WPICTF 2018 guess5

The WPICTF 2018 “guess5” challenge:

The URL presented us with a guessing game, we have to pick 6 numbers. If we picked the correct numbers we’ll get a flag:

However submitting our picks never worked. Investigating this for a bit it looks like this requires to run a local Ethereum node. Before trying to set that up we’ve looked more into what the web application does. Interestingly it fetches the URL https://glgaines.github.io/guess5/Guess6.json (mirror here). In there we can find the ETH contract including in plain text for some reason. Which contains the flag:

The flag is: WPI{All_Hail_The_Mighty_Vitalik}

WPICTF 2018 Shell-JAIL-2

The WPICTF 2018 “Shell-JAIL-2” challenge:

This is almost the same challenge as Shell-JAIL-1 (see my write-up here for explanation of details) with the exception of one extra line in access.c (full mirror here):

        setenv("PATH", "", 1);

This means that before dropping the arguments to system() the $PATH environment variable is unset. Also the blacklist filter of the previous challenge remains the same. With that only built in sh commands will continue to work and since / is also blacklisted we cannot provide full paths to binaries either. For example id will now not work while pwd still executes:

But the . (or source) command still works. With that we can tell the shell to try to execute the flag.txt file and the error message will reveal its content. We still use the ? wildcard to circumvent the other blacklist by passing . "fl?g.t?t" to it:

The flag is: wpi{p0s1x_sh3Lls_ar3_w13rD}

WPICTF 2018 Shell-JAIL-1

The WPICTF 2018 “Shell-JAIL-1” challenge:

After downloading the linked private key and connecting to the remote server we are dropped into a limited user account and the directory /home/pc_owner. In that folder there are only 3 files – including flag.txt to which our user has no access:

The access file is basically a setuid executable which will run as the pc_owner user. The source of the executable is also available in access.c (mirror here). The program will take all arguments and pass it to system() unless it contains blacklisted strings, relevant parts in the source code:

int filter(const char *cmd){
	int valid = 1;
	valid &= strstr(cmd, "*") == NULL;
	valid &= strstr(cmd, "sh") == NULL;
	valid &= strstr(cmd, "/") == NULL;
	valid &= strstr(cmd, "home") == NULL;
	valid &= strstr(cmd, "pc_owner") == NULL;
	valid &= strstr(cmd, "flag") == NULL;
	valid &= strstr(cmd, "txt") == NULL;
	return valid;
}


int main(int argc, const char **argv){
	setreuid(UID, UID);
	char *cmd = gen_cmd(argc, argv);
	if (!filter(cmd)){
		exit(-1);
	}
	system(cmd);
}

This means passing id to it will work but cat flag.txt will not:

Of course circumventing that filter is rather easy, the * wildcard is forbidden, but ? is not. We can use those wildcards to read flag.txt by passing cat "fla?.tx?" to it:

The flag is: wpi{MaNY_WayS_T0_r3Ad}

Nuit du Hack CTF 2018 CoinGame

The Nuit du Hack CTF 2018 CoinGame challenge:

The URL presented us basically only with a simple webform, which fetches a resource we can specify via cURL:

After a bit of trying, we figured out that file:/// URLs also work, like file:///etc/passwd:

Fetching a lot of files from the server yielded not a lot of success. After a while we noticed the text on the main site: “DESIGNED BY TOTHEYELLOWMOON”

Searching for this and CoinGame a GitHub repo was found: https://github.com/totheyellowmoon/CoinGame
The description of that repo read: “Congrats it was the first step ! Welcome on my Github, this is my new game but I haven’t pushed the modifications …”

From the description of the challenge and the GitHub repo we gather that “CoinGame” is being developed on this server and some changes aren’t pushed yet to the repo.
From /etc/passwd and /var/log/dpkg.log on the server we’ve also figured out that probably a tftp server is running on that system.

Requesting http://coingame.challs.malice.fr/curl.php?way=tftp://127.0.0.1/README.md we found the local repository:

Next we cloned the public GitHub repo, with that we had a list of all existing files in the repository. We looped over all the files and downloaded them via tftp from the system. Then simply ran a diff on the checkout and downloaded files. None of the code had any differences, but a few pictures didn’t match:

In any of the gameAnimationImages/background*.png images the flag was visible:

The flag was: flag{_Rends_L'Arg3nt_!}

iOS camera QR code URL parser bug

I’ve learned recently that the iOS 11 camera app will now automatically scan QR codes and interpret them.
This is pretty cool, until now you needed special apps to do that for you on iOS.
When scanning a QR code which contains a URL – in this case https://infosec.rm-it.de/ –  iOS will show a notification like this:

Naturally the first thing I want to try is to construct a QR code which will show an unsuspicious hostname in the notification but then open another URL in Safari.

And this is exactly what I found after a few minutes. Here it is in action:

There is no redirect misuse being done on facebook.com, Safari will only access infosec.rm-it.de.

Details:

If you scan this QR code with the iOS (11.2.1) camera app:

The URL embedded in the QR code is:
https://xxx\@facebook.com:443@infosec.rm-it.de/

It will show this notification:

But if you tap it to open the site, it will instead open https://infosec.rm-it.de/:

The URL parser of the camera app has a problem here detecting the hostname in this URL in the same way as Safari does.
It probably detects “xxx\” as the username to be sent to “facebook.com:443”.
While Safari might take the complete string “xxx\@facebook.com” as a username and “443” as the password to be sent to infosec.rm-it.de.
This leads to a different hostname being displayed in the notification compared to what actually is opened in Safari.

This issue has been reported to the Apple security team on 2017-12-23.
As of today (2018-03-24) this is still not fixed.

Update:
On 2018-04-24 this has been fixed with iOS 11.3.1 and macOS 10.13.4.
CVE-2018-4187 has been assigned to both issues.

 

NeverLAN CTF 2018 JSON parsing 2

The NeverLAN CTF challenge JSON parsing 1:

The linked file can be found here.

The JSON file contains a minute of VirusTotal scan logs. The challenge wants us to provide a SHA256 hash of a PE resource which most commonly by multiple users. In the data there is the unique_sources field, this will show us which file was uploaded the most by unique users.

Basically I use a short Python script to format the JSON to be easier read and find the highest number of unique_sources, then search the full file for that record.

from pprint import pprint
import json

with open('file-20171020T1500') as f:
    for line in f:
        data = json.loads(line)
        pprint(data)

Running this script like this:

python json2.py |fgrep 'unique_sources' | cut -d ' ' -f 3|sort -n | tail -1

Will find that there is one record with a unique_sources count of 128.
Searching for like this in the full file:

fgrep 'unique_sources": 128' file-20171020T1500

We get the full scan record back, submitting any of the PE resources SHA256 hashes will work as the flag.

NeverLAN CTF 2018 JSON parsing 1

The NeverLAN CTF challenge JSON parsing 1:

The linked file can be found here.

The JSON file contains a minute of VirusTotal scan logs. The challenge wants us to find the 5 AV engines which had the highest detection ratio (not detection count) in that timeframe. To solve it I created this quick Python script:

from __future__ import division
import json

result_true = {}
result_false = {}
result_ratio = {}

with open('file-20171020T1500') as f:
    for line in f:
        data = json.loads(line)
        for scanner in data['scans']:
            if data['scans'][scanner]['detected'] == True:
                if scanner in result_true:
                     result_true[scanner] += 1
                else:
                     result_true[scanner] = 1
            else:
                if scanner in result_false:
                     result_false[scanner] += 1
                else:
                     result_false[scanner] = 1

for scanner in result_false:
    result_ratio[scanner] = result_true[scanner] / (result_true[scanner] + result_false[scanner]) * 100

for key, value in sorted(result_ratio.iteritems(), key=lambda (k,v): (v,k)):
    print "%s: %s" % (key, value)

It will count detection for each AV engine and afterwards calculate the detection ratio for all. Running it will print all ratios sorted by lowest to highest. The last 5 separated by commas is the flag:

The flag is: SymantecMobileInsight,CrowdStrike,SentinelOne,Invincea,Endgame

hxp CTF 2017 irrgarten

The hxp CTF 2017 irrgarten challenge:

Running the dig command (with added +short to reduce output) provided the following output:

$ dig -t txt -p53535 @35.198.105.104 950ae439-d534-4b0c-8722-9ddcb97a50f6.maze.ctf.link +short
"try" "down.<domain>"

Playing around with it we figured out you can prepend “up”, “down”, “left” and “right” to the records to navigate a maze:

$ dig -t txt -p53535 @35.198.105.104 down.950ae439-d534-4b0c-8722-9ddcb97a50f6.maze.ctf.link +short
569b8ba8-ac9a-4d60-a816-10d13b3d7021.maze.ctf.link.
$ dig -t txt -p53535 @35.198.105.104 down.569b8ba8-ac9a-4d60-a816-10d13b3d7021.maze.ctf.link +short
b55b6358-6f9a-4a2c-b68a-211f56c88df9.maze.ctf.link.
$ dig -t txt -p53535 @35.198.105.104 left.b55b6358-6f9a-4a2c-b68a-211f56c88df9.maze.ctf.link +short
$

An empty reply probably means that there is a wall in the way otherwise you get the DNS record of the next tile.

To solve it and figure out how big the maze is, this very inefficient Python script was created:

#!/usr/bin/env python
import os
import subprocess

todo = [ '950ae439-d534-4b0c-8722-9ddcb97a50f6.maze.ctf.link.\n' ]
done = [ ]
directions = [ 'up', 'down', 'left', 'right' ]

while True:
  for tile in todo:
    check = subprocess.check_output("/usr/bin/dig +short -t ANY -p53535 @35.198.105.104 " + tile, shell=True)
    print check
    for direction in directions:
      fqdn = direction + '.' + tile
      output = subprocess.check_output("/usr/bin/dig +short -t ANY -p53535 @35.198.105.104 " + fqdn, shell=True)
      if output:
        if output not in done:
          todo.append(output)
          print output

    todo.remove(tile)
    done.append(tile)

  if not todo:
    break

This basically loops over all known tiles and checks if there is an accessible tile next to it in all 4 directions. If there is it adds it to the todo list and moves on. All newly found tiles get written to stdout. The base FQDN without the direction prepended gets also queried, this is where we suspected the flag will be found.

While this was running we were trying to implement a more efficient solution but it captured the flag after around 28’000 tiles:


"Flag:" "hxp{w3-h0p3-y0u-3nj0y3d-dd051n6-y0ur-dn5-1rr364r73n}"

 

Stored XSS in Foreman

Following up a bit on my recent post “Looking at public Puppet servers” I was wondering how an attacker could extend his rights within the Puppet ecosystem especially when a system like Foreman is used. Cross site scripting could be useful for this, gaining access to Foreman would allow an attacker basically to compromise everything.

I’ve focused first on facts. Facts are generated by the local system and can be overwritten given enough permissions. Displaying facts in the table seemed to be secured sufficiently, however there is another function on the /fact_values page: Showing an distribution graph of a specific fact.

When the graph is displayed HTML tags are not removed from facts and XSS is possible. Both in the fact name (as a header in the chart) and fact value (in the legend of the chart).

For example, add two new facts by running:


mkdir -p /etc/facter/facts.d/
cat << EOF >> /etc/facter/facts.d/xss.yaml
---
aaa_test_fact<script>alert(1)</script>: xxx
aab_test_fact: x<script>alert(1)</script>xx
EOF

It will shop up like this in the global /fact_values page:

Clicking on the “Show distribution chart” action on either of those facts will execute the provided alert(1) JavaScript:

That’s fun but not really useful, tricking someone to click on the distribution chart of such a weird fact seems impractical.

But since the XSS is in the value of the fact we can just overwrite more interesting facts on that node and hope that an Administrator wants to see the distribution of that fact. For example, let’s add this to xss.yaml:

kernelversion: x><script>alert(1)</script>xx

Now if an Administrator wants to know the distribution of kernel versions in his environment and he uses this chart feature on any host the alert(1) JavaScript will get executed. This is what any other node will look like:

And after navigating to the kernelversion distribution chart on that page:

Still some interaction needed. I’ve noticed that on the general /statistics page the same graphs are used and facts like “manufacturer” are used in them. Unlike the other graphs these do not have a legend. But when you hover over a portion of the graph you’ll get a tooltip with the fact value. This is again vulnerable to XSS. For example add to xss.yaml:

manufacturer: x<img src='/' onerror='alert(1)'>x

Now when you visit the /statistics page and move the mouse over the hardware graph, the alert(1) will execute:

Still needs interaction. But if you inject a value into all the graphs it may not take long for an Administrator to hover over one of those.

However: By default Foreman uses CSP. Stealing someones session with this setup is not easily possible. So my initial plan to steal an Administrators Foreman session failed in the end.

This was tested on Foreman 1.15.6 and reported to the Foreman security team on 2017-10-31.
CVE-2017-15100 has been assigned to this issue.
A fix is already implemented and will be released with version 1.16.0.

 

HITCON 2017 CTF Data & Mining

The HITCON 2017 CTF “Data & Mining” challenge:

The file attached was a 230MB big pcapng file.

I think I solved this by accident. I was sifting through the data for a bit and started to exclude the flows with a huge amount of data as it was mostly compressed / unreadable to me.

In the remaining data I stumbled over a plaintext TCP stream on port 3333:

This contained the flag in plaintext: hitcon{BTC_is_so_expensive_$$$$$$$}

In retrospective, searching for the string hitcon in the packet data would have worked as well.