No Tools in a CTF

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[@nelsoncorreia7293]

Love it. Never really considered how to use the terminal if the tools in Linux suddenly disappeared.

[@zuberkariye2299]

Same 😂

[@trizedlyza]

This challenge is thrown to some degree on a lot on embedded devices like routers, or an old Linux box called TiVo. TiVo had a shell but no ls (dir) command so you had to use things like "echo *" to get the list of files.

[@TheoryCraftBR]

I'd just like to interject for a moment. What you’re referring to as Linux, is in fact, GNU/Linux, or as I’ve recently taken to calling it, GNU plus Linux. Linux is not an operating system unto itself, but rather another free component of a fully functioning GNU system made useful by the GNU corelibs, shell utilities and vital system components comprising a full OS as defined by POSIX. Many computer users run a modified version of the GNU system every day, without realizing it. Through a peculiar turn of events, the version of GNU which is widely used today is often called “Linux”, and many of its users are not aware that it is basically the GNU system, developed by the GNU Project. There really is a Linux, and these people are using it, but it is just a part of the system they use. Linux is the kernel: the program in the system that allocates the machine’s resources to the other programs that you run. The kernel is an essential part of an operating system, but useless by itself; it can only function in the context of a complete operating system. Linux is normally used in combination with the GNU operating system: the whole system is basically GNU with Linux added, or GNU/Linux. All the so-called “Linux” distributions are really distributions of GNU/Linux.

[@GreatTaiwan]

@@TheoryCraftBR ahhh classic

[@StuartMill14]

John you make this look breezy as all heck! Always a treat to watch you work!

[@movax20h]

To be pedantic, echo is not required to be a built-in into a shell (i.s. posix sh specification says absolutely nothing about echo). It can be just a binary in /bin/ (in fact there is one there just for this occasion).

[@zackarycrider5191]

I understood a few of those words

[@ChikoOof]

those were words?

[@My1xT]

Wait if echo works why not just echo the file?

[@matbeedotcom]

Because echo outputs what’s input, it’d just output the string path to the filename

[@kippers12isOG]

Redirect to echo

[@Jarvx]

I think they gave this particular challenge at the Bsides San Francisco CTF a week ago, pretty fun experience

[@mu11668B]

In some `sh` implementations, trying to execute `sh /path/to/script` will lead to all unavailable "commands" in that script reported in the terminal. Since `sh` is supposed to be available, `sh /home/ctf/flag.txt` might also work.

[@lurifos9576]

I was thinking the same thing

[@EugeneYunak]

@@lurifos9576 $ sh flag.txt sh flag.txt /bin/sh: 16: sh: not found

[@EugeneYunak]

but `.` (aka `source`) works! $ . ./flag.txt . ./flag.txt /bin/sh: 1: ./flag.txt: CTF{where-are-my-tools}: not found

[@trizedlyza]

Sourcing the txt CTF file, nothing could go wrong.

[@mu11668B]

@@trizedlyza Even if it does, it's their VM that gets rekt. Not something I would worry about. 😁

[@huntit4578]

Please more shorts like this love it ❤

[@injuringflea727yeet]

Where can I find these challenges?

[@JeffNoel]

This was BSideSSF's CTF challenge. It's not available anymore I think.

[@RedBigz]

I completely didn't know the read command until yesterday, because I was trying to parallelize some cracking tools, and this gem pops up on my shorts. Awesome timing.

[@mopioline65]

bookmarking this one. I have a feeling I may need it one day lol

[@Mirikant]

I don’t understand a thing that is happening but this all looks and sounds cool

[@BubbleS1]

Very cool solution and challenge!

[@georgerobbins5560]

Good stuff, John.

[@G-man_the-cyber-guy]

Please carry on with ctf and challenge walk through I love that content of yours! ❤

[@_mrcrypt]

‘‘Tis the season to be practicing your CTF 🍷😜🏴‍☠️

[@KP-nc9gk]

Noob question. Does `cat ` works as well?

[@MakerTim]

Cat is a binary, so that is one of the missing features what makes this a challenge

[@lauraprates8764]

​@@MakerTimit depends, by the standard echo is also a file, part of the core utils, but some implementations use it as a built in, and the same can be said about cat and ls

[@CatDevz]

@@lauraprates8764 in bash at least it’s a shell built in.

[@koshakush]

the short answer is no. cat for me is in /usr/bin and in this scenario, everything in usr/bin was deleted.

[@someoneunqualified7715]

That's a super cool CTF and it's also just super simple

[@georgehammond867]

You made it look like easy work 😅

[@blackhavoc1320]

fr

[@righteous-bison]

i love this concept

[@arshiaalidosti8818]

Loved it!

[@elliot-hacks]

Hammond back on CTFs, bro got some awesome tricks

[@robindeboer7568]

Oh hey! I saw you there! That was a fun one! I actually ended up doing this a different way hahaha I literally just did . /home/ctf/flag.txt and it spit out the answer in an error XD Your way is better.

[@xheezy]

Oooh bsides san francisco?

[@rayforever]

Can anyone explain what is that website?

[@hiraelum]

What about the 'cat' command? Is it available for this challenge? I've used it to print file contents to the standard output before.

[@iliealexandru2181]

how can I access those challenges, what is the website?

[@exaltedlord8771]

Thats so cool

[@Baked7DX]

This so cool

[@christiansanchez1097]

Where can You learn this ?

[@wishmaster-productions]

I would like to set something like this up for a LAN party. but im lazy and im wondering if there is a package with vm's ready to use to start playing a set of challenges with friends. Do you know of any that I could use?

[@blankexpression2u]

vulnhub has a bunch of Virtual Machines and the challenges for each

[@seasong7655]

where can we play the challenge? Does anyone know?

[@BlockchainShango]

Dude! 😮

[@snakshayan2132]

hey guys , I am building a CTF and need a lot of help 🙂 , any idea how to create a challenge like this ??

[@Proman4713]

Wouldn't `cat

[@MeTheCoolDude]

cat is binary that was deleted

[@aaronderoberts3273]

I need help with acid server, the PHP script that's supposed to patch me in with the host isn't working

[@fyodor8008]

There are probably a bunch of ways to solve that

[@machacador9000]

ah. yes. i understood everything happening here

[@corners1733]

Cool

[@Cucazikf]

Heyy, live CTFs 😁

[@TehTobis]

This looks really interesting, what is it? Is capture the flag like a game to help you learn Linux commands?

[@nicholasfinch4087]

It's a game, yes, but it's really just ways of Hacking into a machine to find some text in a file.

[@joshuawillis7874]

Is `cat` a binary that was removed?

[@kilgarragh]

yes

[@H3cJP]

yes, because cat is a binary located on /usr/bin, built in functions work because those are already loaded on RAM

[@anirudhsharma2003]

@H3cJP Please tell me why was read not removed if cat was?

[@H3cJP]

@anirudhsharma2003 because read is a bash function, not a binary unlike cat, which is its own binary; /usr/bin/cat you can see it by doing a "whereis read", you will see there isnt a binary associated with the command and if you do whereis cat, you will indeed see that it points to /usr/bin/cat you can also test it by running a virtual machine with some distro installed (or even just using a live distro) then doing rm -rf / --no-preserve-root and seing that "cat" fails but "read" still works if you kept the terminal with bash opened why? because it is still loaded on ram, you are using bash so bash functions are loaded on ram

[@anirudhsharma2003]

@@H3cJP Thanks a lot, that was really helpful for me. I have now a better understanding over these functions and binaries.

[@skylo706]

This is clever. As a linux noob though, I have to ask: Is stuff like ls also removed?

[@EugeneYunak]

yes

[@THATCANADIANDUDE682]

I wouldn't have used the variable, just straight echo out.

[@movax20h]

Like how? Remember cat is not available.

[@koshakush]

think someone else mentioned that would only echo the filename, not the contents so wouldn't work.

[@THATCANADIANDUDE682]

​@@movax20h echo "$(

[@petrlaskevic1948]

Could you use the cat command?

[@F38U]

No binaries, no cat

[@petrlaskevic1948]

​@@F38Uwhat is the difference between read and echo commands and cat?

[@F38U]

@@petrlaskevic1948 well echo just print whatever is after it to the terminal, cat concatenates files and read reads a file. major diff is cat lives in /usr/bin/cat while the others in /opt/

[@lettd0wn]

Can you open the txt file with nano?

[@ARMcPro]

it's NO tools

[@Dxeus]

These are lil kids' games.

[@nakedpotato9894]

Im just going to pretend that i understand every single second of that

[@constantitus]

ok, now read the second line in the file

[@errorcode778]

where can I find the CTF?

[@j0hn4th4nd03]

The URL is visible at the top of the screen

[@hayder775]

Oh the joker forgot remove binary of echo command 🤣

[@planktonfun1]

is cat a builtin tool?

[@andrewpatterson3139]

Yeah he just did cat with extra steps.

[@CatDevz]

@@andrewpatterson3139 cat isn’t a built in

[@OrNah.]

Sir this is a Wendy’s

[@OG-zr3bw]

Basic linux admin skills, nice.

[@xloppyschannel4881]

why cant you just cat it?

[@harshvardhansinha7688]

Because all of the binaries are deleted including the cat command

[@theAymen_dev]

but echo also in bin folder can you explain why it work when he is use it please

[@HansPeter-qg2vc]

​@@theAymen_devecho is both a binary and a shell-built-in. Try executing `type echo` (w/o the backticks) and it will probably tell you that you're using the shell-built-in echo. However, you can also use /bin/echo.

[@domelessanne6357]

how the fk can u learn all that wow?

[@superbellum]

cat

[@DreadArsenol]

Why can’t you just use cd and go to the directory and then use cat to read the file?

[@mewc0re]

because the binaries were deleted...

[@kilgarragh]

both cd and cat were deleted

[@movax20h]

@@kilgarragh only cat. cd is not a binary, it is a builtin (in fact some shells might have cat a builtin, but it is not possible to have cd to not be builtin, because cd modifies state of current process, and that cannot be done by a child process)

[@danthemango]

But you deleted all your binaries then how are you using sh? Huh? Check mate

[@DiverSteenberg]

Kali 😂

[@axelaragon7550]

I mean… can’t you just use cat?

[@CatDevz]

No. The problem description says there’s no programs available, you have to use shell builtins.

[@stipgoddab2087]

cat?

[@meomapxanh]

cat?????????

[@___jd]

y'all need to touch grass fr could be laying in a field eating peaches but instead you're doing absolutely nothing for no reason smh

[@trizedlyza]

Two options: 1. Normally just "touch grass" But all tools are removed, we want a non-destructive "touch" and that's just no command with the append style redirect: 2. ">> grass" Both would allow you to update the file time stamp on the the file called grass.

[@morsikpl]

Nice, altough I would just use simplier: echo $(

[@douwehuysmans5959]

/dev/stdout < /home/ctf/flag.txt

[@douwehuysmans5959]

/home/ctf/flag.txt > /dev/stdout

[@spookycode]

That ain’t gona work, files aren’t executables!

[@lukasz.jagiello]

. /path/to/flag.txt

[@lukasz.jagiello]

I was also the first one who solves it