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Date: Tue, 30 Apr 2019 22:35:58 -0400
From: Nightwatch Cybersecurity Research <research@...htwatchcybersecurity.com>
To: fulldisclosure@...lists.org
Subject: [FD] RCE in CGI Servlet – Apache Tomcat on Windows – CVE-2019-0232

[Original post:
https://wwws.nightwatchcybersecurity.com/2019/04/30/remote-code-execution-rce-in-cgi-servlet-apache-tomcat-on-windows-cve-2019-0232/]

SUMMARY

Apache Tomcat has a vulnerability in the CGI Servlet which can be
exploited to achieve remote code execution (RCE). This is only
exploitable when running on Windows in a non-default configuration in
conjunction with batch files. The vendor released a fix in Tomcat
versions 7.0.94, 8.5.40 and 9.0.19. Users are encouraged to upgrade as
soon as possible. CVE-2019-0232 has been assigned to track this issue.

VULNERABILITY DETAILS

Common Gateway Interface (CGI) is a standard protocol to allow web
servers to execute command line programs / scripts via web requests.
This protocol also allows passing of command line arguments to the
script or program being executed via URL parameters. The protocol
itself is defined in RFC 3875.

Apache Tomcat supports execution of CGI scripts / programs in a
non-default configuration via a special CGI servlet. This servlet also
parses URL parameters and translates them into command line arguments.
The actual execution of the CGI scripts happens via Java Runtime
Environment (JRE)’s java.lang.Runtime class, exec() function.

When CGI support is enabled in Apache Tomcat in Windows, and command
line argument passing is enabled, it is possible to cause command
injection via parameter interpolation when calling a batch file (*.bat
/ *.cmd). This happens because “cmd.exe” performs interpolation on
some special characters before execution which can cause other shell
commands to be called. Neither Apache Tomcat or the Windows JRE
perform any kind of input validation for these special characters.

STEPS TO REPLICATE
1. Install a Java Runtime Environment (JRE) in Windows.
2. Download a vulnerable version of Tomcat and extract.
3. Modify the conf\context.xml file on line 19, to enable privileged context:
<Context privileged="true">

4. Modify conf\web.xml to enable the CGI Servlet by removing the
comments around line 387 as follows and adding the following
parameters (enableCmdLineArguments is only needed for Tomcat 9):
<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-INF/cgi</param-value>
</init-param>
<init-param>
  <param-name>executable</param-name>
  <param-value></param-value>
</init-param>
<init-param>
  <param-name>enableCmdLineArguments</param-name>
  <param-value>true</param-value>
</init-param>
<load-on-startup>5</load-on-startup>
</servlet>

5. Enable the CGI servlet by removing comments around this – you also
need to change the URL pattern to match the one in the previous step
(“cgi”):
<servlet-mapping>
<servlet-name>cgi</servlet-name>
<url-pattern>/cgi/*</url-pattern>
</servlet-mapping>

6. Create a folder for the CGI files:
mkdir webapps\ROOT\WEB-INF\cgi

7. Place the following text into a batch file located in
“webapps\ROOT\WEB-INF\cgi\test.bat”
@echo off
echo Content-Type: text/plain
echo.
echo Hello, World!

8. Run Tomcat via the following command:
cd bin
catalina run

9. Trigger the following URLs and observe the dir command being run:
http://localhost:8080/cgi/test.bat?&dir

ADDITIONAL NOTES – ENVIRONMENT VARIABLES AND PATH

By default, Tomcat doesn’t pass all of the environment variables from
the parent process that runs Tomcat itself. That means that if you run
“set”, you will not see any environment variables other than those set
by Tomcat itself. This also means that you would need to spell out the
directory path of the command you are trying to run. However, if the
“passShellEnvironment” parameter is set to true, the variables from
the parent process will be passed through and you can call any command
in PATH as well as view those variables. If the command cannot be
found, there will be a error in the console log “XXXX is not
recognized as an internal or external command”.

ADDITIONAL NOTES – MEMORY LEAKS / DENIAL OF SERVICE

If the command being executed is a long running command, it maybe
possible to cause a denial of service or a memory leak. This happens
because Tomcat waits for the OS process to complete.

ADDITIONAL NOTES – OTHER COMMANDS AND STDERR

The “executable” parameter indicates which executable should be used
to run the script. By default, this is set to “perl” with the
expectation that the files being executed are Perl scripts. If this is
set to empty, then it is possible to execute batch files since those
are executed by “cmd.exe”. HOWEVER, it seems that the command
interpolation only happens with batch files – if this is set to real
program, then command interpolation doesn’t necessary occur and this
vulnerability may be not exploitable.

Also, if the command being triggered outputs to STDERR instead of
STDOUT, that output doesn’t get piped back to the web request –
instead it goes to the Tomcat console log.

VENDOR RESPONSE

This issue was responsibly reported to the vendor via the EU FOSSA
bounty program operated by Intigriti. Vendor analysis indicated that
the core cause for this issue has to do with the way the Java Runtime
Environment (JRE) interprets command arguments in Windows specifically
and doesn’t impact Apache Tomcat when used with other operating
systems. The vendor assigned CVE-2019-0232 to track this issue and
provided a fix.

The vendor fix consists of two parts:
* Disabling command line arguments from being passed to the CGI
servlet in the default configuration (“enableCmdLineArguments” set to
“false“) for Tomcat 7 and 8  – this was already disabled by default in
Tomcat 9.
* Adding a new configuration parameter (“cmdLineArgumentsDecoded“) to
the default CGI configuration that will be used for input validation
if passing of command line arguments is enabled and will be set to the
following regular expression (OS specific). Note that if the user
changes this parameter, they may become vulnerable again.
*** Windows - [[a-zA-Z0-9\Q-_.\\/:\E]+]
*** Other operating systems - [.*]

AFFECTED VERSIONS AND MITIGATION

Apache Tomcat is only vulnerable to this issue if the following
conditions are met:
* Running on Windows
* CGI support is enabled either via the web.xml for a specific web
application or the server as whole (see documentation). This is
disabled by default.
* The “privileged” setting is set to “true” in the Context element.
This is “false” by default.
* Tomcat 9 only – enableCmdLineArguments is set to “true” (enabled by
default in Tomcat 7 and 8)
* The “executable” parameter is empty, and the the CGI scripts being
executed are batch files (either .bat or .cmd). It is not clear if
other commands that use “cmd.exe” are vulnerable as well.

The vendor indicated that the following versions are vulnerable (no
information is available on earlier versions):
* Tomcat 9 – versions 9.0.0.M1 through 9.0.17 (9.0.18 is not affected)
* Tomcat 8 – versions 8.5.0 to 8.5.39
* Tomcat 7 – versions 7.0.0 to 7.0.93

Users are encouraged to upgrade to the following fixed versions or later:
* Tomcat 9 – version 9.0.19 – details and code commit
* Tomcat 8 – version 8.5.40 – details and code commit
* Tomcat 7 – version 7.0.94 – details and code commit

IMPORTANT NOTE: even when running a fixed version, you SHOULD NOT
change the “cmdLineArgumentsDecoded” configuration parameter to a
different value. If you do, your installation may become vulnerable.

If an upgrade is not possible, users can apply one of the following mitigations:
* Disable CGI support (it is disabled by default)
* Or set the “enableCmdLineArguments” parameter to “false”. This
setting will disable command line arguments from being passed via the
CGI servlet.

BOUNTY INFORMATION

This report satisfied the requirement of the EU FOSSA bounty program
and a bounty has been paid.

REFERENCES

Blog post on JRE behavior: see here (Markus Wulftange)
[https://codewhitesec.blogspot.com/2016/02/java-and-command-line-injections-in-windows.html]
CGI Standard: RFC 3875 [https://tools.ietf.org/html/rfc3875]
CVE-ID: CVE-2019-0232
CVSS v2.0 Score: 9.3 – (AV:N/AC:M/Au:N/C:C/I:C/A:C)
CVSS v3.0 Score: 8.1 – (AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H)
Tomcat CGI Servlet source code: see GitHub
[https://github.com/apache/tomcat/blob/master/java/org/apache/catalina/servlets/CGIServlet.java]
Vendor advisory: see here
[http://mail-archives.us.apache.org/mod_mbox/www-announce/201904.mbox/%3C13d878ec-5d49-c348-48d4-25a6c81b9605%40apache.org%3E]

CREDITS

Text of the advisory written by Yakov Shafranovich.

TIMELINE

2019-02-14: Initial report submitted to the platform
2019-02-17: Initial report validated by the platform
2019-03-03: Report acknowledged by the vendor
2019-03-14: Interim evaluation received from the vendor
2019-03-22: Communication with the vendor
2019-04-10: Public advisory issued by the vendor
2019-04-30: Public disclosure by reporter

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