What is a File Format?

Let's explore what a file format is, and provide a different view on it. We dive into polyglots, file format research and the impact on security.
Funky File Formats Talk: • Ange Albertini: Funky ...
corkami/mitra tool: github.com/cor...
Guessing vs. Not Knowing featuring Steganography: • Guessing vs. Not Knowi...
=[ ❤️ Support ]=
→ per Video: / liveoverflow
→ per Month: / @liveoverflow
=[ 🐕 Social ]=
→ Twitter: / liveoverflow
→ Instagram: / liveoverflow
→ Website: liveoverflow.com/
→ Subreddit: / liveoverflow
→ Facebook: / liveoverflow

[@Sunpy_Emily]

The scene with dark background, a table and a simple t-shirt makes this feel like an interrogation scene where police is asking the criminal questions.

[@SamuelCarreira]

I really like this scenario, for me doesn’t look any like that... but only a minimalist and well filmed scenario. No more that typical youtuber background bullshit with their setup behind

[@tilakmadichettitheappdeveloper]

#hackersroom

[@ultraviolet.catastrophe]

"We know you work with the File Format Cartel! Who is your leader?!?"

[@NStripleseven]

Except instead, he’s answering questions nobody asked.

[@yuxin7440]

This video is pretty fascinating. With many years of experience using Linux and command line, I am already familiar with the fact that file format is just a trick (every file is just a binary stream anyway), but I am sill surprised that you can easily craft files to be interpreted by multiple programs differently. I am not sure whether it's a good thing or a bad thing. From a developer's perspective, we want the file format to be unambiguous, because we know from experience that ambiguity is a common source of bugs and unexpected behaviors. However, sometimes we also want flexibility and tolerance. For example, we want to add more features to file format but not break the older version program, which means we shouldn't be overly strict on recognizing format. These two design principle are sometimes conflicting to each other, and I think it is the main cause of the issue.

[@redpanda31337]

@@00O3O1B a word file is also just a zip file

[@nagitokomaeda3237]

this is why magic numbers exist

[@Blast-Forward]

It also doesn't sound very efficient to put different "files" into the same file for most use cases.

[@philipk883]

Aah yes! the Schrodinger's zip file.

[@thefridge6913]

I laughed at this a little too hard.

[@NestiGaming]

That cat example doesn't seem that random...

[@AjayKumar-fd9mv]

അടിപൊളി

[@chregig7967]

@@thefridge6913 you can never laugh too hard :D

[@mouaztabboush5571]

Schrödinger doesn't like this trick

[@joachimprz]

Roger, LiveOverflow gone Dark Mode

[@rogervanbommel1086]

Why use my name?

[@bruh_5555]

I was searching for this comment🤣🤣

[@TimLF]

I love dark mode. I wonder what % of 0.5M viewers are on OLED and how much energy was saved.

[@machinexa1]

Black lives matter

[@Architector_4]

I honestly like the light mode more. Not only it feels more reminiscent with what I associate with the channel, keeping it light at all times prevents eye whiplash like at 0:55 where the light fills the entire video instantly.

[@SimonHuenecke]

How dark should the background be? Live overflow: YES

[@YotaNinja]

I remember my first time learning about this file format trick was probably about 2005-2006 on 4chan of all places. Someone uploaded an image, it was the cover of a C++ textbook, or some C language. I can't quite remember, but what I do remember was you could download the image, and extract that exact document from it. They embedded the textbook within the image itself, and used any image hosting site to discretely share it with people. I was blown away.

[@MrKristian252]

If anyone has seen this image going around on Discord; "Please don't open me in the browser". Basically 2 image png animation. Renamed to .zip, gives a .mp3 file inside, containing some metadata about opening the audio in a image viewer. Pretty cool, took half of the day to get to the end of it.

[@zekiz774]

Yes. I'm not the only one. I just posted this video in the discord server.

[@atharvavaidya6230]

Looks interesting. Can you post a link here?

[@zekiz774]

@Nigel YING No. It's not quiet the end. Try to do "file" on the happiness file

[@E404NNF]

I saw the one which only works on VLC (not the UWP version)

[@johnny5gr]

The conversation went like this: - WTF did you do? - You dog... - Zip it man!

[@shady4tv]

I'm someone in the category of people freaking out about closed source binwalk so I see files agnostically already. But I thought - "Hey, If I give this video a chance I'm sure LiveOverFlow will teach me something new" - All I can say is WOW The idea of not encapsulating but "programming" a file into the zip format is a complete paradigm shift. I will never be able to look at files in the same way again holy shit bro you just blew my mind.

[@kanucks9]

11 minutes in "I don't know exactly why the pdf isn't shown in the zip file" Dude that's literally the only reason I watched this video.

[@petey5009]

The pdf file is probably contained in a place where the zip program doesn't check, and pdf headers don't need to start at the beginning

[@oODomeeOo]

@@petey5009 Since the PDF is in the Zip record it is probably checked by the Zip program. But since it has no filename it is simply not displayed.

[@giantbee9763]

He does know why, just not precisely what the reason is in this particular case. In this particular case it could be anything, depending on the way the zip format is there might be many ways to hide the pdf. You just need to find a way to make the information redundant, like making things a comment in the earlier polyglot C php bash etc example. :)

[@erickcardozo462]

That's because the PDF file was not zipped, its contents were just combined with the contents of the zip file in the final generated file. So when the zip program reads the final file, it encounters the only thing that was zipped in that file: the text file. It's not that hard to get 🙂

[@31redorange08]

@@erickcardozo462 You should watch the video.

[@solomioist]

LiveOverflow: „You don’t want to do PDF by hand“ Me: *cries in LaTeX*

[@ra1d3r34]

i feel you!

[@blackneos940]

What is Latex, some condom ingredient?

[@int16_t]

A typesetting tool.

[@Cobalt985]

LaTeX is fucking incredible. Also if you like meme ways of writing your documents check out groff/troff. Much simpler than LaTeX. Or, just start converting markdown/emacs org-mode to LaTeX. That takes the pain out of it. I wrote all my Bio notes in org-mode then compiled it into a final LaTeX document without much trouble.

[@flp322]

LaTeX by hand is much easier than PDF by hand.

[@uuuuuhhlettuce3909]

omggg u finally made a dark mode intro. 🙏🏿🙏🏿🙏🏿

[@panagiotispetridis7961]

I really like this type of videos. The explanations where really good the quality is very high and overall I can confidently say that I've learnt something I didn't know before!

[@p.9227]

My mind was absolutely blown away. I've never thought that the same file could be interpreted differently. This is eye-opening for me.

[@frni]

You can do this in windows with the copy command and the /B switch for binary "copy /B picture.jpg+folder.zip new.jpg" I learned this when I heard that a promotional desktop wallpaper for Portal had an Easter egg in it. If you opened it as an archive the ending song "Still Alive" mp3 was in there. This was a triumph!

[@BGroothedde]

The 010 tool is pretty awesome, reminds me of something similar I made for terminals - but more advanced and with a hex editor. Thanks for sharing that!

[@4g3v]

010's template feature is the best one I have seen yet in any hex editor. It's really useful for reversing proprietary file formats.

[@MulleDK19]

Nothing beats Hex Editor Neo. Unfortunately it's not free.

[@stevepoper8073]

I remember being blown away by file extensions when I played DDLC

[@imgladnotu9527]

Dont We All................... just monika ngl LiveOverflow should check out/play games that have neat tricks like what ddlc does, and im pretty sure there are obscure as heck ones out there.

[@otkchk]

LiveOverflow 2016 - finding a parser differential in loading ELF LIveOverflow 2020 - what is a file format just joking. top notch stuff I didn't know.

[@hafidhzouahi7146]

LiveOverFlow: hiding files in files is not fun justCTF: yes

[@WtfAnupam]

This whole channel is a blessing in my Cybersecurity journey! thankyou soo much for creating such level of content...

[@Brlitzkreig]

Wow, you make it so easy for people to understand these complex topics!

[@maddoggLP]

Ideal video to start reading my digital forensic course. As if you know I am procrastinating

[@hypergraphic]

This gives me new appreciation for the mantra: parse don't validate. If you just look for what you are expecting, you might admit more than you were bargaining for.

[@svampebob007]

Files being source code is so blatantly obvious I never though of it, but when you pointed it out it instantly made sense how one could play with the file. and espessially when you showed that ansicphpbash "file format" :) How many of us have struggled with one code trying to parts a bit of another code as a string/variable, or what ever, only to realize you forgot to reformat it so that the thing you're trying to pass is not a being interpreted as actual code.

[@attention_shopping]

love the bit about CTF at the end -- that makes so much sense!

[@helper_bot]

I'd like to point out some flaw in the video 1. The Person A and Person B analogy, rather than just "liking" it should've been "only knowing" or "ignoring except". PDF program would read the PDF code, and not the ZIP code, and the other way around for Zip programs 2. Rather than changing the file name extensions, you could probably just run the file with the program right away. Though I'm not sure since I haven't tried it, but I'm sure it would run as is. Anyway, great video as usual, thanks for sharing this information with us!

[@TheKinGG0ld]

Love the musicians of bremen image and the new videos format!

[@UjjwalKumar-wg4wu]

loving the new brand design

[@nohbdy9634]

This is great, the education was great, and the whole building up to the message about the CTFs was hilarious, but I 100% agree.

[@cannaroe1213]

I like this channel a lot :) I really liked how you explained that zip files actually program zip to make a file, rather than "contain" data. You could have brought in zip-bombs at this point, because then a 1Kb file making a 42Gb file kind of shows how it's generative. Having said that, I think you made one point unclear, which was that programs sometimes "ignore" bytes they don't understand, like scanning for a dog and not seeing the cat. PDF is weird in that it looks for it's magic sequence anywhere in the file, ignoring the zip at the beginning. ZIP, for example, wont do this. Python wont do this. etc etc. Nevertheless, through use of commenting, which is like programming zip to deliberately ignore code, you make polyglots. Polyglots almost always use commenting. If commenting wasn't possible, making polyglots would be WAY harder! Programs don't typically ignore anything, unless you trick them into it. Also this whole video strikes at the heart of a big problem in Europe, what does data privacy/security/illegal information actually mean? What if a picture file, for example, looks like a beautiful sunset in one image viewer application, but child-pron in another. Is the *file* child-pron, or is the image-viewer *making* child-pron when it's displayed? Or both? Do you need to have both on your computer to break the law? TL;DR we all have child-pron and state-secrets on our computers, sometimes in the same file, we just don't have the software to view it.

[@0okaze]

À true hacker spirit, reminds me of my youth. It pleases me to see young talents, there are so few of them, while I thought 30 years ago that there would be countless hackers far better than us in the future. It never happened, everything has gone down, so these videos are refreshing.

[@joshinils]

"grew up with a commandline" yes, you could say that i grew up when i started using linux a few years ago

[@Crux161]

This is one reason why simple things like the `file` command in Linux are *so useful*

[@VeryBlueBot]

Wow your production level got upgraded so much since the last time I watched your vids. Looks even better than before. Wish YT algo would push more of your content to my Recommended.. Now I will to go on and "manually" watch your latest stuff. Super interesting content and approach to digital security one of my favorite channel on the subject for me as a developer/programmer Keep it up :)

[@thecakeredux]

Presentation is excellent, no background and you in the center explaining and gesticulating is a very good idea.

[@TheSam1902]

Great lighting ! Really striking how you made that work despite usually commenting without a camera feed

[@Basepilot]

love the example with the "Town Musicians of Bremen " :)

[@aj2090]

Don’t listen to the weirdos. Loving the low light setup. I see you clearly and nothing else. Minimal and clean

[@Jagnathbaba]

I always used to think that these formats are "strict" as in they wouldn't allow unknowns. Turns out they do and you can play tricks with them.

[@Rudxain]

_[HTML without DOCTYPE has entered the chat]_

[@RichardiOS275]

the thing that went in my head when I see 6:26 is C I didn't realise there's php and bash until you told us

[@mina86]

ZIP file’s ‘header’ is located at the end of file and it includes list of files in the archive. Any file records which are not specified in that header at the end are simply ignored. This way you can easily modify a ZIP archive by appending data at the end of it (e.g. you can remove files from the list, add new file records and new entries in the list etc.). This also means that you can just append a ZIP file at the end of another file, and the result will likely work as both. ZIP program will look at the end of the file to look for list of files to extract and will completely ignore whatever is at the beginning of the file. This is how self-extracting archives work. Your zip tool might warn you if there is some data at the beginning that it doesn’t expect, but that depends on the tool. The way the trick works with PDF is that the PDF header does not need to be located at the very beginning of a file. This means that one can add a prefix which will make ZIP think it is a file record but because the file record is never referenced in the header at the end, the tool will just ignore it. On the other hand, PDF reader will ignore the prefix looking for the PDF header.

[@1Hippo]

Very interesting, it really works, no special tools required, just with: cat tmp.zip >> random.pdf Then random.pdf still works as pdf and can also be read as zip in my filemanager (dolphin) if renamed. But not in other archive tools, it says that it not a supported archive, so that would need the prefix trick as well. "file" reports it as pdf, because it reads the magic number at the start, which was of course not modified by the simple appending.

[@mina86]

@@1Hippo, it may be ignorance or security concerns why some tools don’t open such archives. Info-Zip will complain about extra bytes but otherwise will just go ahead and extract files correctly. By the way, some libraries might offer a ‘streaming’ interface for reading archives (e.g. Java’s ZipInputStream) which will read the file entries as they appear in the data ignoring the directory at the end.

[@FennecTECH]

I think the CTF about finding the hidden stuff in a file would be a great challenge for a stego CTF. And it is valuable experence in identifying stego. And if your getting into information security stego experence is very important.

[@olbluelips]

Great video, I love thinking of zip as interpreter for .zip "source code"! I also just love the concept of weird machines Recently, for educational purposes, I've written a couple of image file formats and also I'm writing an interpreter, so this is right up my alley :)

[@mikee.]

Love the new style of videos. Also the video quality is absolutely stunning, so well produced!

[@l-12343]

wow! this new format is fantastic ... not to mention the video quality! spot on man!

[@logc1921]

The quality of this video is good! Keep it up man! Really appreciate your work.

[@michamarzec9786]

That was very informative and entertaining. Learn something new today. Thanks :D

[@zonbiimusic4750]

Greetings from Bremen. Thanks for the interesting Video. As a linux user and C programmer not much was new for me but it was still interesting to listen to you explaining in detail

[@zakuarbor]

Great video. I recalled how many students were amazed when I had students extract an image from a PDF in my seminar course (a course where students teach the class) where I talked about stegosploit. Makes you think what files could be hidden in a PDF. However, that was only the start of it because the toolkit created by Saumil Shah, the person who created Stegosploit, hid the toolkit inside the image. So you had to rename the image extension to HTML and open it in the browser to obtain the toolkit. I was also very shocked when I first was researching the topic.

[@Yaxqb]

File formats based on extension: Windows virgin. File format based on actual content: Unix file CHAD basedlord

[@MrFram]

MIME types: Our new web overlords

[@funkykong9001]

So cool to see 010 Editor mentioned! I love it :)

[@darske1]

Loved the format, the topic, the explanation... Loved everything, great video

[@samansinaei1426]

Huge thank you, I was really hoping to find a video like this. You couldn't do it better than this

[@HolowatyVlogs]

This is really interesting, I’ve never heard of polyglot computing! This kinda reminds me of steganography.

[@user-vn7ce5ig1z]

Because steganography often uses this as the method of embedding data. That said, not all cases of this are steganography, for example, a self-extracting archive is seen by the shell as an executable file but as an archive by the archive program. (An archive program is usually involved with these.)

[@Danielo515]

I came to this video thinking I will just reaffirm on my knowledge about files, it them I learned a lot of new stuff. Thanks

[@barkeeper7887]

I Love that your Videos can be watched by IT guys but still be understood by beginners :)

[@Sankaritarina89]

I can totally relate to this. Grew up with windows but entirely switched to Linux like 8 years ago. I have a completely different understanding for the filesystem now.

[@LeoStaley]

Over the last twelve years, I have tried to switch over to Linux no less than 6 times. I am driven insane by trying to deal with obscure problems, and have to turn back to windows every time. Perhaps it's just because I'm not a coder and am just a power user. But if a power user like me can't take the frustration of Linux, I can't imagine normal people ever being able to take it.

[@Sankaritarina89]

@@LeoStaley if you are a windows user who needs windows it will be difficult I guess. I'm a software engineer, so I have a big benefit from using Linux and only downsides when using windows so that made the decision easy for me.

[@lilyliao9521]

@@LeoStaley never fully switch to linux, dont get tricked by the masochistic nerds

[@OmegaZ2]

Something I didn't quite understand is: Wikipedia says that polyglots "performs the same operations or output independent of the programming language used to compile or interpret it.", but you showed that when renaming file formats, after "fusing" them with Mitra, the PDF and the TXT showed different things, this isn't the same output. Isn't this a fault on Wikipedia's side?. Very cool and informative video! Thanks as always LiveOverflow

[@LiveOverflow]

I Wouldn’t call it a fault. I would just say that some terms have maybe slightly different meaning depending on who you ask

[@NebMotion]

Thank you for sharing this knowledge, we truly live in the age of information!

[@RahulRaj-pd7gi]

No this type of ctf challenges are not at all annoying infact I solved this type of challenge yesterday and learned alot about file extension and that's how I reached to this video. You explained everything perfectly. Thanks

[@HritikV]

Damn! This is so cool. At first I thought it was about magic bytes but you never fail to surprise!

[@Hyperboid]

"YOU decide what to open the file with" xdg-open: exists

[@nashonightmare]

Great explanation LiveOverflow .

[@lukor-tech]

THANK YOU SO MUCH FOR THE DARK MODE LOGO!

[@bramble-east]

I remember my frustration when first switching from Windows to Ubuntu for work projects. I didn't understand how the Ubuntu file system structure worked, how I should manage individual files, and how to work with them. I asked people questions like "Where should I install programs in Ubuntu?" and similar. At that time I thought to myself self "Gosh, Windows seems like a much cleaner system, everything is neatly organized, I have a dedicated folder for Program Files and the only thing I should do is click shortcuts". But after learning the Ubuntu FS layout, understanding how PATH actually works and what is it intended for, and a lot of other tips and tricks Windows FS principles feel rather restrictive. Although I now daily-drive Windows for home and work stuff (Windows made MAJOR progress towards being a developer-friendly system in the last few years), I still miss some of that clean simplicity and infinite possibilities that a proper GNU/Linux system provides.

[@DM-qm5sc]

Thank you for dark mode!

[@nahu4870]

2:38 There's is magic though! At the first bytes of the file

[@hikingpete]

Often PDFs will be read from the end. PDFs are designed to be written linearly, but a linearly written file is inefficient to access in a random fashion. Thus, after writing out the contents of a PDF, the writer will usually append an index at the end. The index is placed at a known offset from the end of the file, and readers will generally access that first, and then only read the parts of the file that they actually need.

[@aakash-codes]

That was interesting to know! Thanks for making a video about this.

[@backendninja8333]

What an outstanding explanation. Just mind blowing. Keep it up man!

[@orikd5d3xy]

Love the enthusiasm and great content as always! Keep these coming :)

[@MeriaDuck]

2:38 LOL at no magic joke 😀 For those new to this, the linux file format recognizer (the file command) is configured in a file called /etc/magic

[@TimLF]

Anyone ever use that file? I though it just stayed empty and people used defaults, #!, /usr/share/applications/, or whatever.

[@samfoxman7046]

The term "magic" comes from en.wikipedia.org/wiki/Magic_number_(programming)

[@1Hippo]

Just checked on my system, /etc/magic does not exist. So at least for Arch Linux it is in /usr/share/file/misc/magic. It reads a compiled version (.mgc) first.

[@sreejithsubhash7301]

File exists in Ubuntu 😃

[@nahu4870]

Pretty sure the joke was about the magic numbers like Sam said It's the first few bytes of a file / the signature which you can teoad to find out the format and other info like the version of the program uses to create that file

[@nagitokomaeda3237]

This reminds me of that time I concatenated a shell script unzipping itself with a zip file to have basic self-extracting archives.

[@justkarkat9575]

I am glad I actually understood the statue reference!

[@MateHegyhati]

Thanks, I'll bring this example up at my Formal Language Theory classes. This is a fun way to talk about the intersection of formal languages. :-)

[@louisdupont4397]

This video blew my mind ^^ Thanks a lot for sharing and teaching this ! Keep doing your awesome work !

[@GegoXaren]

PoC||GTFO PDFs are all zip polyglot, some of the PDFs are also SNES roms and boot sectors... They also have an exploration of how it is done in the journal volumes. Highly recommended reading that journal.

[@redsus3866]

Epic dark mode intro!

[@afiqzx]

Broo liveoverflow has predicted elden ring culture faster than anyone

[@ozgun228]

A video topic suggestion: how to make your own file format. That would give us more intuition in the topic.

[@aakashadhikari3752]

Ohhhhh man ...i lobh your talks🔥🔥🔥🔥 the last part was funny " please dont give such kinda Ctfs"

[@Null-c7i]

I'm currently creating a custom file type for my company, nearly finished it. Where I don't disagree entirely I would say a better way to look at it is a file type is designed and the tools or libraries that interact with it are coded with an understanding the designed structure. The file type is a concept and needs code to create it or read it, it technically doesn't exist beyond the concept of the structure. In simplest terms it's a structure in which data is stored.

[@bancodrut]

For a second there I thought this was some bare entry-level tutorial ... saw the channel name ... Wait a second 😂 Nice video btw. I was thinking about something similar and it's nice to know that it has been already researched (cus I'm lazy as fuu)

[@clippy3656]

I love how you turned on dark mode in the intro

[@user-vn7ce5ig1z]

These sorts of subjective-filetype files usually involve an archive because they ignore stuff they don't understand more than other types of programs, but steganography can involve most formats (though plain-text is a lot harder to pull off as the container).

[@manotive6094]

Love this filming setup

[@SeaHay]

This seems like a really cool way to install a smallish program while making sure someone reads the README file first and foremost

[@JonJaded]

Awesome video man... Very informative.

[@gokus22]

Yo, thanks. Really like the new bg.

[@Basileuswar]

A really good video as always. The quality on overall looks really sharp. I would prefer to have windows in dark theme to better match the rest of the video

[@otesunki]

I'm digging the dark theme! IDK if it's new or not, I haven't watched ALL of your videos ( yet )

[@_nikeee]

I use binwalk when dissecting a file for its contents.

[@NaveenRajuM]

I rarely like a video. This guy got my Like. :D

[@ΔημητρηςΤ-ζ7ω]

Great explanation!!

[@kmnaniak]

we can go even deeper, it is possible to make file without filename and extension but with content

[@SriHarshaChilakapati]

I used to do this with BMP images. Compress all your files, and combine them with a BMP image of your choice. Just a simple copy command in cmd will work and you can hide some stuff from people. copy /b image.bmp files.zip image2.bmp. I used to hide my games in school lab PC this way.

[@SriHarshaChilakapati]

This is also exactly the way SFX archives work. Open an SFX archive (.exe file) in any Zip program and it will show the contents. Fun stuff indeed.

[@ashupar84]

Very informative thanks 👍🙏

[@you_just]

What really made file extensions click for me was when I started writing files with scripts. In python, for example, the File.write() method only takes a string for the file path. To change the file extension, you just backspace the “.pdf” part of the path and replace it with “.csv”. There’s no magic, your OS just looks at the last three or four characters in the file name and decides how to process it accordingly.

[@narayanbandodker5482]

LiveOverflow has finally enabled Dark mode!