diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index fda2bf447c4..88eb798fb98 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -157,6 +157,14 @@ Other
 
 * LUCENE-10635: Ensure test coverage for WANDScorer by using a test query. (Zach Chen, Adrien Grand)
 
+======================== Lucene 9.4.1 =======================
+
+Bug Fixes
+---------------------
+* GITHUB#11858: Fix kNN vectors format validation on large segments. This
+ addresses a regression in 9.4.0 where validation could fail, preventing
+ further writes or searches on the index. (Julie Tibshirani)
+
 ======================== Lucene 9.4.0 =======================
 
 API Changes
diff --git a/lucene/core/src/java/org/apache/lucene/codecs/lucene94/Lucene94HnswVectorsReader.java b/lucene/core/src/java/org/apache/lucene/codecs/lucene94/Lucene94HnswVectorsReader.java
index 65af461786e..cef594e9be6 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/lucene94/Lucene94HnswVectorsReader.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/lucene94/Lucene94HnswVectorsReader.java
@@ -175,7 +175,8 @@ public final class Lucene94HnswVectorsReader extends KnnVectorsReader {
           case BYTE -> Byte.BYTES;
           case FLOAT32 -> Float.BYTES;
         };
-    int numBytes = fieldEntry.size * dimension * byteSize;
+    long vectorBytes = Math.multiplyExact((long) dimension, byteSize);
+    long numBytes = Math.multiplyExact(vectorBytes, fieldEntry.size);
     if (numBytes != fieldEntry.vectorDataLength) {
       throw new IllegalStateException(
           "Vector data length "