[Q] Doubts about installing modified system .apks - Galaxy S II Q&A, Help & Troubleshooting

For example, let's say, I wanna change a widget that comes in stock roms, let's say digital clock or whatever another widget.
I've read some ways for doing this:
- With root explorer or command line, push the new .apk into the required folder and overwrite the apk (save the old one before!) then reboot.
- Via recovery, install from zip the app and reboot.
- Just install the .apk normally like it was a new app.
I've read that you can't use third method if you have the app installed already as system, so let's ignore that method.
About methods 1 and 2:
Can you choose method for any app? I mean, can I choose to replace an apk that had the instructions to be installed via recovery in a zip? can I do the inverse?
And a big doubt:
Can this affect the phone in any way? I mean replacing system widget or system minor apps, not dialer, settings or anything important.
Even if you put a bad file as the new .apk, the worst thing that can happen is that you wont find the widget or the app right? and its easilly solved by replacing it with the saved copy of the original system apk?
Thanks in advance for the help!

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I am sorry for bumping this, but can't anyone for really ask this noob question? :'(

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Genrauser said:
Even if you put a bad file as the new .apk, the worst thing that can happen is that you wont find the widget or the app right? and its easilly solved by replacing it with the saved copy of the original system apk?
Thanks in advance for the help!
Click to expand...
Click to collapse
Well, worst case scenario is that you install some malware. But that shouldn't be a problem if you trust the source.
If the app/widget does not work, it shouldn't have impact on your system as long as it's not a critical app like the music player.
Before you mess with such files, you should better backup the entire ROM with cwm anyway.
Sent from my GT-I9100 using XDA App

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TheSniperFan said:
Well, worst case scenario is that you install some malware. But that shouldn't be a problem if you trust the source.
If the app/widget does not work, it shouldn't have impact on your system as long as it's not a critical app like the music player.
Before you mess with such files, you should better backup the entire ROM with cwm anyway.
Sent from my GT-I9100 using XDA App
Click to expand...
Click to collapse
Okay, thanks for the reply!
So yeah, you can play with non-critical system apps as I thought.
About the 'malware' stuff, isn't it dependant on which app you're replacing?
For example, if I replace a digital clock widget, malware couldn't do **** since this hasn't any kind of permissions right?

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Not really.
I mean it's easy to write a Keyboard-App-replacement with a build-in keylogger. However the malware might use exploits.
For example you can replace the music player and the infected one uses an exploit for "privilege escalation" to gain root rights.
Unlikely but possible.
It's just one example. You have to understand that malware doesn't need the system to allow anything, if it's well written.
You should just make sure that you get the apps from a trusted source.
Sent from my GT-I(OVER 9000!!!!) using XDA App

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TheSniperFan said:
Not really.
I mean it's easy to write a Keyboard-App-replacement with a build-in keylogger. However the malware might use exploits.
For example you can replace the music player and the infected one uses an exploit for "privilege escalation" to gain root rights.
Unlikely but possible.
It's just one example. You have to understand that malware doesn't need the system to allow anything, if it's well written.
You should just make sure that you get the apps from a trusted source.
Sent from my GT-I(OVER 9000!!!!) using XDA App
Click to expand...
Click to collapse
Okay I see.. thanks a lot for your help!