Code analysis

from aiohttp import web

async def index(request):
    return web.Response(body='''
        <header><h1>static file server</h1></header>
        Here are some files:
        <ul>
            <li><img src="/files/ductf.png"></img></li>
            <li><a href="/files/not_the_flag.txt">not the flag</a></li>
        </ul>
    ''', content_type='text/html', status=200)

app = web.Application()
app.add_routes([
    web.get('/', index),

    # this is handled by https://github.com/aio-libs/aiohttp/blob/v3.8.5/aiohttp/web_urldispatcher.py#L654-L690
    web.static('/files', './files', follow_symlinks=True)
])
web.run_app(app)

According to the code, the route /files gives access to static files. It maybe vulnerable to path traversal, let’s exploit it throught BurpSuite :