do NOT descend: getSelect() would throw.
+        if (violation == null) {
+            if (statement instanceof ParenthesedUpdate) {
+                violation = UPDATE_IS_NOT_PERMITTED;
+            } else if (statement instanceof ParenthesedInsert) {
+                violation = INSERT_IS_NOT_PERMITTED;
+            } else if (statement instanceof ParenthesedDelete) {
+                violation = DELETE_IS_NOT_PERMITTED;
+            } else {
+                violation = STATEMENT_IS_NOT_PERMITTED;
+            }
+        }
+        return null;
+    }
+}
diff --git a/guard/jsqltranspiler/src/main/java/org/eclipse/daanse/sql/guard/jsqltranspiler/TranspilerSqlGuard.java b/guard/jsqltranspiler/src/main/java/org/eclipse/daanse/sql/guard/jsqltranspiler/TranspilerSqlGuard.java
index f916f54..81db60f 100644
--- a/guard/jsqltranspiler/src/main/java/org/eclipse/daanse/sql/guard/jsqltranspiler/TranspilerSqlGuard.java
+++ b/guard/jsqltranspiler/src/main/java/org/eclipse/daanse/sql/guard/jsqltranspiler/TranspilerSqlGuard.java
@@ -16,6 +16,7 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Optional;
 import java.util.Set;
 import java.util.regex.Pattern;
 
@@ -90,6 +91,13 @@ public String guard(String sqlStr) throws GuardException {
 
             // we can test for SELECT, though in practise it won't protect us from harmful statements
             if (st instanceof Select) {
+
+                // A statement can parse as a Select yet still carry a data-modifying
+                // statement wrapped in a WITH item (e.g. WITH x AS (UPDATE ...) SELECT ...).
+                // The resolver does not populate its update/insert column lists for that
+                // case, so reject it explicitly before anything else.
+                validateReadOnly((Select) st);
+
                 resolver.resolve(st);
 
                 // select columns should not be empty
@@ -172,6 +180,18 @@ public String guard(String sqlStr) throws GuardException {
 
     }
 
+    private static void validateReadOnly(Select select) throws GuardException {
+        Optional<String> violation = new ReadOnlyStatementValidator().firstViolation(select);
+        if (violation.isPresent()) {
+            String message = violation.get();
+            LOGGER.atInfo().log(message);
+            if (ReadOnlyStatementValidator.STATEMENT_IS_NOT_PERMITTED.equals(message)) {
+                throw new UnallowedStatementTypeGuardException(message);
+            }
+            throw new GuardException(message);
+        }
+    }
+
     private static JdbcMetaData calculateMetaData(String currentCatalogName, String currentSchemaName,
             DatabaseCatalog databaseCatalog) {
         JdbcMetaData jdbcMetaData = new JdbcMetaData(currentCatalogName, currentSchemaName);
diff --git a/guard/jsqltranspiler/src/test/java/org/eclipse/daanse/sql/guard/jsqltranspiler/integration/SqlGuardTest.java b/guard/jsqltranspiler/src/test/java/org/eclipse/daanse/sql/guard/jsqltranspiler/integration/SqlGuardTest.java
index aa9f51b..7f788dc 100644
--- a/guard/jsqltranspiler/src/test/java/org/eclipse/daanse/sql/guard/jsqltranspiler/integration/SqlGuardTest.java
+++ b/guard/jsqltranspiler/src/test/java/org/eclipse/daanse/sql/guard/jsqltranspiler/integration/SqlGuardTest.java
@@ -242,6 +242,33 @@ public class SqlGuardTest {
             WITH cte1 AS (SELECT foo.id FROM foo),
                  cte2 AS (SELECT fooFact.wrongCol FROM fooFact)
             SELECT * FROM cte1, cte2""";
+
+    private static final String SQL_CTE_UPDATE = """
+            WITH x AS (UPDATE foo SET name='pwned' WHERE id=1 RETURNING id) SELECT id FROM foo""";
+
+    private static final String SQL_CTE_UPDATE_ALL_ROWS = """
+            WITH x AS (UPDATE foo SET name='pwned') SELECT id FROM foo""";
+
+    private static final String SQL_MULTIPLE_CTE_UPDATE = """
+            WITH x AS (UPDATE foo SET name='a' RETURNING id),
+                 y AS (UPDATE foo SET name='b' RETURNING id)
+            SELECT id FROM foo""";
+
+    private static final String SQL_CTE_INSERT = """
+            WITH x AS (INSERT INTO foo (id, name) VALUES (1, 'a') RETURNING id) SELECT id FROM foo""";
+
+    private static final String SQL_CTE_DELETE = """
+            WITH x AS (DELETE FROM foo WHERE id=1 RETURNING id) SELECT id FROM foo""";
+
+    private static final String SQL_NESTED_SUBQUERY_CTE_UPDATE = """
+            SELECT foo.id FROM foo
+            WHERE foo.id IN (SELECT sub.id FROM (WITH y AS (UPDATE foo SET name='z' RETURNING id) SELECT id FROM foo) sub)""";
+
+    private static final String UPDATE_IS_NOT_PERMITTED = "UPDATE is not permitted.";
+
+    private static final String INSERT_IS_NOT_PERMITTED = "INSERT is not permitted.";
+
+    private static final String DELETE_IS_NOT_PERMITTED = "DELETE is not permitted.";
     private static final String SQL_LEFT_JOIN_ON_WRONG_COLUMN = """
             SELECT foo.id FROM foo LEFT JOIN fooFact ON foo.wrongCol = fooFact.id""";
 
@@ -993,6 +1020,75 @@ void testMultipleCteWrongColumn(@InjectService SqlGuardFactory sqlGuardFactory)
         }
     }
 
+    @Nested
+    @DisplayName("CTE Data-Modifying (DML) Security Tests")
+    class CteDmlSecurityTests {
+
+        // A data-modifying CTE parses as a Select and resolves against the whitelisted
+        // table, so it must be rejected by the read-only gate, not the column resolver.
+
+        @Test
+        void testCteUpdate(@InjectService SqlGuardFactory sqlGuardFactory) throws Exception {
+            DatabaseCatalog databaseCatalog = schemaWithTwoTableTwoCol();
+            SqlGuard guard = sqlGuardFactory.create("", SCH, databaseCatalog, List.of(), dialect);
+            assertThatThrownBy(() -> guard.guard(SQL_CTE_UPDATE))
+                    .isInstanceOf(GuardException.class)
+                    .hasMessage(UPDATE_IS_NOT_PERMITTED);
+        }
+
+        @Test
+        void testCteUpdateAllRows(@InjectService SqlGuardFactory sqlGuardFactory) throws Exception {
+            DatabaseCatalog databaseCatalog = schemaWithTwoTableTwoCol();
+            SqlGuard guard = sqlGuardFactory.create("", SCH, databaseCatalog, List.of(), dialect);
+            assertThatThrownBy(() -> guard.guard(SQL_CTE_UPDATE_ALL_ROWS))
+                    .isInstanceOf(GuardException.class)
+                    .hasMessage(UPDATE_IS_NOT_PERMITTED);
+        }
+
+        @Test
+        void testMultipleCteUpdate(@InjectService SqlGuardFactory sqlGuardFactory) throws Exception {
+            DatabaseCatalog databaseCatalog = schemaWithTwoTableTwoCol();
+            SqlGuard guard = sqlGuardFactory.create("", SCH, databaseCatalog, List.of(), dialect);
+            assertThatThrownBy(() -> guard.guard(SQL_MULTIPLE_CTE_UPDATE))
+                    .isInstanceOf(GuardException.class)
+                    .hasMessage(UPDATE_IS_NOT_PERMITTED);
+        }
+
+        @Test
+        void testCteInsert(@InjectService SqlGuardFactory sqlGuardFactory) throws Exception {
+            DatabaseCatalog databaseCatalog = schemaWithTwoTableTwoCol();
+            SqlGuard guard = sqlGuardFactory.create("", SCH, databaseCatalog, List.of(), dialect);
+            assertThatThrownBy(() -> guard.guard(SQL_CTE_INSERT))
+                    .isInstanceOf(GuardException.class)
+                    .hasMessage(INSERT_IS_NOT_PERMITTED);
+        }
+
+        @Test
+        void testCteDelete(@InjectService SqlGuardFactory sqlGuardFactory) throws Exception {
+            DatabaseCatalog databaseCatalog = schemaWithTwoTableTwoCol();
+            SqlGuard guard = sqlGuardFactory.create("", SCH, databaseCatalog, List.of(), dialect);
+            assertThatThrownBy(() -> guard.guard(SQL_CTE_DELETE))
+                    .isInstanceOf(GuardException.class)
+                    .hasMessage(DELETE_IS_NOT_PERMITTED);
+        }
+
+        @Test
+        void testNestedSubqueryCteUpdate(@InjectService SqlGuardFactory sqlGuardFactory) throws Exception {
+            DatabaseCatalog databaseCatalog = schemaWithTwoTableTwoCol();
+            SqlGuard guard = sqlGuardFactory.create("", SCH, databaseCatalog, List.of(), dialect);
+            assertThatThrownBy(() -> guard.guard(SQL_NESTED_SUBQUERY_CTE_UPDATE))
+                    .isInstanceOf(GuardException.class)
+                    .hasMessage(UPDATE_IS_NOT_PERMITTED);
+        }
+
+        @Test
+        void testValidCteStillPasses(@InjectService SqlGuardFactory sqlGuardFactory) throws Exception {
+            DatabaseCatalog databaseCatalog = schemaWithTwoTableTwoCol();
+            SqlGuard guard = sqlGuardFactory.create("", SCH, databaseCatalog, List.of(), dialect);
+            assertNotNull(guard.guard(SQL_VALID_CTE));
+        }
+    }
+
     @Nested
     @DisplayName("JOIN Security Tests")
     class JoinSecurityTests {