msfrognymize

Overview
Background
Enumeration
Exploitation
Conclusion

Overview

Contributor: @siunam
Solved by: @flocto
64 solves / 147 points
Author: jazzpizazz
Overall difficulty for me (From 1-10 stars): ★★★☆☆☆☆☆☆☆

Background

At CoR we care greatly about privacy (especially FizzBuzz). For this reason we anonymize any selfies before sharing them on Discord. We even encrypt the metadata using a special key!

msfrognymize.be.ax

Enumeration

Home page:

In the index page (/), we can upload some images.

Let’s try to upload one:

Our uploaded image’s faces has been anonymized by frogs to a certain degrees.

Burp Suite HTTP history:

When we uploaded an image, it’ll send a POST request to / with name=file, filename, Content-Type: image/jpeg, and the raw bytes of the image.

Once the processing is finished, it’ll redirect us to /anonymized/<UUIDv4>.png.

In this challenge, we can download a file:

┌[siunam♥Mercury]-(~/ctf/corCTF-2023/web/msfrognymize)-[2023.07.31|14:04:44(HKT)]
└> file msfrognymize.tar.gz 
msfrognymize.tar.gz: gzip compressed data, from Unix, original size modulo 2^32 101867520
┌[siunam♥Mercury]-(~/ctf/corCTF-2023/web/msfrognymize)-[2023.07.31|14:04:46(HKT)]
└> tar xf msfrognymize.tar.gz                 
┌[siunam♥Mercury]-(~/ctf/corCTF-2023/web/msfrognymize)-[2023.07.31|14:04:54(HKT)]
└> ls -lah msfrognymize
total 88K
drwxr-xr-x 7 siunam nam 4.0K Jul 26 17:15 .
drwxr-xr-x 3 siunam nam 4.0K Jul 31 14:04 ..
-rw-r--r-- 1 siunam nam 2.6K Jul 26 17:15 app.py
-rw-r--r-- 1 siunam nam  479 Jul 26 17:15 celery_config.py
drwxr-xr-x 2 siunam nam 4.0K Jul 26 17:15 data
-rw-r--r-- 1 siunam nam  485 Jul 26 17:15 Dockerfile
-rw-r--r-- 1 siunam nam   18 Jul 26 17:15 flag.txt
-rw-r--r-- 1 siunam nam  315 Jul 26 17:15 Pipfile
-rw-r--r-- 1 siunam nam  32K Jul 26 17:15 Pipfile.lock
drwxr-xr-x 2 siunam nam 4.0K Jul 26 17:15 src
drwxr-xr-x 3 siunam nam 4.0K Jul 26 17:15 static
-rw-r--r-- 1 siunam nam  772 Jul 26 17:15 supervisord.conf
-rw-r--r-- 1 siunam nam  657 Jul 26 17:15 tasks.py
drwxr-xr-x 2 siunam nam 4.0K Jul 26 17:15 templates
drwxr-xr-x 2 siunam nam 4.0K Jul 26 17:15 uploads

Dockerfile:

FROM python:3.9

RUN apt-get update && apt-get install -y --no-install-recommends \
    libgl1-mesa-glx \
    redis-server \
    supervisor \
    && rm -rf /var/lib/apt/lists/*

WORKDIR /app

COPY Pipfile Pipfile.lock /app/

RUN pip install pipenv && \
    pipenv install --system --deploy --ignore-pipfile

COPY . /app

RUN mv flag.txt /

EXPOSE 4444

COPY supervisord.conf /etc/supervisor/conf.d/supervisord.conf

CMD ["supervisord", "-c", "/etc/supervisor/conf.d/supervisord.conf"]

The flag file is in /flag.txt.

After fumbling around, the app.py is the main web application source code.

Route /:

@app.route('/', methods=['GET', 'POST'])
def upload_file():
    if request.method == 'POST':
        file = request.files['file']
        if file:
            try:
                img = Image.open(file)
                if img.format != "JPEG":
                    return "Please upload a valid JPEG image.", 400

                exif_data = img._getexif()
                encrypted_exif = None
                if exif_data:
                    encrypted_exif = piexif.dump(encrypt_exif_data(exif_data))
                filename = secure_filename(file.filename)
                temp_path = os.path.join(tempfile.gettempdir(), filename)
                img.save(temp_path)

                unique_id = str(uuid.uuid4())
                new_file_path = os.path.join(UPLOAD_FOLDER, f"{unique_id}.png")
                process_image.apply_async(args=[temp_path, new_file_path, encrypted_exif])

                return render_template("processing.html", image_url=f"/anonymized/{unique_id}.png")

            except Exception as e:
                return f"Error: {e}", 400

    return render_template("index.html")

As you can see, when POST request is sent:

It’ll first check the image format is JPEG or not
Retrieve all the Exif (Exchangeable image file format) data and encrypt it
Then save the image to a temporary path, with secure_filename() to prevent path traversal
Finally anonymize the image’s faces

After reading this route’s code, it seems like it’s not possible to upload arbitrary files and overwrite some files.

Route /anonymized/<image_file>:

from urllib.parse import unquote
[...]
UPLOAD_FOLDER = 'uploads/'
[...]
@app.route('/anonymized/<image_file>')
def serve_image(image_file):
    file_path = os.path.join(UPLOAD_FOLDER, unquote(image_file))
    if ".." in file_path or not os.path.exists(file_path):
        return f"Image {file_path} cannot be found.", 404
    return send_file(file_path, mimetype='image/png')

When a GET request is sent to /anonymized/<image_file>, it’ll:

URL decode the image filename (image_file), and join the path UPLOAD_FOLDER, which will then become uploads/<image_file>
Check if .. and the file path (uploads/<image_file>) exist or not
Finally, send the image to the client with MIME type (Media type) image/png

Hmm… Looks like route /anonymized/<image_file> is vulnerable to path traversal and Local File Inclusion (LFI)?

Exploitation

But first, we need to bypass the .. filter.

According to urllib.parse library’s documentation, unquote() URL decode only one layer.

That being said, we can bypass the .. filter by double URL encoding!!

According to os.path.join() documentation:

“If a segment is an absolute path (which on Windows requires both a drive and a root), then all previous segments are ignored and joining continues from the absolute path segment.”

┌[siunam♥Mercury]-(~/ctf/corCTF-2023/web/msfrognymize)-[2023.07.31|17:00:45(HKT)]
└> python3
[...]
>>> import os
>>> os.path.join('uploads/', '/flag.txt')
'/flag.txt'
>>> os.path.join('uploads/', 'flag.txt')
'uploads/flag.txt'

That being said, we can double URL encode / (%252F): (From CyberChef)

%252F -> %2F -> /

Hence, we can use %252Fflag.txt to get the flag:

Flag: corctf{Fr0m_Priv4cy_t0_LFI}

Conclusion

What we’ve learned:

Local File Inclusion (LFI) & Filter Bypass Via Double URL Encoding

Siunam's Website