mirror of
https://github.com/carlospolop/hacktricks
synced 2024-11-26 22:52:06 +00:00
69 lines
2.1 KiB
Markdown
69 lines
2.1 KiB
Markdown
|
# Google CTF 2018 - Shall We Play a Game?
|
||
|
|
|
||
|
|
Download the APK here:
|
||
|
|
|
||
|
|
I am going to upload the APK to [https://appetize.io/](https://appetize.io/) \(free account\) to see how the apk is behaving:
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
Looks like you need to win 1000000 times to get the flag.
|
||
|
|
|
||
|
|
Following the steps from [pentesting Android](./) you can decompile the application to get the smali code and read the Java code using jadx.
|
||
|
|
|
||
|
|
Reading the java code:
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
It looks like the function that is going print the flag is **m\(\).**
|
||
|
|
|
||
|
|
## **Smali changes**
|
||
|
|
|
||
|
|
### **Call m\(\) the first time**
|
||
|
|
|
||
|
|
Lets make the application call m\(\) if the variable _this.o != 1000000_ to do so, just cange the condition:
|
||
|
|
|
||
|
|
```text
|
||
|
|
if-ne v0, v9, :cond_2
|
||
|
|
```
|
||
|
|
|
||
|
|
to:
|
||
|
|
|
||
|
|
```text
|
||
|
|
if-eq v0, v9, :cond_2
|
||
|
|
```
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
Follow the steps of [pentest Android](./) to recompile and sign the APK. Then, upload it to [https://appetize.io/](https://appetize.io/) and lets see what happens:
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
Looks like the flag is written without being completely decrypted. Probably the m\(\) function should be called 1000000 times.
|
||
|
|
|
||
|
|
**Other way** to do this is to not change the instrucction but change the compared instructions:
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
**Another way** is instead of comparing with 1000000, set the value to 1 so this.o is compared with 1:
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
A forth way is to add an instruction to move to value of v9\(1000000\) to v0 _\(this.o\)_:
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
## Solution
|
||
|
|
|
||
|
|
Make the application run the loop 100000 times when you win the first time. To do so, you only need to create the **:goto\_6** loop and make the application **junp there if** _**this.o**_ **does not value 100000**:
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
You need to do this inside a physical device as \(I don't know why\) this doesn't work in an emulated device.
|
||
|
|
|