2022-07-30: Treebox - Python AST sandbox challenge from Google CTF 2022

While writing an article on how "Hello World" actually works in Python (written with j00ru and Adam Sawicki, and published in 100th issue of the Polish Programista magazine; we'll publish the English translation on our blogs around September/October 2022) I've played a bit with Python's ast module (as in Abstract Syntax Tree) and decided it would make a cool CTF challenge if I would make some restrictions on AST level and have folks try to bypass it.

This wasn't of course the first challenge using AST on a CTF, though I did think to check only after I've already implemented it. Thankfully other challenges use different restrictions, so there was no collisions. Here are some of them though (leave a comment in case I've missed some):

• pysandbox @ TokyoWesterns CTF 4th 2018 (example write-up by hawkcurry),
• Tree of danger @ HTB Uni CTF 2021 - Quals (example write-up by Ratman),
• Finance Calculat0r 2021 @ CyberSecurityRumble CTF 2021 (example write-up by Zeyu).

Eventually the challenge was published at Google CTF 2022 in the Sandbox category under the name of Treebox and was solved 268 times, making it the easiest (or most popular? ;>) challenge of the CTF.

The challenge is likely still online when you're reading this blog post (if it's not, let me know) and you need only netcat to enjoy it. Just follow the link above, download the source code and have fun!

There are only 3 AST-level restrictions in Treebox:

• you can't call a function,
• you can't use import,
• and you can't use import from.

What was wonderful about the way players solved it, was that every solution was unique in some way. There were of course clusters of solutions converging around this or that feature, but at the end of the day the solutions were pretty different.

Since the solutions contain spoilers, I've posted them in a separate note in case readers would like to try their luck first.

Warning SPOILERS: Treebox solutions (it's at the bottom of this set of notes)

Whether you try the challenge first or now, if you enjoy Python I greatly recommend looking at the solutions. They are extremely clever in some cases, and fun in every case.

Have fun!

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2022-05-28: Mega Sekurak Hacking Party - Czerwiec 2022

Coś mi mówi, że muszę trochę częściej coś wrzucać na bloga. Poprzedni post był o grudniowym Mega Sekurak Hacking Party 2021, a ten jest o... czerwcowym Mega Sekurak Hacking Party 2022. Zapewne stali czytelnicy mojego bloga kojarzą ten organizowany przez Sekurak/Securitum event - jest to jednodniowe wydarzenie, z serią wykładów z różnych działek hakingu. Tym razem, jak poprzednio, jest to event on-line.

Kiedy/gdzie: 13 czerwca 2022 r., On-line
Więcej informacji: https://hackingparty.pl/
Sklep z biletami biletu: https://sklep.securitum.pl/mshp-edycja-zdalna-czerwiec-2022
Cena biletu: ~370 PLN (jest też opcja VIP za ~2300 PLN)
Ma być taniej: mshp-gyn-30 (30% zniżki)

Have Fun!

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Five newest or recently updated notes (these are unfinished posts, code snippets, links or commands I find useful but always forget, and other notes that just don't fit on the blog):

• 2022-07-31: Python "sandbox" escape
• 2021-09-15: C++ Hello World!
• 2021-08-02: 8-bit number to binary string ("01011010") [C/C++]
• 2020-06-16: Commands I forget
• 2019-10-26: Windows 10 file type association

Click here for a list of all notes.

• DLL shared sections: a ghost of the past - research notes taken while revisiting the infamous DLL shared sections; also, a description of finding and exploiting a Cygwin vulnerability. See also this post and these tools.
• PHP LFI to arbitratry code execution via rfc1867 file upload temporary files is an attempt to bring a specific LFI exploitation technique to common knowledge. You might also want to check out a follow-up paper by Brett Moore.
• Exploiting the otherwise non-exploitable - Windows Kernel-mode GS Cookies subverted described the internal mechanisms of generating GS cookies in drivers and a way to predict them.
• GDT and LDT in Windows kernel vulnerability exploitation is another paper I've written with j00ru, this time about using call-gates in kernel exploitation - quite a cool method since it works even if you have only a 1-byte write-what-where condition.

Some conference slides are linked at the bottom of this page.

• A few Microsoft Windows privilege escalations and some DoSes patched by MS10-021 and MS10-011 - this research was presented on HITB Dubai 2010 and CONFidence 2010 (video available here).
• Well, actually a few more bugs in Windows kernel and drivers discovered during our Bochspwn research (see this post and this one) using our kfetch-toolkit. They were patched in MS13-016, MS13-017, MS13-031 and MS13-036.
• Adobe Reader 9.5.1 and 10.1.3 multiple vulnerabilities - 62 unique crashes, from that 31 trivially exploitable and 9 more potentially exploitable, 11 CVE's assigned (CVE-2012-4149 to CVE-2012-4160). Some of these bugs were fixed for Windows and OSX releases of Adobe Reader in APSB12-16.
• Adobe Flash had also quite a lot of fixes (around 60 CVEs assigned). Some details can be found in these bulletins (in random order): APSB12-27, APSB12-24, APSB12-22, APSB13-17, APSB13-14, APSB13-11, APSB13-09, APSB13-05 and APSB13-01.
• Contributed to discovery of multiple low-to-high vulnerabilities in Google Chrome (CVE-2012-2851, CVE-2012-2855, CVE-2012-2856, CVE-2012-2862, CVE-2012-2863 and some other) - some of these were mentioned in this post.
• A lot of bugs in ffmpeg and libav which resulted in 892 (sic!) patches in ffmpeg and 299 patches in libav (CVE-2011-3930 to CVE-2011-3952 and some other).
• Cygwin cygwin1.dll shared section local privilege escalation (demo video) - discovered while revisiting old-school classes of bugs (see paper above).
• Two minor bugs in PuTTY and aterm and rxvt found while playing with terminal control codes. Put here to create some illusion of diversity.
• Mozilla Firefox 2.0.0.11 and Opera 9.50 information leak, also midly affected Safair, Konqueror and some other products (CVE-2007-6524, CVE-2008-0420, CVE-2008-0894, CVE-2008-1573). A demo video is also available.
• A small but funny bug in Total Commander 7.01 - an FTP client gets attacked by the server, leading to a path traversal.
• And there were also these two: a local privilege escalation that required a USB stick of death, and a funny compiler bug.

The full list of vulnerabilities discovered by me (including collaborative work) can be found here (please note that the list might be out of date).

The Google Application Security / Research site might also contain some of my findings.

• Unicode fun: PHP preg_match and UTF-8 with analysis of what happens when you forget that UTF-8 is multibyte; and a String-to-Integer vs Unicode - yes, the standard 0-9 digits are not the only ones supported by programming languages or unicode (see also this post).
• Digging deeper into C/C++: internals of static variables initialization, a discussion with myself about NULL/nullptr, the curious case of int a=5; a=a++ + ++a;, and what happens when scanf/atoi/strtol get an overly large number. Also, a way to magically make a list of functions at compilation time, and a way to create naked functions in gcc/g++ (or rather pure-assembly functions with no additional prologs/epilogs).
• Low-level goodies: a Hello World in C without using any headers / library functions, a process that frees almost all the memory it owns, and embedding GDB in assembly source code.
• Do not do this at work kids - OOP in .bat and OpenGL in .bat.

• PiXieServ is a simplified PXE (network boot) server for Windows and Linux-based OS, created for testing of very small home-made OS. See also the post about it.
• ExcpHook, a system-wide exception monitor for Windows XP 32-bit. Useful if you're fuzzing something that doesn't like having a debugger attached.
• Ent is an entropy measuring tool for reverse engineering reconnaissance (see also a post explaining how to use it).
• HiperDrop is a simple command line process memory dumper for Windows, with a few different work modes.
• asmloader - this little app executes headerless machine code (compiled assembly code). It's meant to be an aid in learning/teaching and playing with assembly, as well as the right tool when you just need to execute some machine code.
• NetSock is a simple socket/networking lib/wrapper for C++ I've wrote back in 2003 and update from time to time - I use it for most of my network-enabled projects.

W wolnym czasie prowadzę videocasty na żywo o programowaniu, reverse engineeringu oraz hackingu/security:

Livestream | Kanał na YT | Archiwum starszych odcinków
Najnowszy odcinek: Gynvael's Livestream #75: Implementujemy serwer FTP
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Dodatkowo: ReverseCraft - starsza seria podcastów o reverse engineeringu i assembly.

Dla programistów:

• Poradnik Początkującego Programisty - czyli jaki język wybrać, z czego się uczyć, jak samodzielnie rozwiązywać problemy i co dalej jak już się zna podstawy.
• Zmienne i stałe: wartość, kodowanie, reprezentacja - rozłożenie zmiennych na części pierwsze, od wysokiego poziomu abstrakcji, do kodowania na poziomie RAMu i CPU.

Security / hacking:

• Hacking - jak uczyć się security/hackingu i spać spokojnie.

Dodatkowo, kilka przemyśleń na temat odnajdywania się na rynku pracy w IT:

• Wybór studiów a szanse na rynku pracy - mój punkt widzenia na wybór studiów, rynek pracy, oczywiście w kontekście informatyki, programowania, itp.
• Spis sposobów na dokumentowanie swojej wiedzy, czyli co można wpisać w CV oprócz praktyk i papierka z uczelni.

• Trochę o PHP, preg_match i UTF-8, czyli o problemach związanych z niespójnym kodowaniem znaków.
• Kilka przemyśleń na temat intów, do tego kilka eksperymentów z wrzucaniem dużych liczb (większych od INT_MAX) w funkcje scanf/atoi/strtol, oraz różnice w wynikach w dzieleniu z udziałem liczb ujemnych w różnych językach.
• int a = 5; a = a++ + ++a;, ile wynosi "a"? Wynik jest oczywiście niezdefiniowany, ale jednak jakiś musi wypaść.
• Sposób na stworzenie funkcji naked w gcc/g++.
• Z przymróżeniem oka - programowanie obiektowe oraz OpenGL w języku skryptowym batch (aka .bat).

← trochę więcej postów jest po angielskojęzycznej stronie.

Grafika generowana proceduralnie:

• .S.k.y., czyli software'owy rendering heightmapy w trochę niekonwencjonalny sposób.
  
  
  
• SIN*COS, czyli co wychodzi z mnożenia sin(x) * cos(x) z różnymi współczynnikami.
  
  
  
• Graficzna interpretacja plików znalezionych na dysku - Art of file.
• Prosty raytracer napisany w Go, wraz z wersją unowocześnioną by mulander.