diff --git a/grub-core/fs/jfs.c b/grub-core/fs/jfs.c
index e2fe2850c..7a68fcbe3 100644
--- a/grub-core/fs/jfs.c
+++ b/grub-core/fs/jfs.c
@@ -46,6 +46,7 @@ GRUB_MOD_LICENSE ("GPLv3+");
  * https://jfs.sourceforge.net/project/pub/jfslayout.pdf
  */
 #define GRUB_JFS_INODE_INLINE_ENTRIES	8
+#define GRUB_JFS_DIR_MAX_SLOTS		128
 
 struct grub_jfs_sblock
 {
@@ -481,6 +482,14 @@ grub_jfs_opendir (struct grub_jfs_data *data, struct grub_jfs_inode *inode)
       return 0;
     }
 
+  if (inode->dir.header.sorted[0] >= GRUB_JFS_DIR_MAX_SLOTS)
+    {
+      grub_error (GRUB_ERR_BAD_FS, N_("invalid directory slot index"));
+      grub_free (diro->dirpage);
+      grub_free (diro);
+      return 0;
+    }
+
   blk = grub_le_to_cpu32 (de[inode->dir.header.sorted[0]].ex.blk2);
   blk <<= (grub_le_to_cpu16 (data->sblock.log2_blksz) - GRUB_DISK_SECTOR_BITS);