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Merge pull request #4947 from porcupineyhairs/DexLoading
Java : add query to detect insecure loading of Dex File
2 parents 7d2a60e + 11bf982 commit abeefca

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‎java/ql/src/experimental/Security/CWE/CWE-094/InsecureDexLoading.qhelp‎

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<!DOCTYPE qhelp PUBLIC "-//Semmle//qhelp//EN" "qhelp.dtd">
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<qhelp>
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<overview>
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<p>
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It is dangerous to load Dex libraries from shared world-writable storage spaces. A malicious actor can replace a dex file with a maliciously crafted file
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which when loaded by the app can lead to code execution.
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</p>
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</overview>
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<recommendation>
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<p>
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Loading a file from private storage instead of a world-writable one can prevent this issue,
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because the attacker cannot access files stored there.
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</p>
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</recommendation>
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<example>
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<p>
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The following example loads a Dex file from a shared world-writable location. in this case,
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since the `/sdcard` directory is on external storage, anyone can read/write to the location.
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bypassing all Android security policies. Hence, this is insecure.
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</p>
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<sample src="InsecureDexLoadingBad.java" />
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<p>
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The next example loads a Dex file stored inside the app's private storage.
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This is not exploitable as nobody else except the app can access the data stored there.
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</p>
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<sample src="InsecureDexLoadingGood.java" />
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</example>
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<references>
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<li>
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Android Documentation:
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<a href="https://developer.android.com/training/data-storage/">Data and file storage overview</a>.
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</li>
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<li>
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Android Documentation:
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<a href="https://developer.android.com/reference/dalvik/system/DexClassLoader">DexClassLoader</a>.
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</li>
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</references>
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</qhelp>

‎java/ql/src/experimental/Security/CWE/CWE-094/InsecureDexLoading.ql‎

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/**
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* @name Insecure loading of an Android Dex File
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* @description Loading a DEX library located in a world-writable location such as
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* an SD card can lead to arbitrary code execution vulnerabilities.
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* @kind path-problem
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* @problem.severity error
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* @precision high
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* @id java/android-insecure-dex-loading
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* @tags security
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* external/cwe/cwe-094
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*/
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import java
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import InsecureDexLoading
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import DataFlow::PathGraph
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from DataFlow::PathNode source, DataFlow::PathNode sink, InsecureDexConfiguration conf
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where conf.hasFlowPath(source, sink)
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select sink.getNode(), source, sink, "Potential arbitrary code execution due to $@.",
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source.getNode(), "a value loaded from a world-writable source."

‎java/ql/src/experimental/Security/CWE/CWE-094/InsecureDexLoading.qll‎

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import java
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import semmle.code.java.dataflow.FlowSources
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/**
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* A taint-tracking configuration detecting unsafe use of a
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* `DexClassLoader` by an Android app.
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*/
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class InsecureDexConfiguration extends TaintTracking::Configuration {
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InsecureDexConfiguration() { this = "Insecure Dex File Load" }
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override predicate isSource(DataFlow::Node source) { source instanceof InsecureDexSource }
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override predicate isSink(DataFlow::Node sink) { sink instanceof InsecureDexSink }
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override predicate isAdditionalTaintStep(DataFlow::Node pred, DataFlow::Node succ) {
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flowStep(pred, succ)
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}
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}
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/** A data flow source for insecure Dex class loading vulnerabilities. */
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abstract class InsecureDexSource extends DataFlow::Node { }
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/** A data flow sink for insecure Dex class loading vulnerabilities. */
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abstract class InsecureDexSink extends DataFlow::Node { }
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private predicate flowStep(DataFlow::Node pred, DataFlow::Node succ) {
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// propagate from a `java.io.File` via the `File.getAbsolutePath` call.
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exists(MethodAccess m |
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m.getMethod().getDeclaringType() instanceof TypeFile and
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m.getMethod().hasName("getAbsolutePath") and
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m.getQualifier() = pred.asExpr() and
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m = succ.asExpr()
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)
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or
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// propagate from a `java.io.File` via the `File.toString` call.
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exists(MethodAccess m |
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m.getMethod().getDeclaringType() instanceof TypeFile and
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m.getMethod().hasName("toString") and
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m.getQualifier() = pred.asExpr() and
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m = succ.asExpr()
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)
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or
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// propagate to newly created `File` if the parent directory of the new `File` is tainted
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exists(ConstructorCall cc |
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cc.getConstructedType() instanceof TypeFile and
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cc.getArgument(0) = pred.asExpr() and
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cc = succ.asExpr()
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)
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}
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/**
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* An argument to a `DexClassLoader` call taken as a sink for
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* insecure Dex class loading vulnerabilities.
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*/
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private class DexClassLoader extends InsecureDexSink {
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DexClassLoader() {
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exists(ConstructorCall cc |
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cc.getConstructedType().hasQualifiedName("dalvik.system", "DexClassLoader")
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|
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this.asExpr() = cc.getArgument(0)
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)
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}
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}
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/**
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* A `File` instance which reads from an SD card
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* taken as a source for insecure Dex class loading vulnerabilities.
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*/
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private class ExternalFile extends InsecureDexSource {
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ExternalFile() {
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exists(ConstructorCall cc, Argument a |
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cc.getConstructedType() instanceof TypeFile and
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a = cc.getArgument(0) and
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a.(CompileTimeConstantExpr).getStringValue().matches("%sdcard%")
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|
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this.asExpr() = a
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)
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}
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}
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/**
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* A directory or file which may be stored in an world writable directory
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* taken as a source for insecure Dex class loading vulnerabilities.
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*/
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private class ExternalStorageDirSource extends InsecureDexSource {
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ExternalStorageDirSource() {
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exists(Method m |
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m.getDeclaringType().hasQualifiedName("android.os", "Environment") and
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m.hasName("getExternalStorageDirectory")
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or
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m.getDeclaringType().hasQualifiedName("android.content", "Context") and
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m.hasName([
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"getExternalFilesDir", "getExternalFilesDirs", "getExternalMediaDirs",
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"getExternalCacheDir", "getExternalCacheDirs"
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])
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|
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this.asExpr() = m.getAReference()
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)
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}
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}

‎java/ql/src/experimental/Security/CWE/CWE-094/InsecureDexLoadingBad.java‎

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import android.app.Application;
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import android.content.Context;
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import android.content.pm.PackageInfo;
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import android.os.Bundle;
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import dalvik.system.DexClassLoader;
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import dalvik.system.DexFile;
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public class InsecureDexLoading extends Application {
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@Override
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public void onCreate() {
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super.onCreate();
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updateChecker();
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}
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private void updateChecker() {
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try {
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File file = new File("/sdcard/updater.apk");
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if (file.exists() && file.isFile() && file.length() <= 1000) {
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DexClassLoader cl = new DexClassLoader(file.getAbsolutePath(), getCacheDir().getAbsolutePath(), null,
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getClassLoader());
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int version = (int) cl.loadClass("my.package.class").getDeclaredMethod("myMethod").invoke(null);
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if (Build.VERSION.SDK_INT < version) {
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Toast.makeText(this, "Loaded Dex!", Toast.LENGTH_LONG).show();
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}
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}
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} catch (Exception e) {
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// ignore
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}
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}
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}

‎java/ql/src/experimental/Security/CWE/CWE-094/InsecureDexLoadingGood.java‎

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public class SecureDexLoading extends Application {
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@Override
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public void onCreate() {
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super.onCreate();
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updateChecker();
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}
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private void updateChecker() {
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try {
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File file = new File(getCacheDir() + "/updater.apk");
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if (file.exists() && file.isFile() && file.length() <= 1000) {
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DexClassLoader cl = new DexClassLoader(file.getAbsolutePath(), getCacheDir().getAbsolutePath(), null,
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getClassLoader());
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int version = (int) cl.loadClass("my.package.class").getDeclaredMethod("myMethod").invoke(null);
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if (Build.VERSION.SDK_INT < version) {
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Toast.makeText(this, "Securely loaded Dex!", Toast.LENGTH_LONG).show();
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}
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}
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} catch (Exception e) {
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// ignore
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}
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}
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}

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