diff --git a/libavcodec/cbs_h264_syntax_template.c b/libavcodec/cbs_h264_syntax_template.c
index 106da0630e..1c8d7d5eae 100644
--- a/libavcodec/cbs_h264_syntax_template.c
+++ b/libavcodec/cbs_h264_syntax_template.c
@@ -725,7 +725,7 @@ static int FUNC(sei_user_data_registered)(CodedBitstreamContext *ctx, RWContext
     *payload_size = i + current->data_length;
 #endif
 
-    allocate(current->data, current->data_length);
+    allocate(current->data, current->data_length + AV_INPUT_BUFFER_PADDING_SIZE);
     for (j = 0; j < current->data_length; j++)
         xu(8, itu_t_t35_payload_byte[i], current->data[j], 0x00, 0xff, 1, i + j);
 
diff --git a/libavcodec/cbs_mpeg2_syntax_template.c b/libavcodec/cbs_mpeg2_syntax_template.c
index 8b5d35437e..88cf453b17 100644
--- a/libavcodec/cbs_mpeg2_syntax_template.c
+++ b/libavcodec/cbs_mpeg2_syntax_template.c
@@ -71,7 +71,7 @@ static int FUNC(user_data)(CodedBitstreamContext *ctx, RWContext *rw,
     av_assert0(k % 8 == 0);
     current->user_data_length = k /= 8;
     if (k > 0) {
-        current->user_data_ref = av_buffer_alloc(k);
+        current->user_data_ref = av_buffer_allocz(k + AV_INPUT_BUFFER_PADDING_SIZE);
         if (!current->user_data_ref)
             return AVERROR(ENOMEM);
         current->user_data = current->user_data_ref->data;