diff --git a/java/change-notes/2021-03-18-commons-tostring-builder.md b/java/change-notes/2021-03-18-commons-tostring-builder.md
new file mode 100644
index 000000000000..41ccfb95237b
--- /dev/null
+++ b/java/change-notes/2021-03-18-commons-tostring-builder.md
@@ -0,0 +1,2 @@
+lgtm,codescanning
+* Added models for Apache Commons Lang's `ToStringBuilder` class. This may lead to more results from any data-flow query where ToStringBuilder operations fall between the relevant untrusted source and vulnerable sink.
diff --git a/java/ql/src/semmle/code/java/frameworks/apache/Lang.qll b/java/ql/src/semmle/code/java/frameworks/apache/Lang.qll
index 670014e0f8fb..516b3c5588ed 100644
--- a/java/ql/src/semmle/code/java/frameworks/apache/Lang.qll
+++ b/java/ql/src/semmle/code/java/frameworks/apache/Lang.qll
@@ -637,3 +637,29 @@ private class ApacheObjectUtilsModel extends SummaryModelCsv {
       ]
   }
 }
+
+private class ApacheToStringBuilderModel extends SummaryModelCsv {
+  override predicate row(string row) {
+    row =
+      [
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;toString;;;Argument[-1];ReturnValue;taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;append;(java.lang.Object);;Argument[0];Argument[-1];taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;append;(java.lang.Object[]);;ArrayElement of Argument[0];Argument[-1];taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;append;(java.lang.String,java.lang.Object[]);;Argument[0];Argument[-1];taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;append;(java.lang.String,java.lang.Object[]);;ArrayElement of Argument[1];Argument[-1];taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;append;(java.lang.String,boolean);;Argument[0];Argument[-1];taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;append;(java.lang.String,java.lang.Object);;Argument[0..1];Argument[-1];taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;append;(java.lang.String,java.lang.Object[],boolean);;Argument[0];Argument[-1];taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;append;(java.lang.String,java.lang.Object[],boolean);;ArrayElement of Argument[1];Argument[-1];taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;build;;;Argument[-1];ReturnValue;taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;getStringBuffer;;;Argument[-1];ReturnValue;taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;appendToString;;;Argument[0];Argument[-1];taint",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;appendSuper;;;Argument[0];Argument[-1];taint",
+        // The following are value-preserving steps for fluent methods:
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;append;;;Argument[-1];ReturnValue;value",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;appendAsObjectToString;;;Argument[-1];ReturnValue;value",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;appendSuper;;;Argument[-1];ReturnValue;value",
+        "org.apache.commons.lang3.builder;ToStringBuilder;false;appendToString;;;Argument[-1];ReturnValue;value"
+      ]
+  }
+}
diff --git a/java/ql/test/library-tests/frameworks/apache-commons-lang3/ToStringBuilderTest.java b/java/ql/test/library-tests/frameworks/apache-commons-lang3/ToStringBuilderTest.java
new file mode 100644
index 000000000000..ed2e4400dd7c
--- /dev/null
+++ b/java/ql/test/library-tests/frameworks/apache-commons-lang3/ToStringBuilderTest.java
@@ -0,0 +1,37 @@
+import org.apache.commons.lang3.builder.ToStringBuilder;
+
+class ToStringBuilderTest {
+    String taint() { return "tainted"; }
+
+    void sink(Object o) {}
+
+    void test() throws Exception {
+
+        ToStringBuilder sb1 = new ToStringBuilder(null); sb1.append((Object)taint()); sink(sb1.toString()); // $hasTaintFlow
+        ToStringBuilder sb2 = new ToStringBuilder(null); sb2.append(new Object[] { taint() }); sink(sb2.toString()); // $hasTaintFlow
+        ToStringBuilder sb3 = new ToStringBuilder(null); sb3.append(taint(), true); sink(sb3.toString()); // $hasTaintFlow
+        ToStringBuilder sb4 = new ToStringBuilder(null); sb4.append("fieldname", taint()); sink(sb4.toString()); // $hasTaintFlow
+        ToStringBuilder sb5 = new ToStringBuilder(null); sb5.append("fieldname", new Object[] { taint() }); sink(sb5.toString()); // $hasTaintFlow
+        ToStringBuilder sb6 = new ToStringBuilder(null); sb6.append("fieldname", new Object[] { taint() }, true); sink(sb6.toString()); // $hasTaintFlow
+        // GOOD: this appends an Object using the Object.toString style, which does not expose fields or String content.
+        ToStringBuilder sb7 = new ToStringBuilder(null); sb7.appendAsObjectToString(taint()); sink(sb7.toString());
+        ToStringBuilder sb8 = new ToStringBuilder(null); sb8.appendSuper(taint()); sink(sb8.toString()); // $hasTaintFlow
+        ToStringBuilder sb9 = new ToStringBuilder(null); sb9.appendToString(taint()); sink(sb9.toString()); // $hasTaintFlow
+        ToStringBuilder sb10 = new ToStringBuilder(null); sb10.append((Object)taint()); sink(sb10.build()); // $hasTaintFlow
+        ToStringBuilder sb11 = new ToStringBuilder(null); sb11.append((Object)taint()); sink(sb11.getStringBuffer().toString()); // $hasTaintFlow
+
+        // Test fluent methods:
+        ToStringBuilder fluentTest = new ToStringBuilder(null);
+        sink(fluentTest.append("Harmless").append(taint()).append("Also harmless").toString()); // $hasTaintFlow
+
+        ToStringBuilder fluentBackflowTest = new ToStringBuilder(null);
+        fluentBackflowTest.append("Harmless").append(taint()).append("Also harmless");
+        sink(fluentBackflowTest.toString()); // $hasTaintFlow
+
+        // Test the case where the fluent method contributing taint is at the end of a statement:
+        ToStringBuilder fluentBackflowTest2 = new ToStringBuilder(null);
+        fluentBackflowTest2.append("Harmless").append(taint());
+        sink(fluentBackflowTest2.toString()); // $hasTaintFlow
+
+    }
+}
\ No newline at end of file
diff --git a/java/ql/test/stubs/apache-commons-lang3-3.7/org/apache/commons/lang3/builder/ToStringBuilder.java b/java/ql/test/stubs/apache-commons-lang3-3.7/org/apache/commons/lang3/builder/ToStringBuilder.java
new file mode 100644
index 000000000000..886d59637f03
--- /dev/null
+++ b/java/ql/test/stubs/apache-commons-lang3-3.7/org/apache/commons/lang3/builder/ToStringBuilder.java
@@ -0,0 +1,275 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.lang3.builder;
+
+
+public class ToStringBuilder implements Builder<String> {
+    public static ToStringStyle getDefaultStyle() {
+      return null;
+    }
+
+    public static void setDefaultStyle(final ToStringStyle style) {
+    }
+
+    public static String reflectionToString(final Object object) {
+      return null;
+    }
+
+    public static String reflectionToString(final Object object, final ToStringStyle style) {
+      return null;
+    }
+
+    public static String reflectionToString(final Object object, final ToStringStyle style, final boolean outputTransients) {
+      return null;
+    }
+
+    public static <T> String reflectionToString(
+        final T object,
+        final ToStringStyle style,
+        final boolean outputTransients,
+        final Class<? super T> reflectUpToClass) {
+      return null;
+    }
+
+    public ToStringBuilder(final Object object) {
+    }
+
+    public ToStringBuilder(final Object object, final ToStringStyle style) {
+    }
+
+    public ToStringBuilder(final Object object, ToStringStyle style, StringBuffer buffer) {
+    }
+
+    public ToStringBuilder append(final boolean value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final boolean[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final byte value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final byte[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final char value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final char[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final double value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final double[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final float value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final float[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final int value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final int[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final long value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final long[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final Object obj) {
+      return null;
+    }
+
+    public ToStringBuilder append(final Object[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final short value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final short[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final boolean value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final boolean[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final boolean[] array, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final byte value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final byte[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final byte[] array, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final char value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final char[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final char[] array, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final double value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final double[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final double[] array, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final float value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final float[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final float[] array, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final int value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final int[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final int[] array, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final long value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final long[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final long[] array, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final Object obj) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final Object obj, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final Object[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final Object[] array, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final short value) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final short[] array) {
+      return null;
+    }
+
+    public ToStringBuilder append(final String fieldName, final short[] array, final boolean fullDetail) {
+      return null;
+    }
+
+    public ToStringBuilder appendAsObjectToString(final Object srcObject) {
+      return null;
+    }
+
+    public ToStringBuilder appendSuper(final String superToString) {
+      return null;
+    }
+
+    public ToStringBuilder appendToString(final String toString) {
+      return null;
+    }
+
+    public Object getObject() {
+      return null;
+    }
+
+    public StringBuffer getStringBuffer() {
+      return null;
+    }
+
+    public ToStringStyle getStyle() {
+      return null;
+    }
+
+    @Override
+    public String toString() {
+      return null;
+    }
+
+    @Override
+    public String build() {
+      return null;
+    }
+
+}
diff --git a/java/ql/test/stubs/apache-commons-lang3-3.7/org/apache/commons/lang3/builder/ToStringStyle.java b/java/ql/test/stubs/apache-commons-lang3-3.7/org/apache/commons/lang3/builder/ToStringStyle.java
new file mode 100644
index 000000000000..eef4db088044
--- /dev/null
+++ b/java/ql/test/stubs/apache-commons-lang3-3.7/org/apache/commons/lang3/builder/ToStringStyle.java
@@ -0,0 +1,102 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.lang3.builder;
+
+import java.io.Serializable;
+
+public abstract class ToStringStyle implements Serializable {
+    public static final ToStringStyle DEFAULT_STYLE = null;
+
+    public static final ToStringStyle MULTI_LINE_STYLE = null;
+
+    public static final ToStringStyle NO_FIELD_NAMES_STYLE = null;
+
+    public static final ToStringStyle SHORT_PREFIX_STYLE = null;
+
+    public static final ToStringStyle SIMPLE_STYLE = null;
+
+    public static final ToStringStyle NO_CLASS_NAME_STYLE = null;
+
+    public static final ToStringStyle JSON_STYLE = null;
+
+    public void appendSuper(final StringBuffer buffer, final String superToString) {
+    }
+
+    public void appendToString(final StringBuffer buffer, final String toString) {
+    }
+
+    public void appendStart(final StringBuffer buffer, final Object object) {
+    }
+
+    public void appendEnd(final StringBuffer buffer, final Object object) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final Object value, final Boolean fullDetail) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final long value) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final int value) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final short value) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final byte value) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final char value) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final double value) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final float value) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final boolean value) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final Object[] array, final Boolean fullDetail) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final long[] array, final Boolean fullDetail) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final int[] array, final Boolean fullDetail) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final short[] array, final Boolean fullDetail) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final byte[] array, final Boolean fullDetail) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final char[] array, final Boolean fullDetail) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final double[] array, final Boolean fullDetail) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final float[] array, final Boolean fullDetail) {
+    }
+
+    public void append(final StringBuffer buffer, final String fieldName, final boolean[] array, final Boolean fullDetail) {
+    }
+
+}