diff --git a/jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/FormFields.java b/jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/FormFields.java
index eaf96ccb5b87..07251165e885 100644
--- a/jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/FormFields.java
+++ b/jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/FormFields.java
@@ -32,8 +32,9 @@
 import static org.eclipse.jetty.util.UrlEncoded.decodeHexByte;
 
 /**
- * A {@link CompletableFuture} that is completed once a {@code application/x-www-form-urlencoded}
- * content has been parsed asynchronously from the {@link Content.Source}.
+ * <p>A {@link CompletableFuture} that is completed once a {@code application/x-www-form-urlencoded}
+ * content has been parsed asynchronously from the {@link Content.Source}.</p>
+ * <p><a href="https://url.spec.whatwg.org/#application/x-www-form-urlencoded">Specification</a>.</p>
  */
 public class FormFields extends ContentSourceCompletableFuture<Fields>
 {
@@ -209,98 +210,103 @@ private FormFields(Content.Source source, Charset charset, int maxFields, int ma
     @Override
     protected Fields parse(Content.Chunk chunk) throws CharacterCodingException
     {
-        String value = null;
         ByteBuffer buffer = chunk.getByteBuffer();
 
-        do
+        while (BufferUtil.hasContent(buffer))
         {
-            loop:
-            while (BufferUtil.hasContent(buffer))
+            byte b = buffer.get();
+            switch (_percent)
             {
-                byte b = buffer.get();
-                switch (_percent)
+                case 1 ->
                 {
-                    case 1 ->
-                    {
-                        _percentCode = b;
-                        _percent++;
-                        continue;
-                    }
-                    case 2 ->
-                    {
-                        _builder.append(decodeHexByte((char)_percentCode, (char)b));
-                        _percent = 0;
-                        continue;
-                    }
+                    _percentCode = b;
+                    _percent++;
+                    continue;
+                }
+                case 2 ->
+                {
+                    _percent = 0;
+                    _builder.append(decodeHexByte((char)_percentCode, (char)b));
+                    continue;
                 }
+            }
 
-                if (_name == null)
+            if (_name == null)
+            {
+                switch (b)
                 {
-                    switch (b)
+                    case '&' ->
                     {
-                        case '=' ->
-                        {
-                            _name = _builder.build();
-                            checkLength(_name);
-                        }
-                        case '+' -> _builder.append((byte)' ');
-                        case '%' -> _percent++;
-                        default -> _builder.append(b);
+                        String name = _builder.build();
+                        checkMaxLength(name);
+                        onNewField(name, "");
                     }
-                }
-                else
-                {
-                    switch (b)
+                    case '=' ->
                     {
-                        case '&' ->
-                        {
-                            value = _builder.build();
-                            checkLength(value);
-                            break loop;
-                        }
-                        case '+' -> _builder.append((byte)' ');
-                        case '%' -> _percent++;
-                        default -> _builder.append(b);
+                        _name = _builder.build();
+                        checkMaxLength(_name);
                     }
+                    case '+' -> _builder.append(' ');
+                    case '%' -> _percent++;
+                    default -> _builder.append(b);
                 }
             }
-
-            if (_name != null)
+            else
             {
-                if (value == null && chunk.isLast())
+                switch (b)
                 {
-                    if (_percent > 0)
+                    case '&' ->
                     {
-                        _builder.append((byte)'%');
-                        _builder.append(_percentCode);
+                        String value = _builder.build();
+                        checkMaxLength(value);
+                        onNewField(_name, value);
+                        _name = null;
                     }
-                    value = _builder.build();
-                    checkLength(value);
+                    case '+' -> _builder.append(' ');
+                    case '%' -> _percent++;
+                    default -> _builder.append(b);
                 }
+            }
+        }
 
-                if (value != null)
-                {
-                    Fields.Field field = new Fields.Field(_name, value);
-                    _name = null;
-                    value = null;
-                    if (_maxFields >= 0 && _fields.getSize() >= _maxFields)
-                        throw new IllegalStateException("form with too many fields > " + _maxFields);
-                    _fields.add(field);
-                }
+        if (!chunk.isLast())
+            return null;
+
+        // Append any remaining %x.
+        if (_percent > 0)
+            throw new IllegalStateException("invalid percent encoding");
+        String value = _builder.build();
+
+        if (_name == null)
+        {
+            if (!value.isEmpty())
+            {
+                checkMaxLength(value);
+                onNewField(value, "");
             }
+            return _fields;
         }
-        while (BufferUtil.hasContent(buffer));
 
-        return chunk.isLast() ? _fields : null;
+        checkMaxLength(value);
+        onNewField(_name, value);
+        return _fields;
     }
 
-    private void checkLength(String nameOrValue)
+    private void checkMaxLength(String nameOrValue)
     {
         if (_maxLength >= 0)
         {
             _length += nameOrValue.length();
             if (_length > _maxLength)
-                throw new IllegalStateException("form too large");
+                throw new IllegalStateException("form too large > " + _maxLength);
         }
     }
+
+    private void onNewField(String name, String value)
+    {
+        Fields.Field field = new Fields.Field(name, value);
+        _fields.add(field);
+        if (_maxFields >= 0 && _fields.getSize() > _maxFields)
+            throw new IllegalStateException("form with too many fields > " + _maxFields);
+    }
 }
diff --git a/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/FormFieldsTest.java b/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/FormFieldsTest.java
index cfd0b1ec650c..40b6ff53217b 100644
--- a/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/FormFieldsTest.java
+++ b/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/FormFieldsTest.java
@@ -13,11 +13,13 @@
 
 package org.eclipse.jetty.server;
 
+import java.nio.charset.CharacterCodingException;
 import java.nio.charset.Charset;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.TimeUnit;
 import java.util.stream.Stream;
 
@@ -32,31 +34,41 @@
 import org.junit.jupiter.params.provider.MethodSource;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.instanceOf;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 public class FormFieldsTest
 {
-    public static Stream<Arguments> tests()
+    public static Stream<Arguments> validData()
     {
         return Stream.of(
+            Arguments.of(List.of(""), UTF_8, -1, -1, Map.of()),
+            Arguments.of(List.of("name"), UTF_8, -1, -1, Map.of("name", "")),
+            Arguments.of(List.of("name&"), UTF_8, -1, -1, Map.of("name", "")),
+            Arguments.of(List.of("name="), UTF_8, -1, -1, Map.of("name", "")),
+            Arguments.of(List.of("name%00="), UTF_8, -1, -1, Map.of("name\u0000", "")),
+            Arguments.of(List.of("n1=v1&n2"), UTF_8, -1, -1, Map.of("n1", "v1", "n2", "")),
+            Arguments.of(List.of("n1=v1&n2&n3=v3&n4"), UTF_8, -1, -1, Map.of("n1", "v1", "n2", "", "n3", "v3", "n4", "")),
             Arguments.of(List.of("name=value"), UTF_8, -1, -1, Map.of("name", "value")),
+            Arguments.of(List.of("name=%0A"), UTF_8, -1, -1, Map.of("name", "\n")),
             Arguments.of(List.of("name=value", ""), UTF_8, -1, -1, Map.of("name", "value")),
             Arguments.of(List.of("name", "=value", ""), UTF_8, -1, -1, Map.of("name", "value")),
             Arguments.of(List.of("n", "ame", "=", "value"), UTF_8, -1, -1, Map.of("name", "value")),
             Arguments.of(List.of("n=v&X=Y"), UTF_8, 2, 4, Map.of("n", "v", "X", "Y")),
             Arguments.of(List.of("name=f¤¤&X=Y"), UTF_8, -1, -1, Map.of("name", "f¤¤", "X", "Y")),
-            Arguments.of(List.of("n=v&X=Y"), UTF_8, 1, -1, null),
-            Arguments.of(List.of("n=v&X=Y"), UTF_8, -1, 3, null)
+            Arguments.of(List.of("na+me=", "va", "+", "lue"), UTF_8, -1, -1, Map.of("na me", "va lue")),
+            Arguments.of(List.of("=v"), UTF_8, -1, -1, Map.of("", "v"))
         );
     }
 
     @ParameterizedTest
-    @MethodSource("tests")
-    public void testFormFields(List<String> chunks, Charset charset, int maxFields, int maxLength, Map<String, String> expected)
-        throws Exception
+    @MethodSource("validData")
+    public void testValidFormFields(List<String> chunks, Charset charset, int maxFields, int maxLength, Map<String, String> expected)
     {
         AsyncContent source = new AsyncContent();
         Attributes attributes = new Attributes.Mapped();
@@ -66,8 +78,9 @@ public void testFormFields(List<String> chunks, Charset charset, int maxFields,
         int last = chunks.size() - 1;
         FutureCallback eof = new FutureCallback();
         for (int i = 0; i <= last; i++)
+        {
             source.write(i == last, BufferUtil.toBuffer(chunks.get(i), charset), i == last ? eof : Callback.NOOP);
-
+        }
 
         try
         {
@@ -89,4 +102,45 @@ public void testFormFields(List<String> chunks, Charset charset, int maxFields,
             assertNull(expected);
         }
     }
+
+    public static Stream<Arguments> invalidData()
+    {
+        return Stream.of(
+            Arguments.of(List.of("%A"), UTF_8, -1, -1, IllegalStateException.class),
+            Arguments.of(List.of("name%"), UTF_8, -1, -1, IllegalStateException.class),
+            Arguments.of(List.of("name%A"), UTF_8, -1, -1, IllegalStateException.class),
+
+            // TODO: these 2 should throw the same exception.
+            Arguments.of(List.of("name%A="), UTF_8, -1, -1, CharacterCodingException.class),
+            Arguments.of(List.of("name%A&"), UTF_8, -1, -1, IllegalArgumentException.class),
+
+            Arguments.of(List.of("name=%"), UTF_8, -1, -1, IllegalStateException.class),
+            Arguments.of(List.of("name=A%%A"), UTF_8, -1, -1, IllegalArgumentException.class),
+            Arguments.of(List.of("name=A%%3D"), UTF_8, -1, -1, IllegalArgumentException.class),
+            Arguments.of(List.of("%="), UTF_8, -1, -1, IllegalStateException.class),
+            Arguments.of(List.of("name=%A"), UTF_8, -1, -1, IllegalStateException.class),
+            Arguments.of(List.of("name=value%A"), UTF_8, -1, -1, IllegalStateException.class),
+            Arguments.of(List.of("n=v&X=Y"), UTF_8, 1, -1, IllegalStateException.class),
+            Arguments.of(List.of("n=v&X=Y"), UTF_8, -1, 3, IllegalStateException.class),
+            Arguments.of(List.of("n%AH=v"), UTF_8, -1, -1, IllegalArgumentException.class),
+            Arguments.of(List.of("n=v%AH"), UTF_8, -1, -1, IllegalArgumentException.class),
+            Arguments.of(List.of("n=v%FF"), UTF_8, -1, -1, CharacterCodingException.class)
+        );
+    }
+
+    @ParameterizedTest
+    @MethodSource("invalidData")
+    public void testInvalidFormFields(List<String> chunks, Charset charset, int maxFields, int maxLength, Class<? extends Exception> expectedException)
+    {
+        AsyncContent source = new AsyncContent();
+        CompletableFuture<Fields> futureFields = FormFields.from(source, new Attributes.Mapped(), charset, maxFields, maxLength);
+        assertFalse(futureFields.isDone());
+        int last = chunks.size() - 1;
+        for (int i = 0; i <= last; i++)
+        {
+            source.write(i == last, BufferUtil.toBuffer(chunks.get(i)), Callback.NOOP);
+        }
+        Throwable cause = assertThrows(ExecutionException.class, futureFields::get).getCause();
+        assertThat(cause, instanceOf(expectedException));
+    }
 }
diff --git a/jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/CharsetStringBuilder.java b/jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/CharsetStringBuilder.java
index 8ac5b800caf8..0959e7406206 100644
--- a/jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/CharsetStringBuilder.java
+++ b/jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/CharsetStringBuilder.java
@@ -95,6 +95,14 @@ default void append(ByteBuffer buf)
      */
     String build() throws CharacterCodingException;
 
+    /**
+     * @return the length in characters
+     */
+    int length();
+
+    /**
+     * <p>Resets this sequence to be empty.</p>
+     */
     void reset();
 
     /**
@@ -168,6 +176,12 @@ public String build()
             return s;
         }
 
+        @Override
+        public int length()
+        {
+            return _builder.length();
+        }
+
         @Override
         public void reset()
         {
@@ -207,6 +221,12 @@ public String build()
             return s;
         }
 
+        @Override
+        public int length()
+        {
+            return _builder.length();
+        }
+
         @Override
         public void reset()
         {
@@ -319,6 +339,12 @@ public String build() throws CharacterCodingException
             }
         }
 
+        @Override
+        public int length()
+        {
+            return _stringBuilder.length();
+        }
+
         @Override
         public void reset()
         {
diff --git a/jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/Utf8StringBuilder.java b/jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/Utf8StringBuilder.java
index 3da2f322c1c7..77c17b673ffe 100644
--- a/jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/Utf8StringBuilder.java
+++ b/jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/Utf8StringBuilder.java
@@ -92,6 +92,7 @@ protected Utf8StringBuilder(StringBuilder buffer)
         _buffer = buffer;
     }
 
+    @Override
     public int length()
     {
         return _buffer.length();