png_get_uint_31(png_structp png_ptr, png_bytep buf)
{
#ifdef PNG_READ_BIG_ENDIAN_SUPPORTED
    png_uint_32 i = png_get_uint_32(buf);
 #else
    /* Avoid an extra function call by inlining the result. */
   png_uint_32 i = ((png_uint_32)((*(buf    )) & 0xff) << 24) +
                   ((png_uint_32)((*(buf + 1)) & 0xff) << 16) +
                   ((png_uint_32)((*(buf + 2)) & 0xff) <<  8) +
                   ((png_uint_32)((*(buf + 3)) & 0xff)      );
 #endif
    if (i > PNG_UINT_31_MAX)
      png_error(png_ptr, "PNG unsigned integer out of range.");
    return (i);
 }

The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.