2020-08-10: Just another Null Byte Poison via Unicode variant (MuPDF mutool RCE)

Null Byte Poison is a neat little attack that usually can be applied when "length+data"-type strings get converted into "zero-terminated"-type strings. It's a well known problem though that haunted PHP scripts for several years, and even visited the browser world. Nowadays a lot of languages (or rather: runtime environments of these languages) have built-in protections against it (including PHP!) - for instance see this Python example:

>>> open("\0") Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: embedded null character

Unicode brought another similar problem to the table in the form of two ways (one invalid-yet-working) to encode a Null Byte without using an actual \x00 byte - this allows to, depending on the scenario, either bypass a Null Byte Poison detection, or actually inject a Null Byte into a "zero-terminated"-type string at a later processing stage (which is sometimes useful):

• UTF-8 overlong sequence: \xC0\x80
• UTF-7 being UTF-7: +AAA-

In the above cases when the strings get decoded to Unicode we (might) see Null Bytes popping up in the data. Thankfully all decent UTF-8 decoders deal properly with overlong sequences and nothing modern uses UTF-7 anyway (with the notable exception of Express.JS in some scenarios).

While playing with a path traversal bug in mutool (details below) I've found yet another Unicode-related way to inject a Null Byte into a string. This method actually relies on a decoder bug and is pretty case-specific, but I think it's worth testing for as I wouldn't be surprised to find it again in similar scenarios in the future.

The bug in question resided in this code (mupdf/source/fitz/time.c):

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2020-07-11: PWNing Online 2020 CFP

Piąta edycja PWNingu, czyli bardzo technicznej konferencji, którą organizujemy wspólnie z Wydawnictwem Naukowym PWN, odbędzie się standardowo w listopadzie. Z tą różnicą, że w tym roku wyjątkowo przenosimy ją online z oczywistych powodów. Drugą nowością są dwie ścieżki – tj. tradycyjna SEC/HACK (bezpieczeństwo komputerowe), i nowa CODE (programowanie). Obie ścieżki mają osobne rady programowe, i osobne Call For Papers, czyli zaproszenie do zgłaszania prelekcji. A skoro już mowa o CFP...

• SEC/HACK: CFP (CFP trwa do 31 lipca)
• CODE: CFP (CFP trwa do 31 sierpnia)

Formularze w obu przypadkach są dostosowane do specyfiki danej branży (tj. są bardzo różne). W przypadku zaakceptowanych prelegentów z Polski będziemy zachęcać do wystąpienia w studiu w Warszawie – myślimy m.in. o dodatkowym panelu/panelach dyskusyjnych z udziałem prelegentów w studiu (zobaczymy jak wyjdzie i jak sytuacja będzie się rozwijać).

Więcej informacji można znaleźć na stronie konferencji (hint hint: do 31 lipca bilety są tańsze).

P.S. Osobno i niezależnie będzie CFP na Lightning Talks (5-10 minutowe wystąpienia) – ale o tym więcej koło sierpnia/września.

P.S.2. PWNing rok temu wyglądał tak:

Czytaj dalej... [ 2 komentarze ]

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):

• 2020-06-16: Commands I forget
• 2019-10-26: Windows 10 file type association
• 2018-06-18: PHP quirks/security/CTF tricks/tips
• 2018-01-04: Python 3 BrokenPipeError pipe test
• 2017-11-12: IDA PYTHONHOME wrapper

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.