diff --git a/libavformat/mov.c b/libavformat/mov.c
index f2a66b8a95be9eb182d39d02dda98173bd4551f7..a157d603309ded599fb5ce50155b121e043627d5 100644
--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -3471,7 +3471,7 @@ static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom)
                 }
             }
             total_size += 8;
-            if (a.size == 1) { /* 64 bit extended size */
+            if (a.size == 1 && total_size + 8 <= atom.size) { /* 64 bit extended size */
                 a.size = avio_rb64(pb) - 8;
                 total_size += 8;
             }