This is some research I developed for OnSecurity based around Jinja2 Server Side Template Injections. Monday 15 February 2021 (2021-02-15) writeups. Now the job is to identify which SSTI engine .Using the payload. In your application, you will use templates to render HTML which will display in the user’s browser. Doctor was about attacking a message board-like website. During an attack-defense CTF, like the iCTF, there is usually not much time to think about in-depth fixes when other teams actively exploit your sevice. Decoded the string and found source code of a html page within which flag was hidden. Example 17 from Flask-VueJs-Template. *CTF (StarCTF) 2021 Happy New Year, everyone! TASK 6: Flask Injection. Doctor is an easy difficulty rated Linux machine from Hack the Box. Use whitelists! ... HTTP Methods and Template Rendering; 2.4. As usual, https://247ctf.com delivers excellent challenges. Template injection is injecting user inputs unsafely inside the template being used by the website. Monday 22 February 2021 (2021-02-22) writeups. Github. Replacing request[request.args.param] with |attr(request.args.param) will bypass both checks. After the CTF was over and I published the writeup, @busbauen asked if I could bypass his __ filter: @0daywork could you bypass our fix: filtering __ out? by Elber "f0lds" Tavares. The previously discussed join-function handles both lists in the same way. As security professionals, we are in the business of helping organizations make risk-based decisions. If you’re unfamiliar check out the whitepaper (PDF) by James Kettle. In our case, if the signature step passes, the application tries to deserialize the provided user cookie; that’s our injection point. In Flask, Jinja is configured to autoescape any data that is rendered in HTML templates. We can use this awesome github tool jwt-tool to bruteforce the pass key used for signing the token. Indeed, it’s a follow up from the previous challenge. Next we discover the user has privileges to read logs, where we find a password sent over password reset url, resulting in gaining access to next user. In our case we define the name using the l parameter and the content of the list with several a parameters. Server-Side Template Injection is possible when an attacker injects template directive as user input that can execute arbitrary code on the server. The core issue was that a template was rendered twice and the first rendering step contained user-controlled input. The 2015 Black Hat talk from James Kettle established the foundations for the exploitation techniques in multiple template engines. A look into JinJa's API comes to our rescue with a functiona called |join that will concatenate a list of strings. Doctor starts off with attacking a health service message board website where we discover two vulnerabilities, Server-side Template injection and Command injection both of which leads to initial foothold on the box. A script to generate flask sessions and exploit a server side template injection - ASIS CTF 2017 - flask-custom-sessions.py I’ll find two vulnerabilities in the site, Server-Side Template injection and command injection. Asis CTF Quals 2019 - Fort Knox. Byte Bandits CTF 2018 - hard_to_hack (400) This was another Jinja2 template injection challenge (they've been showing up a lot recently). 3. In our case the quick'n'dirty fix implemented by a colleague for this particular attack was to block requests that contain [. Now since I had to keep characters minimum and I know the flag file will contain ‘{‘ command “grep -r {“ worked for me and printed the flag. In conclusion, I massively enjoyed the challenge. This weekend, apart from participating to CodeGate 2020 CTF Qualifier (and hopefully qualifying in the finals), I had the pleasure of playing FooBarCTF 2020, an interesting competition held by students from NIT Durgapur, India. In most examples we used request.args to access GET parameters, but there are other dictionaries that can be populated with custom values: The following notations can be used to access attributes of an object: Although most of the bypasses could be further locked down by introducing stricter blacklists (e.g. This paper … Jewel - Write-up - HackTheBox. The project provides some sensible defaults that are easy to continue building on, and the source code is open source under the MIT license. A server side template injection is a vulnerability that occurs when a server renders user input as a template of some sort. ... One of particular interest is the Flask app instance. There were two cookies events_sesh_cookies and user.I tried forging the events_sesh_cookies first, setting the id in the session data to 1 (most likely to be the admin). As suggested by the name jwt token based challenge.Since none attack and other encryption related attack did’nt worked over here our option was to guess the key that was used for jwt signature. MySQL-Python: This is a Python interface to MySQL. It will help us connect the MySQL database to the app. Unsafe input in the template results in Server-Side Template Injection. 3. For example, depending on the IP that accesses a site, the site may look like: Exploit. My teammates were amazing, and together we made some amazing progress. Overview: The box starts with us finding a python flask jinja 2 webapp on port 80 and we have splunk running on port 8089 , We do Server-Side Template Injection to get remote code execution. In that challenge, this webpage was running the Jinja2 engine and was vulnerable to Server-Side Template Injection (STTI). We use both tricks to get the following bypass: Here's a short breakdown of what will happen: To answer your question, Christian, there is a way to bypass your __ blacklist! At this point we know our goal: Craft a custom pickle user object that when deserialized gives us RCE (preferably a reverse shell) After trying several things, Server-Side Template Injection (SSTI) came to mind. This time they denied access to properties such as '__mro__' and '__class__' which show up in top python SSTI tutorials. So we have to find another way. Now open this request in this extension. Viewing the source code of the page could indicate the possibilities for the LFI. Having achieved the previous bypass, I was curious if I could manage to bypass our very own fix of filtering "[" and "]". The Future of Cybersecurity in the Hands of AI, Exfiltrate data Through RGB color of IoT device in Air gapped Network using Tuya API, Vulnerability Management, Taking a Wide View, The CIAM Industry Is Projected to Reach $15.3 Billion in a 5-Year Span at a 15.1% CAGR, Cloud KMS Fundamentals for Enterprise: Part 2, Glossary of Common Privacy & Technology Terms. 3.1 Read. Advent of CTF Challenge 16 Start Page Well, let’s start with finding something interesting. A server side template injection is a vulnerability that occurs when a server renders user input as a template of some sort. I went back to find an old copy of the challenge's sourcecode and built a small testbed. A major vulnerability was found in Flask’s template rendering. Now we have to find a way to smuggle __class__ past both blacklists and into our exploit. The %s identifiers will be replaced with the passed string. It successfully authenticated me as another user but didn't grant me admin authorization. After some digging around , I came up with “KnCM6ogsNA1W" whose hash is 00e73414578113850089230341919829. Facebook CTF 2019 Writeup: events – Template Injection and Cookie Forgery. Also, multiplication of a string with a number'n' duplicates it 'n' times. The last couple of bypasses all relied on the |join function. The first step after finding SSTI vulnerability and identifying the template engine is … Luckily, there is another way to access attributes without . by Elber "f0lds" Tavares. Flask Unsign is a penetration testing utility that attempts to uncover a Flask server's secret key by taking a signed session verifying it against a wordlist of commonly used and publicly known secret keys (sourced from books, GitHub, StackOverflow and various other sources). Change user to admin and set the settings to recalculate the signature and enter the key as “fuckit” .Now hit go and we will get our flag in response. For the wordlists use our favourite ‘rockyou.txt’. picoCTF is a CTF hosted by CMU targeted at high school students, which is a great opportunity for beginner to improve their skill. I could bet that there are or going to be cleverer hackers that are capable of discoverying a bypasses for the other blacklists as well. While the latter wasn’t listed on CTFTime, it was still full of interesting challenges. Conclusion. At this point, it looks like Flask is a great framework for young developers. H4rryp0tt3r / flask-custom-sessions.py Created Sep 10, 2017 A script to generate flask sessions and exploit a server side template injection - ASIS CTF 2017 So, we can predict that we will retrive the flag with string like {{config.items()}} in the template file. Now using intruder i ran all my payloads for LFI but none of them produced positive results. Exploit. The first step after finding SSTI vulnerability and identifying the template engine is to read the documentation. By messing around with the admin panel, we can find that there is a custom 404 page (as seen when clicking on Charts or Tables). In Flask web applications using Jinja2’s templating language, this can often lead to an SSTI, or Server-Side Template Injection. Let's add request[request. This is Jinja2 Template Engine. As usual, https://247ctf.com delivers excellent challenges. Server Side Template Injection - A Crash course! A possible bypass would be: It is a bit ugly and long, though. I enjoy this CTF a lot. A major vulnerability was found in Flask’s template rendering. With function write_log (rip) we could write arbitrary string from X-Forwarded-For to the ticket file. The very first thing that should caught our attention is that it is a flask application , therfore one should check for SSTI(server side template injection). As Flask-Injector uses Injector under the hood you should find the Injector documentation, including the Injector API reference, helpful.The Injector README provides a tutorial-level introduction to using Injector.. The objective is exploiting SSTI (server side template injection) Flask/Jinja2 , ii)-Level 1 ( JWT ) After registering and opening our account. After registering and logging in, we are shown the following navbar info: 3.1 Read. Capture The Flag, CTF teams, CTF ratings, CTF archive, CTF writeups Server-Side Template Injection is possible when an attacker injects template directive as user input that can execute arbitrary code on the server. However, we still haven't bypassed a stricter blacklist that checks all parameters and the __ string. Capture The Flag, CTF teams, CTF ratings, CTF archive, CTF writeups Answer: THM ... HTTP Methods and Template Rendering; 2.4. As we can see in source code, the FLAG was kept in the flask's config variable. Here’s What You Should Learn. After trying some payloads, we can find that this website is vulnerable for a Server Side Template Injection, with this payload: {{7*'7'}} It will be result in 7777777. We can render arbitrary templates now! After exploring several tricks to bypass the blacklists, we still need to form the final, blacklist-bypassing RCE exploit. Based on CTF playing experience, the challenge and flavourtext always gives hints about the vulnerability or at least a starting point. Advent of CTF Challenge 17 Start Page. After trying some payloads, we can find that this website is vulnerable for a Server Side Template Injection, with this payload: … It traverse over child attributes of request recursively. In this case it is a dead giveaway that it has something to do with the Flask webserver. Inferno - Write-up - TryHackMe. | we45 Have you noticed how some emails, even though looks like it’s from a mass email campaign, are addressed to your […] www.we45.com Shows us the source code for the flask application itself. The call to render_template_string now includes the dir, help, and locals built-ins, which adds them to the template context so we can use them to conduct introspection through the vulnerability and find out what is programmatically available to the template.. Let’s pause briefly and talk about what the documentation says about the template context. Either way, the shell I get back has access to read logs, where I’ll find a password sent to a password reset url, which works for both the next user and to log into the Splunk Atom Feed. From the challenge name it can be hinted that it is talking about the concept of php magic hashes.in PHP two strings matching the regular expression 0+e[0-9]+ compared with == returns true. Server Side template injections are not a vulnerability in Frameworks. So I looked to the other cookie, user.Decoding it reveals that it is actually storing the user's username. So, I assume that the same template engine is being used, but that there are some security improvements done. The reason why we are entring string not any numeric value is because it has a filter for numeric value. Template injection After we learned to send request with arbitrary headers, we can retrive the flag. But using quotes (") will result in an exception, because it will be converted to " in the first rendering step. So You Want to Get Into Cybersecurity This Year? In theory, this should be easy, but it turned out to be a bit harder. i)- Reading & Analysing the given code. Advent of CTF Challenge 16 Start Page. If you’ve never heard of Server-Side Template Injection (SSTI) or aren’t exactly sure what it is, then read this article by James Kettle before continuing. Flask uses the Jinja2 template engine, so we have a Jinja2 template injection! Actually, i followed three sub-steps in this step: read, explore then attack. This time it is about bypassing blacklist filtering approaches by our and other teams as well as some useful tricks. Template injection that is present in this extension is for Flask/Django websites only. Initially seeing the source code I tried to execute commands like ls , sent through parameter. First, I had to modify the testbed a bit to introduce a simple blacklist which only checks if the exploit argument matches bad words: As you can see, I first wanted to circumvent the use of __class__ in the exploit parameter. In conclusion, I massively enjoyed the challenge. http://dwoo.org/what-dwoo.html. Shrine challenge, TokyoWesterns CTF 2018 Link; Exploring SSTI in Flask/Jinja2 Part 1 / Part 2 the output 7777777 in response box indicates that it is jinja template engine.Each of flask object has its configuration stored in configuration attribute.Therefor using the simple payload for displaying the configuration will display the flag. My teammates were amazing, and together we made some amazing progress. For the first CTF of the year, my team (Crusaders of Rust) played in *CTF 2021, and it the challenges were very interesting but also very difficult. Well, let’s start with finding something interesting. Motivation. Tuesday 16 February 2021 (2021-02-16) writeups. Flask Injection# What's inside /home/flask/flag.txt ? Not really a Team, just me. For the first CTF of the year, my team (Crusaders of Rust) played in *CTF 2021, and it the challenges were very interesting but also very difficult. Flask uses the Jinja template library to render templates. Not only did I manage to bypass the __, but also my team's [ filter. Flask uses the Jinja2 template engine, so we have a Jinja2 template injection! For eg -'0e1' == '00e2', In order to get the flag we just have to enter a string whose md5 hash starts with 0e or 00e. It gave me a list of file including flag.txt. With the same query-string paramters &a=_ we can form a format string that will result in __class__: %s%sclass%s%s Flask Injection; recents. Asis CTF Quals 2019 - Fort Knox. It definitely is a great tool but a simple misconfiguration may lead to severe security consequences. A script to generate flask sessions and exploit a server side template injection - ASIS CTF 2017 - flask-custom-sessions.py Flask-VueJs-Template is a minimal Flask boilerplate starter project that combines Flask, Vue.js, and Flask-RESTPlus. 'SECRET_KEY': 'LLS{server_side_template_injection_unmasked}', http://jh2i.com:50014/index.php?page=FLAG, What is Data Privacy and Data Trust are very different in Software Decisions. This was another Jinja2 template injection challenge (they've been showing up a lot recently). Here's a quite readable exploit that will bypass the [, ] checks, but not the __ check: As you can see, we have to use the set function to get access to the necessary object (i) class. I've read about template injections (i.e in Uber's websites), but have never found one in-the-wild or exploited one. Documentation. At this point, it looks like Flask is a great framework for young developers. Thus we need to think of a command less than 10 characters that could print the shell. They appear due to insecure code. In essence, we only have to get rid of the brackets. The alternative template is the error string and the error string contains our supplied (malicious) file name.
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