Santa has setup a web service for you to receive your flag for today. Unfortunately, the flag doesn't seem to reach you.
For this challenge we are given a website that shows the text "Thanks for visiting the Flag service. Your Flag is: ". At that point the text cuts off. Lookin at the response in the network tab we can see something interesting:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=IBM+Plex+Mono&display=swap" rel="stylesheet">
<style>
body{font-family: 'IBM Plex Mono', monospace;height: 100vh !important;background-image: url("https://source.unsplash.com/random");-webkit-background-size: cover;-moz-background-size: cover;-o-background-size: cover;background-size: cover;background-color:#131627;color:#fff;overflow:hidden;}
::selection{background-color:rgba(0, 0, 0, 0);}
#flex-wrapper{position:absolute;top:0;bottom:0;right:0;left:0;-ms-flex-direction:row;-ms-flex-align:center;display:-webkit-flex;display:flex}#container{margin:auto; z-index: 10; padding:25px;}#container *{margin:0}h1{text-align:center;font-size:60px;color:#131627;text-shadow:0 0 5px #fff;opacity:0;-webkit-animation:fade-in 3s ease-in 0s forwards;-moz-animation:fade-in 3s ease-in 0s forwards;-o-animation:fade-in 3s ease-in 0s forwards;animation:fade-in 3s ease-in 0s forwards}h2{font-size:50px;text-shadow:0 0 5px orange;text-align:center;opacity:0;-webkit-animation:fade-in 3s ease-in .5s forwards;-moz-animation:fade-in 3s ease-in .5s forwards;-o-animation:fade-in 3s ease-in .5s forwards;animation:fade-in 3s ease-in .5s forwards}@-webkit-keyframes fade-in{from{opacity:0}to{opacity:1}}@-moz-keyframes fade-in{from{opacity:0}to{opacity:1}}@-o-keyframes fade-in{from{opacity:0}to{opacity:1}}@keyframes fade-in{from{opacity:0}to{opacity:1}}
</style>
<title>Flag Service</title>
</head>
<body>
<div id="flex-wrapper">
<div id="container">
<h1>Thanks for using the Flag service.<br/> Your Flag is:</h1>
<h2>The tags aren't closed which means that part of the response is being cut off. After trying around with curl a bit I
found the option --ignore-content-length. This prevents the response to stop once the length of the Content-Lenght
response header has been reached. Using said option, we get the response:
<div id="container">
<h1>Thanks for using the Flag service.<br/> Your Flag is:</h1>
<h2>HV21{4lw4y5_c0un7_y0ur53lf_d0n7_7ru57_7h3_53rv3r}</h2>
</div>And the flag is HV21{4lw4y5_c0un7_y0ur53lf_d0n7_7ru57_7h3_53rv3r}.