Tomcat Cgi

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Tomcat is an extremely stable platform to build on – and using it to run your applications will contribute to your server’s stability, as well. This is because Tomcat runs independently of your Apache installation – even if a significant failure in Tomcat caused it to stop working, the rest of your server would run just fine.

Exploits & Vulnerabilities

  • Welcome to the Apache Tomcat ® 10.x software download page. This page provides download links for obtaining the latest version of Tomcat 10.0.x software, as well as links to the archives of older releases.
  • Apache Tomcat is an open-source web server and servlet container that is used to serve Java applications. It is developed by the Apache Software Foundation, written in Java and released under Apache License 2.0. It is a top level project of the Apache foundation.
  • CGI的定义是:外部应用程序与Web服务器之间的接口。1.Tomcat7支持CGI,但是默认配置是关闭的需要进行如下配置 修改Tomcat conf/web.xml中两处代码,默认是注释掉的,去掉注释即可 1 2 cgi 3.

CVE-2019-0232 is a vulnerability in Apache Tomcat that could allow attackers to execute arbitrary commands by abusing an operating system command injection brought about by a Tomcat CGI Servlet input validation error.

Apache Tomcat, colloquially known as Tomcat Server, is an open-source Java Servlet container developed by a community with the support of the Apache Software Foundation (ASF). It implements several Java EE specifications, including Java Servlet, JavaServer Pages (JSP), Java Expression Language (EL), and WebSocket, and provides a 'pure Java' HTTP web server environment in which Java code can run.

On April 15, Nightwatch Cybersecurity published information on CVE-2019-0232, a remote code execution (RCE) vulnerability involving Apache Tomcat’s Common Gateway Interface (CGI) Servlet. This high severity vulnerability could allow attackers to execute arbitrary commands by abusing an operating system command injection brought about by a Tomcat CGI Servlet input validation error. This blog entry delves deeper into this vulnerability by expounding on what it is, how it can be exploited, and how it can be addressed.

Understanding CVE-2019-0232

The CGI is a protocol that is used to manage how web servers interact with applications. These applications, called CGI scripts, are used to execute programs external to the Tomcat Java virtual machine (JVM). The CGI Servlet, which is disabled by default, is used to generate command line parameters generated from a query string. However, Tomcat servers running on Windows machines that have the CGI Servlet parameter enableCmdLineArguments enabled are vulnerable to remote code execution due to a bug in how the Java Runtime Environment (JRE) passes command line arguments to Windows.

In Apache Tomcat, the file web.xml is used to define default values for all web applications loaded into a Tomcat instance. The CGI Servlet is one of the servlets provided as default. This servlet supports the execution of external applications that conform to the CGI specification. Typically, the CGI Servlet is mapped to the URL pattern “/cgi-bin/*”, meaning any CGI applications that are executed must be present within the web application.

Tomcat

A new process in Windows OS is launched by calling the CreateProcess() function, which takes the following command line as a string (the lpComandLine parameter to CreateProcess):

  • int CreateProcess( .., lpComandLine, .. )

In Windows, arguments are not passed separately as an array of strings but rather in a single command-line string. This requires the program to parse the command line itself by extracting the command line string using GetCommandLine() API and then parsing the arguments string using CommandLineArgvW() helper function.

This is depicted in the flowchart shown below:

Cmdline = “program.exe hello world”

Figure 1. Command line string for Windows

Argv[0]->program.exe
Argv[1]->hello
Argv[2]->world

The vulnerability occurs due to the improper passing of command line arguments from JRE to Windows.

For Java applications, ProcessBuilder() is called before CreateProcess() function kicks in. The arguments are then passed to the static method start of ProcessImpl(), which is a platform-dependent class. In the Windows implementation of ProcessImpl(), the start method calls the private constructor of ProcessImpl(), which creates the command line for the CreateProcess call.

Figure 2. Command line string for Java apps

ProcessImpl() builds the Cmdline and passes it to the CreateProcess() Windows function, after which CreateProcess() executes the .bat and .cmd files in a cmd.exe shell environment.

If the file that is to be run contains a .bat or .cmd extension, the image to be run then becomes cmd.exe, the Windows command prompt. CreateProcess() then restarts at Stage 1, with the name of the batch file being passed as the first parameter to cmd.exe.

This results in a 'hello.bat …' becoming 'C:Windowssystem32cmd.exe /c 'hello.bat …''. Because the quoting rules for CommandLineToArgvW differ from those of cmd’s, this means that an additional set of quoting rules would need to be applied to avoid command injection in the command line interpreted by cmd.exe.

Since Java (ProcessImpl()) does no additional quoting for this implicit cmd.exe call promotion on the passed arguments, arguments processed by cmd.exe is now used to execute, presenting inherent issues if arguments are not passed to cmd.exe properly.

Argument parsing by cmd.exe

We begin with the understanding that cmd is essentially a text preprocessor: Given a command line, it makes a series of textual transformations then hands the transformed command line to CreateProcess().

Tomcat Chimney Sweep

Some transformations replace environment variable names with their values. Transformations such as those triggered by the &, , && operators, split command lines into several parts. All of cmd’s transformations are triggered by the presence of one of the following metacharacters: (, ), %, !, ^, ', <, >, &, and .

The metacharacter ' is particularly interesting: When cmd is transforming a command line and sees a ', it copies a ' to the new command line then begins copying characters from the old command line to the new one without seeing whether any of these characters is a metacharacter. This continues until cmd either reaches the end of the command line, runs into a variable substitution, or sees another '.

If we rely on cmd’s '-behavior to protect arguments, using quotation marks will produce unexpected behavior. By passing untrusted data as command line parameters, the bugs caused by this convention mismatch become a security issue.

Tomcat Chipmunk

Take for example, the following:

hello.bat “dir ”&whoami”
0: [hello.bat]
1: [&dir]

Here, cmd is interpreting the & metacharacter as a command separator because, from its point of view, the &character lies outside the quoted region. In this scenario, 'whoami’ can be replaced by any number of harmful commands. When running the command shown above with hello.bat, we get the following output.

Figure 3. The resulting output when running “hello.bat”

The issue shown in the screenshot is used in Apache Tomcat to successfully perform command execution, which is shown in the following image:

Figure 4. Performing command execution in Apache Tomcat

To successfully perform command injection, we need to add a few parameters and enable CGI Servlet in the web.xml file.

Figure 5. Snapshot of web.xml

The Apache Software Foundation has introduced a new parameter, cmdLineArgumentsDecoded, in Apache Tomcat CGI Servlet that is designed to address CVE-2019-0232. cmdLineArgumentsDecoded is only used when enableCmdLineArguments is set to true. It defines a regex pattern “[[a-zA-Z0-9Q-_./:E]+]” that individual decoded command line arguments must match or else the request will be rejected. The introduced patch will eliminate the vulnerability that arises from using spaces and double quotes in command line arguments.

Figure 6. The Apache Tomcat patch, which can be found in the codebase

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Recommendations and Trend Micro Solutions

Apache Software Foundation recommends that users running Apache Tomcat upgrade their software to the latest versions:

VersionRecommended Patch
Apache Tomcat 9Apache Tomcat 9.0.18 or later
Apache Tomcat 8Apache Tomcat 8.5.40 or later
Apache Tomcat 7Apache Tomcat 7.0.93 or later

Furthermore, users should set the CGI Servlet initialization parameter enableCmdLineArguments to false to prevent possible exploitation of CVE-2019-0232.

Developers, programmers, and system administrators using Apache Tomcat can also consider multilayered security technology such as Trend Micro Deep Security and Vulnerability Protection solutions, which protect user systems from threats that may exploit CVE-2019-0232 via the following Deep Packet Inspection (DPI) rule:

  • 1009697 - Apache Tomcat Remote Code Execution Vulnerability (CVE-2019-0232)

Trend MicroTippingPoint® Threat Protection System customers are protected from attacks that exploit CVE-2019-0232 via the following MainlineDV filter:

  • 315387 - HTTP: Apache Tomcat Remote Code Execution on Windows
Cgi

Using Perl on Your Personal

Installation of Tomcat

CGI processing is handled in Tomcat with a servlet,CGIServlet, which supplied with the Tomcat installation.It is not enabled by default, however.You must install the JAR file containing the CGIServletclass file and set up use of the servlet in your web application descriptorfile.This can be done for a particularweb application or it for the entire Tomcat installation.

An apparent bug in CGIServlet prevents it from accessingPerl scripts in any directory whose path contains a space character.Since Tomcat is often installed in Windowsunder the Program Files directory, this can be a problem.If your Tomcat is installed in such a place,you should either

·Re-install Tomcat elsewhere

·Or set up a web application in a location inanother location.

It is convenient to have your web applications under the webappsdirectory under the Tomcat installation directory because you do not have toconfigure them: just create a new subdirectory containing a WEB-INF/web.xmlfile and you’re ready to go.The URLpath to access the new web application is simply the name of the directory youcreate. Therefore, I recommendinstalling Tomcat in a directory without spaces in its path to avoid the complicationsinvolved with setting up a web application in a non-default location.

The JAR file is already present in the correct directory,but it does not have the correct name.You need to look in the directory server/lib under the Tomcatinstallation directory, and rename the file whose name begins with servlets-cgi.The new name of the file should be servlets-cgi.jar.

To set up CGIServlet for the entire Tomcat installation,modify the default web application descriptor file, conf/web.xml, under theTomcat installation directory.To set itup for a single web application, modify the file WEB-INF/web.xml under the webapplication’s root directory.

Your default web.xml file probably has the tags forCGIServlet already present, but commented out.You should remove the commenting and adjust parameters as needed.

The Servlet Tag

Here is the servlet tag I use in my home installation.You can also find such a tag in your web.xmlfile on your Einstein account.

<servlet>

<servlet-name>cgi</servlet-name>

<servlet-class>org.apache.catalina.servlets.CGIServlet</servlet-class>

<init-param>

<param-name>cgiPathPrefix</param-name>

<param-value>WEB-INFcgi</param-value>

</init-param>

<init-param>

<param-name>debug</param-name>

<param-value>6</param-value>

</init-param>

</servlet>

Explanation:

·The servlet-name tag defines a name that will beused later for mapping a URL pattern to the servlet.

·The servlet-class is found in the JAR file yourenamed above.

Tomcat Cgi Support

·The init-param tags define parameters that theservlet will use:

oThe parameter cgiPathPrefix tells where to lookfor your Perl scripts.It is a directoryrelative to the web application root.

oThe parameter debug causes debug output to belogged, which I found useful in debugging my configuration.(It’s how Idiscovered the problem with spaces in the path name mentioned above.)The debug output appears in the webapplication log file.Normally, this islocated in the logs directory under the Tomcat installation root.Look for a file containing the name of yourweb application.

Note: in your installationon Einstein, I have set up logging for your web application so it appears in afile under the logs directory under your web application root (webapp orwebappw).You will probably have to makethis directory accessible by “group” (mode 770) for logging to work.

The Servlet Mapping Tag

Recall that you also need to set up URL mapping for theservlet.Here is the tag I use in myhome installation:

<servlet-mapping>

<servlet-name>cgi</servlet-name>

<url-pattern>/cgi-bin/*</url-pattern>

</servlet-mapping>

Explanation:

·The servlet-name tag specifies the servlet name,which must match the name given in the servlet tag above.

·The url-pattern tagspecifies the URLs that will map to this servlet.The above tag in a web application named “MyApp” would access the Perl script myperl.pl using the URLhttp://localhost:8080/MyApp/cgi-bin/myperl.pl(assuming you are accessing Tomcat locally and are using the default port,8080.)

Tomcat Chipmunk Poison

Download Perl from http://www.perl.org/or other location of your choice.Makesure that the directory containing the Perl executable appears in your systemenvironment variable %PATH% (for Windows).You can check this using Start->Control Panel->System, for classicstyle control panel.If you enable theglitzy new style control panel, you’ll find System under Performance andMaintenance on the main control panel window.Click on the Advanced tab and use theEnvironment Variables button at the bottom.

Tomcat Cgi Servlet

Using the servlet tags described above, you should installyour Perl scripts in the WEB-INF/cgi directory under your web application rootdirectory.They should have the .pl fileextension.I was unable to get CGIServletto use the first line of the file to determine which CGI command to use, sothis extension seems to be required.