fix: blob sidecar validation to ensure KZG commitment count matches

CHANGELOG.md

@@ -32,6 +32,7 @@ The format is based on Keep a Changelog, and this project adheres to Semantic Ve
 ### Fixed
 
 - Added check to prevent nil pointer deference or out of bounds array access when validating the BLSToExecutionChange on an impossibly nil validator.
+* [Beacon Chain] Fixed blob sidecar validation to ensure the number of available sidecars matches KZG commitment count
 
 ### Security
 

beacon-chain/sync/rpc_blob_sidecars_by_range.go

@@ -32,6 +32,26 @@ func (s *Service) streamBlobBatch(ctx context.Context, batch blockBatch, wQuota
 			s.writeErrorResponseToStream(responseCodeServerError, p2ptypes.ErrGeneric.Error(), stream)
 			return wQuota, errors.Wrapf(err, "could not retrieve sidecars for block root %#x", root)
 		}
+
+		// Get the number of KZG commitments in the block
+		kzgCommitments := len(b.Block().Body().BlobKzgCommitments())
+
+		// Count available blob sidecars
+		availableSidecars := 0
+		for _, hasIndex := range idxs {
+			if hasIndex {
+				availableSidecars++
+			}
+		}
+
+		// Check if we have all required blob sidecars
+		if kzgCommitments > 0 && availableSidecars < kzgCommitments {
+			s.writeErrorResponseToStream(responseCodeServerError, errMissingBlobsForBlockCommitments.Error(), stream)
+			return wQuota, errors.Wrapf(errMissingBlobsForBlockCommitments, 
+				"block root %#x has %d KZG commitments but only %d available sidecars", 
+				root, kzgCommitments, availableSidecars)
+		}
+
 		for i, l := uint64(0), uint64(len(idxs)); i < l; i++ {
 			// index not available, skip
 			if !idxs[i] {

beacon-chain/sync/rpc_blob_sidecars_by_range_test.go

@@ -147,6 +147,24 @@ func TestBlobByRangeOK(t *testing.T) {
 			},
 			total: func() *int { x := int(params.BeaconConfig().MaxRequestBlobSidecars); return &x }(),
 		},
+		{
+			name: "missing blob sidecars for block with KZG commitments",
+			nblocks: 1,
+			requestFromSidecars: func(scs []blocks.ROBlob) interface{} {
+				return &ethpb.BlobSidecarsByRangeRequest{
+					StartSlot: scs[0].Slot(),
+					Count:     1,
+				}
+			},
+			defineExpected: func(t *testing.T, scs []blocks.ROBlob, req interface{}) []*expectedBlobChunk {
+				return []*expectedBlobChunk{
+					{
+						code:    responseCodeServerError,
+						message: errMissingBlobsForBlockCommitments.Error(),
+					},
+				}
+			},
+		},
 	}
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {