diff -up libwebp-0.3.0/src/mux/muxread.c.rhbz-1956919 libwebp-0.3.0/src/mux/muxread.c
--- libwebp-0.3.0/src/mux/muxread.c.rhbz-1956919	2021-05-27 09:52:07.953914353 +0200
+++ libwebp-0.3.0/src/mux/muxread.c	2021-05-27 09:52:59.325863123 +0200
@@ -118,6 +118,7 @@ static int MuxImageParse(const WebPChunk
         wpi->is_partial_ = 1;  // Waiting for a VP8 chunk.
         break;
       case WEBP_CHUNK_IMAGE:
+        if (wpi->img_ != NULL) goto Fail;  // Only 1 image chunk allowed.
         if (ChunkSetNth(&subchunk, &wpi->img_, 1) != WEBP_MUX_OK) goto Fail;
         wpi->is_partial_ = 0;  // wpi is completely filled.
         break;