We know that the website files live in
/usr/share/nginx/html/ and the flag is at /flag.txt
but the website is filtering absolute file paths. Can you get past the filter to read the flag?题目给出了绝对路径无法使用，那么
../../../../flag.txt 就可以访问了。
picoCTF{7h3_p47h_70_5ucc355_32e3a320}