diff --git a/libavcodec/cbs_vp9.c b/libavcodec/cbs_vp9.c
index 42e4dcf5ac..98730e03e3 100644
--- a/libavcodec/cbs_vp9.c
+++ b/libavcodec/cbs_vp9.c
@@ -428,6 +428,9 @@ static int cbs_vp9_split_fragment(CodedBitstreamContext *ctx,
         index_size = 2 + (((superframe_header & 0x18) >> 3) + 1) *
                           ((superframe_header & 0x07) + 1);
 
+        if (index_size > frag->data_size)
+            return AVERROR_INVALIDDATA;
+
         err = init_get_bits(&gbc, frag->data + frag->data_size - index_size,
                             8 * index_size);
         if (err < 0)