diff --git a/CHANGES.txt b/CHANGES.txt
index 2c6d1522c8..baa7eee95c 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -6,6 +6,7 @@
  * Equality check for Paxos.Electorate should not depend on collection types (CASSANDRA-19935)
  * Fix race condition in DecayingEstimatedHistogramReservoir during rescale (CASSANDRA-19365)
 Merged from 4.0:
+ * CBUtil serialization of UTF8 does not handle all UTF8 properly (CASSANDRA-20234)
  * Make hint expiry use request start time rather than timeout time for TTL (CASSANDRA-20014)
  * Do not attach rows and partitions to CheckForAbort when already attached (CASSANDRA-20135)
  * Allow hint delivery during schema mismatch (CASSANDRA-20188)
diff --git a/src/java/org/apache/cassandra/db/TypeSizes.java b/src/java/org/apache/cassandra/db/TypeSizes.java
index 8df83b6ba2..54ed3eeac8 100644
--- a/src/java/org/apache/cassandra/db/TypeSizes.java
+++ b/src/java/org/apache/cassandra/db/TypeSizes.java
@@ -45,6 +45,16 @@ public final class TypeSizes
         return sizeof((short) length) + length;
     }
 
+    /**
+     * Java uses a Modified UTF-8 (see {@link java.io.DataOutput#writeUTF(String)}), and this method attempts to
+     * calculate the modified utf-8 length; this method only works when the utf-8 writing logic is java's modified utf-8
+     * and will not work when normal utf-8 is written.
+     *
+     * If normal utf-8 is written, then {@link org.apache.cassandra.transport.CBUtil#encodedUTF8Length(String)} should be
+     * used instread of this one.
+     *
+     * @see <a href="https://docs.oracle.com/en/java/javase/23/docs/api/java.base/java/io/DataInput.html#modified-utf-8">Modified UTF 8</a>
+     */
     public static int encodedUTF8Length(String st)
     {
         int strlen = st.length();
diff --git a/src/java/org/apache/cassandra/transport/CBUtil.java b/src/java/org/apache/cassandra/transport/CBUtil.java
index 6cab63855e..dab6b49d9c 100644
--- a/src/java/org/apache/cassandra/transport/CBUtil.java
+++ b/src/java/org/apache/cassandra/transport/CBUtil.java
@@ -41,7 +41,6 @@ import io.netty.buffer.UnpooledByteBufAllocator;
 import io.netty.util.concurrent.FastThreadLocal;
 import org.apache.cassandra.config.Config;
 import org.apache.cassandra.db.ConsistencyLevel;
-import org.apache.cassandra.db.TypeSizes;
 import org.apache.cassandra.utils.ByteBufferUtil;
 import org.apache.cassandra.utils.Pair;
 import org.apache.cassandra.utils.TimeUUID;
@@ -150,14 +149,28 @@ public abstract class CBUtil
 
     public static void writeString(String str, ByteBuf cb)
     {
-        int length = TypeSizes.encodedUTF8Length(str);
+        int length = encodedUTF8Length(str);
         cb.writeShort(length);
         ByteBufUtil.reserveAndWriteUtf8(cb, str, length);
     }
 
     public static int sizeOfString(String str)
     {
-        return 2 + TypeSizes.encodedUTF8Length(str);
+        return 2 + encodedUTF8Length(str);
+    }
+
+    /**
+     * Java uses a Modified UTF-8, whereas Netty uses UTF-8 proper... this means that the encoded UTF8 lengths
+     * do not match, and you must be careful to use the correct length method...
+     *
+     * When using {@link ByteBufUtil#reserveAndWriteUtf8(ByteBuf, CharSequence, int)} or similiar logic, you must use
+     * this method.  When using {@link java.io.DataOutput#writeUTF(String)} you must use {@link org.apache.cassandra.db.TypeSizes#encodedUTF8Length(String)}.
+     *
+     * @see <a href="https://docs.oracle.com/en/java/javase/23/docs/api/java.base/java/io/DataInput.html#modified-utf-8">Modified UTF 8</a>
+     */
+    public static int encodedUTF8Length(String str)
+    {
+        return ByteBufUtil.utf8Bytes(str);
     }
 
     /**
@@ -185,14 +198,14 @@ public abstract class CBUtil
 
     public static void writeLongString(String str, ByteBuf cb)
     {
-        int length = TypeSizes.encodedUTF8Length(str);
+        int length = encodedUTF8Length(str);
         cb.writeInt(length);
         ByteBufUtil.reserveAndWriteUtf8(cb, str, length);
     }
 
     public static int sizeOfLongString(String str)
     {
-        return 4 + TypeSizes.encodedUTF8Length(str);
+        return 4 + encodedUTF8Length(str);
     }
 
     public static byte[] readBytes(ByteBuf cb)
diff --git a/test/unit/org/apache/cassandra/transport/CBUtilTest.java b/test/unit/org/apache/cassandra/transport/CBUtilTest.java
index c20efec8f8..9e0cd74b83 100644
--- a/test/unit/org/apache/cassandra/transport/CBUtilTest.java
+++ b/test/unit/org/apache/cassandra/transport/CBUtilTest.java
@@ -25,6 +25,10 @@ import org.junit.Test;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufAllocator;
 import io.netty.buffer.PooledByteBufAllocator;
+import org.assertj.core.api.Assertions;
+import org.quicktheories.generators.SourceDSL;
+
+import static org.quicktheories.QuickTheory.qt;
 
 public class CBUtilTest
 {
@@ -38,6 +42,21 @@ public class CBUtilTest
             buf.release(buf.refCnt());
     }
 
+    @Test
+    public void stringList()
+    {
+        qt().forAll(SourceDSL.lists().of(SourceDSL.strings().allPossible().ofLengthBetween(0, 20)).ofSizeBetween(0, 10)).checkAssert(list -> {
+            int expectedSize = CBUtil.sizeOfStringList(list);
+            ByteBuf body = CBUtil.allocator.buffer(expectedSize * 2);
+            CBUtil.writeStringList(list, body);
+            Assertions.assertThat(body.readableBytes()).isEqualTo(expectedSize);
+            // In CASSANDRA-20234 the sizeOf method now reflects the write method, but the work to do this in read
+            // was not done; read took a stance that it wants to limit things in CASSANDRA-8101 but that never made
+            // it to the other methods... write and read are not compatable with the full domain of UTF-8, and fixing
+            // that was higher bar than CASSANDRA-20234 wanted to tackle out of fear of unknown regressions it would cause.
+        });
+    }
+
     @Test
     public void writeAndReadString()
     {