BSD 3-Clause License
Jan 8, 2017
See also
Dex 65536


archive/dynamic (Nov 5, 2015)
Aug 14, 2012
May 20, 2021 (Retired)
Programming Languages at University of Maryland (plum-umd)
Moshe Kaplan (moshekaplan)
Jinseong Jeon (jsjeon)
Kristopher Micinski (kmicinski)
Nikolaos Kofinas (eldr4d)
Taehun Kim (TaeHunKim)
Kubo Kováč (kuk0)
Source code



Redexer is a reengineering tool that manipulates Android app binaries. This tool is able to parse a DEX file into an in-memory data structure; to infer with which parameters the app uses certain permissions (we name this feature RefineDroid); to modify and unparse that data structure to produce an output DEX file (we name these features Dr. Android, which stands for Dalvik Rewriting for Android).



  • OCaml and Ruby

This tool is tested under OCaml 4.09.0 and Ruby 1.8.6(7), so you need to install them (or higher versions of them).

  • OCaml package/library manager and SHA library

To manipulate a SHA-1 signature (hash) in the DEX format, we utilize OCaml SHA library via ocamlfind/findlib, an OCaml library manager. The easiest way to install both is using OPAM, an OCaml package manager, which has both packages---OPAM ocamlfind and OPAM sha.

You can also build and/or install both packages directly. If you're using a linux machine, you can easily find distributions.

If not, e.g., using a Mac, you need to build it by yourself. You can find the original source codes at here. Build it by running make, and link the resulting directory into ocamlfind's root site-lib directory; or sudo make install.

If you're using a PC, you need to install ocamlfind/findlib and FlexDLL first. Please make sure that your environment variables are set correctly as follows:

FLEXLINKFLAGS=-L%MinGW%\lib -L%MinGW%\lib\gcc\mingw32\N.N.N
  • OPAM packages:

    • sha
    • str
    • ppx_deriving
    • ppx_deriving_yojson
    • yojson
  • Android SDK (or sources)

To unpack and repack apk files, we use apktool, an open source APK reengineering tool. Since it uses aapt, Android Asset Packaging Tool, you need to install Android SDK or sources. Besides, we use zipalign, which also comes from Android SDK, to optimize rewritten apps.

You can set paths to Android base tools by adding the followings to your profile:

ANDROID_HOME=$HOME/android-sdk    # your own path here!

PATH=$PATH:$ANDROID_HOME/build-tools/19.0.0    # installed version number
export PATH
  • RubyGems and Nokogiri

The main scripts are written in Ruby and require RubyGems, a Ruby package manager, and Nokogiri, an XML library to manipulate manifest files.

  • (optional) graphviz dot

If you want to see graphs (e.g. call graph, control-flow graph, dominator tree, etc.), you need to install graphviz dot.


To build redexer, just make! You can see redexer binary at the top level.

$ make (clean)

Before using the tool, installing the most recent platform file for apktool is up to users. For instance, you need to do like

$ java -jar tools/apktool.jar if [proper platform file]

You can generate API documents in html format as well.

$ make api


  • help

You can see all the options the tool provides:

$ ruby scripts/cmd.rb -h
$ ruby scripts/cmd.rb --help
  • unparse

Like dexdump at Android SDK, redexer allows you to view the internals of the given dex file in a YAML format.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd unparse [--to blah.yml]
  • dump method

This option dumps instructions for a specified method.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd dump_method --mtd cls.mtd
  • identity

This feature is to test parsing and dumping modules of redexer. It probably generates an identical dex file.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd id [--to blah.dex]
  • info

You can also see basic statistics about the dex file, e.g., # instr.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd info
  • classes

This option prints out all class names defined in the dex file.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd classes

It may be useful to search specific third-party libraries, e.g.,

$ ruby scripts/cmd.rb target.(apk|dex) --cmd classes | egrep 'apache'
  • api

This option prints out API usage in the dex file.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd api [--sdk android.]
$ ruby scripts/cmd.rb target.(apk|dex) --cmd api --sdk com.facebook.
  • opcode statistics

Aren't you curious how rarely some opcodes are used in Dalvik bytecodes? It will show you the histogram of entire opcodes, or you can look up how often the exact opcode is used in the given app.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd opstat [--op opcode1,opcode2,...]

For instance,

$ ruby scripts/cmd.rb ~/apps/top24/com.whatsapp.apk --cmd opstat
$ ruby scripts/cmd.rb ~/apps/top24/com.whatsapp.apk --cmd opstat --op div-int/lit16,nop
  • Intent resolution analysis

This option conducts propagation-based Intent resolution analysis, and prints out transitions amongst Activity classes.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd intent
  • call graph

This option generates a pdf file that depicts a call graph of the given file. If you don't specify the pdf name, cg.pdf will be used.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd cg [--to blah.pdf] [--no-pdf]
  • control-flow graph

This option generates a pdf file that shows a control-flow graph of the given method. Append a method name to a class name with dot: class_name.method_name

$ ruby scripts/cmd.rb target.(apk|dex) --cmd cfg --mtd cls.mtd [--to blah.pdf] [--no-pdf]
  • (post) dominator tree

This option is similar to the above feature, except for that it depicts (post) dominator tree.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd (p)dom --mtd cls.mtd [--to blah.pdf] [--no-pdf]
  • liveness analysis

This option performs a classic backward data-flow analysis.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd live --mtd cls.mtd
  • constant-propagation analysis

This option conducts a classic forward data-flow analysis.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd const --mtd cls.mtd
  • reaching definition analysis

This option conducts a classic forward data-flow analysis.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd reach --mtd cls.mtd
  • dependants

This option finds class dependency.

$ ruby scripts/cmd.rb target.(apk|dex) --cmd dependants --mtd cls.mtd
  • launcher activity

This option prints out the launching activity name of the given apk.

$ ruby scripts/cmd.rb target.apk --cmd launcher
  • exported

This option lets you know what components are exposed to outside.

$ ruby scripts/cmd.rb target.apk --cmd exported
  • activity, service, provider, receiver

These options print out Android basic components declared in the manifest.

$ ruby scripts/cmd.rb target.apk --cmd [activity | service | provider | receiver]
  • custom_views

This option explores layout definitions in resource and prints out custom views.

$ ruby scripts/cmd.rb target.apk --cmd custom_views
  • fragments

This option explores layout definitions in resource and prints out Fragment elements.

$ ruby scripts/cmd.rb target.apk --cmd fragments
  • buttons

This option explores layout definitions in resource and prints out Buttons, along with their id (or text) as well as callback method (if exists).

$ ruby scripts/cmd.rb target.apk --cmd buttons
  • permissions declared in the app's manifest

This option prints out permissions the apk uses.

$ ruby scripts/cmd.rb target.apk --cmd permissions
  • target SDK version

This option prints out what SDK version the apk requires.

$ ruby scripts/cmd.rb target.apk --cmd sdk

If you don't want to unpack the apk file, actually, you can do the same thing with a combination of commands:

$ aapt dump badging target.apk | grep 'targetSdkVersion' | tr -dc 0-9.\\n
  • Hello, DEX

This option would generate a dex file that prints out a simple message. That dex file is made only using redexer's APIs.

$ ruby scripts/cmd.rb --cmd hello

Check its internal.

$ dexdump -d results/classes.dex

If you're interested, you can test that dex file as follows. Assume path to ANDROID_SDK is set.

// create a temporary jar suitable for dalvik VM
$ aapt add temp.jar results/classes.dex

// (optional) if you didn't make an avd, then make it.
$ android create avd -n myAVD1 -t android-8

// launch your emulator
$ emulator -avd myAVD1 &

// push the temporary jar
$ adb push temp.jar /data

// connect to adb shell
$ adb shell

// at last, run the dex
# /system/bin/dalvikvm -Xbootclasspath:/system/framework/core.jar \
 -classpath /data/temp.jar Hello
Hello, DEX
  • logging

This is a variant of the rewrite feature. Using this feature, you can log apps behavior from specific points of view. The pre-built dex file for logging library is provided: data/logging.dex. If you want to add more features or utilities, build it as follows:

$ cd logging
$ gradle copyDex
$ cd ..

Then, use the following command:

$ ruby scripts/cmd.rb target.apk --cmd logging can capture the call-return sequences of the instrumented app. (You should first instrument the app under test using redexer.)

If those logs are short enough, i.e., the phone (or emulator) can hold all information in the memory, you may use the offline mode of the script:

$ ./scripts/ -d

Note that all command-line parameters will be passed to adb logcat, and by default, org.umd.logging:I *:S is passed to filter out irrelevant logs.

If logs overflow, you should use the online mode:

$ ./scripts/

The script catches key interrupt, so you can finish logging via Ctrl+C.

In either mode, logs are saved in log.txt and shown to the screen at once. Thus, after collecting logs, you may need to move that file, e.g.:

$ mv log.txt app.scenario.txt
  • logging user interactions

The logging feature above is general in that you can specify what to log at a method level. (See logging module for more details.) However, this is sometimes too verbose and may induce performance degradation. This feature is designed to log only user interactions. Using this feature, you can capture UI-related events only. Similarly, the pre-built dex file for logging library is provided: data/logging-ui.dex. If you want to modify the verbosity of UI information, build it as follows:

$ cd logging-ui
$ gradle copyDex
$ cd ..

Then, use the following command:

$ ruby scripts/cmd.rb target.apk --cmd logging_ui

The logging library is inherited from android a11y service, which requires user's explicit consent. Thus, after installing the rewritten apk, go to Settings/Accessibility and turn on UI Logging service. (This step can be viewed similar to turning on device debugging mode.) In the logcat, messages with tags org.umd.logging_ui.* are interactions between the user and the app under test.

  • directed exploration

This option finds component transition paths towards target method calls.

$ ruby scripts/cmd.rb target.apk --cmd directed

You can specify target methods to be invoked in data/directed.txt

Those component transition paths are used to drive apps to test security vulnerabilities in 3rd party libraries. More details are described at the following paper:

* Brahmastra: Driving Apps to Test the Security of Third-Party Components.
R. Bhoraskar, et al., In 23rd Usenix Security Symposium (Security '14).

withTimeout.rb can build a skip file for an app automatically. This script runs cmd.rb with a specific timeout, used to limit the time spent instrumenting any single class. This is a temporary work-around for the occasional class that gets stuck in a loop during instrumentation. When this script finds a class that crashes, it adds it to the skip file and continues from where it left off. Once withTimeout has terminated, there will be a file called [apk name]-skip.txt in the data directory, which can be used to build a fully-instrumented app for that apk. To use withTimeout, simply call

$ ruby scripts/withTimeout.rb TIMEOUT COMMANDS

Where TIMEOUT is the length of the timeout you want in seconds (300 is recommended) and COMMANDS are all the regular command-line inputs you would pass to scripts/cmd.rb for the apk.