diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index b0cad04de19..0cab7fbf7aa 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -236,6 +236,8 @@ Bug Fixes
 * GITHUB#12555: Fix bug in TermsEnum#seekCeil on doc values terms enums
   that causes IndexOutOfBoundsException. (Egor Potemkin)
 
+* GITHUB#12571: Fix HNSW graph read bug when built with excessive connections. (Ben Trent).
+
 Other
 ---------------------
 
diff --git a/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsReader.java b/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsReader.java
index a97ce43f896..b16f874e5c3 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsReader.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsReader.java
@@ -40,6 +40,7 @@ import org.apache.lucene.store.DataInput;
 import org.apache.lucene.store.IndexInput;
 import org.apache.lucene.store.RandomAccessInput;
 import org.apache.lucene.util.Accountable;
+import org.apache.lucene.util.ArrayUtil;
 import org.apache.lucene.util.Bits;
 import org.apache.lucene.util.IOUtils;
 import org.apache.lucene.util.RamUsageEstimator;
@@ -457,7 +458,7 @@ public final class Lucene95HnswVectorsReader extends KnnVectorsReader {
     private final DirectMonotonicReader graphLevelNodeOffsets;
     private final long[] graphLevelNodeIndexOffsets;
     // Allocated to be M*2 to track the current neighbors being explored
-    private final int[] currentNeighborsBuffer;
+    private int[] currentNeighborsBuffer;
 
     OffHeapHnswGraph(FieldEntry entry, IndexInput vectorIndex) throws IOException {
       this.dataIn =
@@ -491,6 +492,9 @@ public final class Lucene95HnswVectorsReader extends KnnVectorsReader {
       dataIn.seek(graphLevelNodeOffsets.get(targetIndex + graphLevelNodeIndexOffsets[level]));
       arcCount = dataIn.readVInt();
       if (arcCount > 0) {
+        if (arcCount > currentNeighborsBuffer.length) {
+          currentNeighborsBuffer = ArrayUtil.grow(currentNeighborsBuffer, arcCount);
+        }
         currentNeighborsBuffer[0] = dataIn.readVInt();
         for (int i = 1; i < arcCount; i++) {
           currentNeighborsBuffer[i] = currentNeighborsBuffer[i - 1] + dataIn.readVInt();