Nebula exploit exercises walkthrough – level12

There is a backdoor process listening on port 50001.

My experience with Lua is minimal at best, but it’s pretty obvious that the hash() function calls a shell command, and allows for command injection.

To run getflag is very simple:

And if you want to pass the check for the hash for fun, it is also simple:

Nebula exploit exercises walkthrough – level11

The /home/flag11/flag11 binary processes standard input and executes a shell command.

There are two ways of completing this level, you may wish to do both :-)

Now it gets interesting. This is the first bit of code where it isn’t obvious what the intent is from a quick glance.

I think I have found three ways to get this to execute getflag, though one is just a variation of another.

The code reads from stdin, then checks for “Content-Length: “, reads a length integer, and then processes this.

There are a number of paths from this point. If the length is less than the buf length (1024), then fread is called. Then there is a bug.

This is what happens on this code path:

But later on:

From the man page of fread:

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

The function fread() reads nmemb elements of data, each size bytes
long, from the stream pointed to by stream, storing them at the loca‐
tion given by ptr.

fread() and fwrite() return the number of items successfully read or
written (i.e., not the number of characters).

Whilet both read in the same data, the return values will be different. The first will return 1, the second will return the number of chars read.

This means the only way to get to process with the length less than 1024 is to set the length to 1. This restricts our options a fair bit.

We’ll try it out though:

As expected, the value we pass (E, arbitrary choice) gets “processed” to become D. system is then called, but because we can only provide a single character, we can’t null terminate the command, and we get some random values after.

We can see these values vary each time we run it:

One thing that does happen though is that, by chance, we end up with a null being in the right place:

This is pure luck. The rest of buffer is uninitialized and nulls are common in uninitialised memory.

If we now symbolic link D to /bin/getflag, and alter the path so it runs D when the null is in the right place:

Hmmph. Why is it not the flag account? I think this is a bug – the call to system isn’t preceded by setresuid/setresgid, so anything it runs will run as the real UID (level11) instead of the effective UID (flag11).

Co-incidentally, I had recently read of a technique to fill uninitialised memory. It’s virtually useless in the real world – using uninitalised memory is indicative of much bigger issues. It’s interesting though, so let’s try it here.

This technique uses an environment variable called LD_PRELOAD. This is commonly used to override library functions for debugging (or exploits!). When the linker starts up, it reads the entirity of LD_PRELOAD onto the stack and then doesn’t clean up afterwards. This means we can initialise the memory to something under out control:

i.e. fill the stack with one thousand /bin/getflags.

Then when we run flag11 with length of 1, it will almost certainly have this in the buffer already:

Again the same issue with suid/system, but I think it counts.

Now we need to come back to the length being 1024 or more. What happens here?

There is a really simple encryption function:

We can easily build the reverse of this in Python and output a string:

(note, that I terminated the command with newline (x0a) to start with, which was causing this to fail)

We can then pipe this into the executable, to run the command:

Simple!

Aside

Whilst playing around with this level, I thougut there might be something I could do with the random path/filename that is used when the content length is 2014 or greater.

The filename is normally of the form:

As seen from strace. This is PID (process ID) with a “random” string.

We can gain control of this string, the filename, and stop it from being deleted. This uses LD_PRELOAD, but for it’s genuine use.

First, we must check that the executable is dynamically linked:

Good.

Now we need to create a c file to override the functions we want – random(), unlink() and getpid():

The we compile it into a library, set LD_PRELOAD, and then run the executable:

And now we have control of the filename, and it is preserved rather than deleted.

Not of any real use, but a handy technique.

Nebula exploit exercises walkthrough – level10

The setuid binary at /home/flag10/flag10 binary will upload any file given, as long as it meets the requirements of the access() system call.

I think I can already see the problem.

Firstly, we can see that the token file we need to read out is permissioned such that level10 cannot see it:

On line x above, we have the following:

From the man page of access:

access() checks whether the calling process can access the file path‐
name.

The check is done using the calling process’s real UID and GID, rather
than the effective IDs as is done when actually attempting an operation
(e.g., open(2)) on the file.

So we check the file permissions using the real UID (level10), but then later on we do:

and open uses the effective UID, and as the executable has suid, this means flag10.

This is commonly called a time-of-use to time-of-check or TOCTOU bug (Wikipedia’s example is pretty much exactly the same issue)

If we can swap out the file between the time-of-check and the time-of-use, we should be able to send token.

First, let’s just check the program works as expected.

Setup a listening netcat on my host using:

And then run it on nebula with a file we have access to:

And we receive it at the other end, plus a little banner:

Ok – so how do we explout the race condition? The best way to swap the file about is to use symolic links again. How do we time that though? I’m fundamentally a lazy person, so let’s try and just swap out the files as quickly as we can and hope it works.

First, let’s setup a loop that flips a symbolic link from the real token to a fake one repeatedly:

The f switch on ln makes sure we overwrite the existing symbolic link. The &amp at the end puts the job into the background.

Then let’s setup the listening netcat to keep on listening rather than exit using the k switch.

And finally, let’s run flag10 repeatedly using another bash one-liner:

Go back to netcat and we have the token:

There we go – the password for flag10.

Nebula exploit exercises walkthrough – level09

There’s a C setuid wrapper for some vulnerable PHP code…

I’m no PHP expert – this one took me a long time. There are two functions that look dubious there – file_get_contents and preg_replace. Let’s see what it is meant to do.

It looks like it reads the file provided as the first argument ($filename) and does nothing with a second argument ($use_me). The file read in is expected to be in the format:

and it returns a string like so:

You can use the command to get an arbitrary file that flag09 is permissioned for:

But we need to execute something, not read something.

Look closely at one of the preg_replace lines:

This looks like, for the 2nd matching term, run the spam function on it. The second term is substituted inside the spam() function, then executed. Maybe we can inject a command here.

I’ve recently done a couple of XSS tutorials/games, which have given me a fair bit of practice at command injection (in Javascript, though), and felt I was getting quite natural and good at it. However, this PHP one ended up being just a big case of trial and error.

I started trying to execute phpinfo() – it nearly always works and doesn’t need any parameters passing to it.

Right – this just echos the command.

Ok – it’s now treating phpinfo as a variable, but that variable isn’t defined.

Now we have passed an expression with invalid syntax…

Yes! Ok – so this strange notation with curly braces works. I’m not quite sure why it needs to be like this, but now I have it, I can find examples of people using it.

Now we need to run getflag. PHP has system to do system calls.

Hmm. It is escaping the inverted commas so it doesn’t work. In fact, it seems to escape anything helpful

Coming back to one of the examples above – we managed to get it to treat phpinfo as a variable. What happens if we try to use the unused parameter, use_me?

Right – so we can use that to pass in a string. Let’s combine the two.

Excellent! I got there in the end. It felt a little painful. If the second parameter hadon’t been called use_me, and this wasn’t an exploit wargame, I would have given up. Not happy with this level.

Nebula exploit exercises walkthrough – level08

World readable files strike again. Check what that user was up to, and use it to log into flag08 account.

A readable pcap file in the flag08 home directory. This is a network capture, so might have some interesting traffic.

Now… we can read this on the terminal using tcpdump:

Even when it is this prettied up, it’s still hard work – especially if it is a keyboard interactive process. People using the keyboard expect instant feedback – they press a key, they what to see the screen change. This means that there is a lot of back and forth. Compare this to, say, a request for a web page, which is machine generated and will fit neatly into packets.

So I want to get this file into Wireshark on my local machine. How can we do that? netcat!

(note that these instructions have OS X as the remote end – the command name and options syntax vary from OS to OS)

On the host machine, we do the following:

Listen on port 2001, and pipe any output to the file capture.pcap.

and on the client (Nebula machine) we do this:

Connect to port 2001 and pipe capture.pcap down the connection.

Now we have our file at the other end, it is an easy taste to run Wireshark and open the capture.Wireshark

There is a single connection between two given IPs here. The trace is still hard to follow though, so go to Analyze -> Follow TCP stream. This gives us a nice, coherent conversation:
Conversation

We can see a login to another machine. We are just going to have to hope for some password re-use. The password bit looks like:

However, those . are not . – they are characters not represented by display characters. Switch the view to hex view and we can see:

Hex view

Hex view

x7f – DEL (well, backspace). That makes the password backd00Rmate

Nebula exploit exercises walkthrough – level07

The flag07 user was writing their very first perl program that allowed them to ping hosts to see if they were reachable from the web server.

The code of the CGI script is provided (and can be viewed in /home/flag07):

“);

}

# check if Host set. if not, display normal page, etc

ping(param(“Host”));

Immediately you can see this is not sanitising or validating the input parameter Host that it passes to a command - ping. We can therefore pass it another command for it to execute.

Let's test the script out, from the command line to start with:

(I've stripped out HTML as I am lazy and can't be bothered getting it to format correctly).

It just runs ping against localhost, as expected.

Run it without parameters, and we get the help:

And then let's check we can inject a command:

Excellent.

The challenge now is that, for the first time, this script isn't set to run suid. If I try running getflag, it isn't going to work.

That thttpd.conf file in flag07's home directory looks interesting. Could he be running a test web server?

Excellent - a web server on port 7007.

So, we need to:

  • Connect to the web server running on localhost at port 7007
  • Request a index.cgi
  • Pass a Host parameter with a command being careful to URL escape all of the special chars

wget is a simple utility present on nearly all Linux boxes that allows us to get a webpage.

We just need to escape the semi-colon to be %3B.

Check the content of the file and we have run getflag as a flag07.

Nebula exploit exercises walkthrough – level06

The flag06 account credentials came from a legacy unix system.

Most Linux systems use a shadow password file. The normal /etc/passwd file is visible in the open (it is used to map userid -> name etc.), but it has no password hashs. These are stored in /etc/shadow, which is permissioned such that unprivileged users can’t see the hashes.

So, let’s take a look at /etc/passwd:

Compare level06 (a normal account) to flag06 (legacy). ueqwOCnSGdsuM is the hash of their password.

It’s been a long time since I have done this, but the go-to password cracker was always John the Ripper, and it still appears to be that way.

This is available as a package in Ubuntu, so it could be installed with sudo apt-get install john. I don’t know the sudo password, so I can’t install this in the Nebula VM without using the admin account they give you. It’s perfectly possible to install it on your local machine, copy the passwd file across, and crack it there though.

I ran it on my Mac and it got the password very quickly – it’s just hello. Login and run getflag.

Aside

I haven’t managed to find an online password cracker that deals with this type of password hash, which is surprising. It is quite old-school though.

iSmartAlarm – quick “teardown”

I noticed this post on the alarm forum at DIYnot. It mentions the iSmartAlarm – an alarm I’ve heard nothing about before. Smart tends to mean “connected to the Internet” which tends to mean “massive attack surface”, so I though I would have a quick look at the system and what is inside it.

The iSmartAlarm homepage is fairily standard. The alarm seems to comprise of a CubeOne at the heart of the system, with PIRs and door contacts connecting to it. The CubeOne appears to connect to your home router using a wired Ethernet connection.

iSmartAlarm

On the site, their is a manual for the alarm. It’s pretty sparse really – it doesn’t seem to have many features. There is also a section called “Port forwarding” on the support section. This only seems to be concern the iCamera they provide. Port forwarding is generally a Bad Thing™ as it lets someone outside of your NAT/firewall inside.

There is nothing really juicy there, we really need some pictures of PCBs to work out what is going on. No-one seems to have torn down one yet, so I’ve got to hope the FCC have something. Most wireless devices sold in the US need to be FCC compliant, which involves submitting test reports and internal photos to the FCC. There are a few ways out of this – using ready made wireless boards like the ElectricImp, and ticking all the boxes that make the documents confidential. Thankfully, most companies only request that the schematic, block diagram, and description of operation is confidential.

How to get the FCC ID? It’s not in the manual (it rarely is), so let’s look for a photo of one of the units.

cnet oblige with a photo of the bottom of one of the units.

FCC ID

Off we go to the FCC OET ID search. I haven’t worked out how to link to results on here, so just enter SENIPU3 as the ID and look for yourself.

The only really interesting document is the internal photos (reposted here).

One photo really stands out – the close-up of the PCB.

PCB shot

What do we have on here then?

First thing I see, upper right, is the venerable TI CC1110 chip. This is the same chip used in the IM-ME toy that has been changed into a spectrum analyser. It’s a combined microprocessor and sub-GHz RF frontend and is very flexible. To the right of it there is a pin header labelled CLK, D, RST, GND. These are the in-circuit programming pins for the CC1110 – you can see this on Travis Goodspeed’s page about hacking the IM-ME. Firmware recovery might be possible.

There is a Ralink RT5350F which appears to be a SoC used in a lot of wireless routers. It will be providing the wireless, Ethernet and USB. The datasheet indicates this boots from SPI serial flash, which means firmware recovery is almost certainly possible. I’m guessing the slightly frazzled looking 8-pin U10 is this device.

The big Winbond chip is SDRAM.

The funny little blue blob-on-board seems to be some kind of LED driver, judging by the proximity to the connections to the LED in the casing.

Not sure what the pin header at the top of the board is. JTAG on the Ralink chip is 5-pin. Could be a serial debug port.

Searching for the first three letters of the FCC ID brings up the other components – the PIR, door contact and fob. All using the CC1110. That means two-way comms is possible.

PIR

PIR

Door contact

Door contact

Fob

Fob

This system looks fairly hackable. The CC1110 data can be sniffed no doubt, firmware recovery may be possible. The Ralink firmware almost certainly can be recovered.

I wonder if it is worth getting hold of one?

Nebula exploit exercises walkthrough – level05

Check the flag05 home directory. You are looking for weak directory permissions

Let’s start looking in /home/flag05:

Compare to the home directory of level05:

So we have .ssh – the store of SSH keys for the user – and .backup. The .ssh directory is locked down so we can’t see it.

Let’s look in .backup:

A single backup .tgz. Let’s copy it out to our own home directory and unpack.

That’s the private (id_rsa) and public (id_rsa.pub) keys for flag05. They may well work on the local machine:

Simple. That’s why you should keep your private key private!

Nebula exploit exercises walkthrough – level04

This level requires you to read the token file, but the code restricts the files that can be read. Find a way to bypass it :)

This program looks like it will read the file passed to it by the first argument. Let’s test that out:

Everything as expected then. The problem is that it explicitly forbids opening of files called token. How can we get round this?

Symbolic links to the rescue again!

Just create a symbolic link to a name that doesn’t match “token”.

So what is this long string? Seems sensible to try and login to the flag04 account with it: