I thought granting internet access to apps is not avoidable on Android.
When an app does not request internet, is it really guaranteed that it cannot talk to the outside world? Or is it having other avenues like opening a browser or some other component with a custom url or something?
Update: I just asked Gemini, and it does not look good:
An Android app without the INTERNET permission is not guaranteed to be isolated from the outside world. While it cannot make direct network connections itself, it can use several other mechanisms to transmit data externally:
Intent-Based Communication (The Browser) An app can launch an explicit or implicit Intent to hand data over to another app that does have internet access.
That means the app can open a system browser using a URL containing the data it wants to transmit.
It can also load a Chrome Custom Tab inside its own UI task, passing data through the URL string.
There is also Inter-Process Communication: If two apps from the same developer share a User ID (sharedUserId), they run in the same process and share all permissions, including internet access.
There is also the concept of Content Providers: Content Providers allow apps to share data. An offline app can write data into a shared database or a public Content Provider. A secondary, online-enabled app can then read that database and upload the contents.
> That means the app can open a system browser using a URL containing the data it wants to transmit.
This can be mitigated by using an app like URLCheck [1], which you set as your default browser, and it shows and lets you edit the URL being opened before handing it off to your real browser. It also has neat features like choosing to open in incognito (if you use Firefox) and automatically removing tracking parameters.
This is all very true, unfortunately. Google designed Android as a an OS for internet appliances. They're not interested in local-only apps working on local files.
But I think the OP is to be complimented for what they've built and their intention in making it local-only. I've long felt that apps that *genuinely" respect privacy are one of the few areas of opportunity still left in the mobile space. I wish there were more develpers building things like this.
I guess that makes sense. Since most non-privacy-focused Android distributions don't let users turn off the internet permission, keeping the permission secure likely ceased to be a priority.
The full list of bypasses is likely much larger because it doesn't fall in the scope of bug bounties.
"I just asked Gemini, and it turns out that Mossad can steal my data anyways".
- An app without the INTERNET permission will crash the moment you try to access the internet. It's like a rite of passage of every android developer with every new project, you forgot the permission.
- Launching an Intent is EXTREMELY visible. It opens a full on browser. It's limited to a GET with a dedicated URL, so what are they going to do, stuff your data in query params ?
- Even in the case of another app being installed that would silently receive this intent and not pop an activity, you need a different app. It cannot be an activity added by the same package.
- Loading a Chrome Custom Tab opens a whole ass browser in front of you, think you're going to miss it ?
- Shared user IDs also require two different app installations and you cannot declare multiple. You also cannot have sharedUserId if you do not sign the apps with the same key. you cannot sharedUserId with Facebook, you need once again a dedicated app installation.
- A ContentProvider needs a dedicated permission on the writing app AND on the reading app, which is once again very visible.
I'll add more to your list: an app can request to write a file inside your downloads folder, and another one can show a popup asking you to open it! And if someone shows up with a hammer at your door, he can also hit your fingers really hard to make you tell him the data.
Android apps are, by default, very well isolated. INTERNET is a permission like any other, just not surfaced as a dangerous one (like permanent access to your background location would be), or a runtime one (like access to your current location while the app is in foreground)
Yeah, Android's sandbox deals with low-level file access and socket APIs and stuff. But Android also, intentionally, allows apps to expose their data and functionality to other apps via the higher-level "intent" system.
Apps get to choose what permissions are needed to access their intents, so under Android's security model, really it's Chrome's fault (or whatever browser the user has installed) for exposing an intent that allows apps that don't have the INTERNET permission to exfoltrate data.
Similarly, apps are also allowed to collude to share data with each other if they want; that's how stuff like Google Play Services works.
> Apps get to choose what permissions are needed to access their intents, so under Android's security model, really it's Chrome's fault (or whatever browser the user has installed) for exposing an intent that allows apps that don't have the INTERNET permission to exfoltrate data.
You might as well blame the phone for not being encased in concrete and thrown into a well. You might not want your text message application to have the internet permission, but you'd certainly want to be able to open links from it.
As I understand it, this "Application Sandbox" is to prevent app A seeing what app B is doing.
The problem here is a different one: The user granting an app access to their files and then having no way to prevent that app from sending that data to the developer of the app.
Literally there for more protection. Split your app into different apps, each handling a component. Want a banking app, without access to registering your card for NFC payments ? Split the app.
sharedUserIds cannot be applied blindly, they need the app to be signed with the same key, both need to declare their sharedUserId to be the same, etc.
Yeah fair I was too flat in the post... No INTERNET means my process can't reach the network. It doesn't stop an app passing data to something that can, so your list is right.
It's not a guarantee then, it's just a lot less to check. There's no silent route out so all that's left is what the app chooses to send via intents and Gander sends two, both when you tap something ("Share" and "Show in file manager")
Nothing anywhere builds a URL out of the file contents. No browser launch, no Custom Tabs and no shared user ID. The only content provider is closed to other apps. The one time a file goes anywhere is when you hit Share and pick the app, and what it gets is read access to that one file.
In case you grep it there's an https:// in ViewerActivity.kt. That's the local virtual host WebViewAssetLoader serves the bundled renderers from, it never leaves the process.
Thanks for sharing. Could it be possible to add optional force-dark mode for documents as well? The feature list mentioned dark mode, but that's for app's own UI. The really awesome feature would be to be able to invert the document colors so that the document itself is shown in dark mode. Thanks.
The app is pleasingly very responsive. I don't think I've used anything else on Android that lets me open and close PDFs rapidly without getting bogged down. Nice!
ods actually already works.. it goes through the same SheetJS path as xlsx and csv. But your question made me check properly and there's a bug... I never registered the OpenDocument mime type so Gander wont show up in the "open with" list when you tap a .ods. Opening it from inside the app works fine.
Fixed in the manifest just now, it'll be in the next build.
odt and odp dont work at all. odt is a fair ask.. its the same shape of problem as docx so it's doable. Less sure about odp though.. the pptx renderer is already the weakest part of this.
And yeah, fewer apps on the phone was pretty much the whole reason I started building this.
pptx by a mile. A slide is absolutely positioned shapes with a whole layout inheritance thing going on so the JS renderer ends up reimplementing a whole layout engine, and it gets you to "approximately right" and stops there. Charts I gave up on.
docx was annoying in a different way. Bullets came out as junk characters for a while and I couldnt work out why. I then found that Word writes them as private use area codepoints in the Symbol font so unless you have that exact font you get some junk. I just map them back to unicode after the render.
xlsx was the easy one, SheetJS does the hard part.
On faithful though none of this is faithful the way LibreOffice is. Its more that "you can read the document and it looks roughly right" which is what I wanted out of an app of 14MB.
You don’t need internet to run JS though. And the JS seems to be pretty sandboxed in normal viewers too, so a pdf file shouldn’t make network requests on its own. As long as there is no iframe-like content I don’t see how rendering a document would ever need network.
I had the goal to eliminate annoying ads in some apps that I use. I deleted the user data for those apps and also applied restrictions so they could no longer access internet. Ads still showed up.
I mostly stopped using these types of apps and switched to F-Droid alternatives.
Sorry, that's on my README. That line is under "Build from source" so it's what you need to compile it, not to run it. minSdk is 26, so Android 8 and up.
Android 14 is well past the floor, you're good to go!
Great. You built an excellent tool to solve a real-world problem: being able to open files on the fly without granting internet access and without having to install 400 MB bloated suites that track everything.
> A tiny, open source, fully offline file viewer for Android that opens PDF, Word,
Perhaps correct Word to DOCX. Word is not just DOCX and DOCX is not just Word. This user needs DOC viewing and would be disappointed having installed this app.
I thought granting internet access to apps is not avoidable on Android.
When an app does not request internet, is it really guaranteed that it cannot talk to the outside world? Or is it having other avenues like opening a browser or some other component with a custom url or something?
Update: I just asked Gemini, and it does not look good:
An Android app without the INTERNET permission is not guaranteed to be isolated from the outside world. While it cannot make direct network connections itself, it can use several other mechanisms to transmit data externally:
Intent-Based Communication (The Browser) An app can launch an explicit or implicit Intent to hand data over to another app that does have internet access.
That means the app can open a system browser using a URL containing the data it wants to transmit.
It can also load a Chrome Custom Tab inside its own UI task, passing data through the URL string.
There is also Inter-Process Communication: If two apps from the same developer share a User ID (sharedUserId), they run in the same process and share all permissions, including internet access.
There is also the concept of Content Providers: Content Providers allow apps to share data. An offline app can write data into a shared database or a public Content Provider. A secondary, online-enabled app can then read that database and upload the contents.
This can be mitigated by using an app like URLCheck [1], which you set as your default browser, and it shows and lets you edit the URL being opened before handing it off to your real browser. It also has neat features like choosing to open in incognito (if you use Firefox) and automatically removing tracking parameters.
[1] https://f-droid.org/packages/com.trianguloy.urlchecker
But I think the OP is to be complimented for what they've built and their intention in making it local-only. I've long felt that apps that *genuinely" respect privacy are one of the few areas of opportunity still left in the mobile space. I wish there were more develpers building things like this.
Edit: also that LLMs are tuned for "engagement" not for answers like "it's secure enough, move along"?
The full list of bypasses is likely much larger because it doesn't fall in the scope of bug bounties.
- An app without the INTERNET permission will crash the moment you try to access the internet. It's like a rite of passage of every android developer with every new project, you forgot the permission.
- Launching an Intent is EXTREMELY visible. It opens a full on browser. It's limited to a GET with a dedicated URL, so what are they going to do, stuff your data in query params ?
- Even in the case of another app being installed that would silently receive this intent and not pop an activity, you need a different app. It cannot be an activity added by the same package.
- Loading a Chrome Custom Tab opens a whole ass browser in front of you, think you're going to miss it ?
- Shared user IDs also require two different app installations and you cannot declare multiple. You also cannot have sharedUserId if you do not sign the apps with the same key. you cannot sharedUserId with Facebook, you need once again a dedicated app installation.
- A ContentProvider needs a dedicated permission on the writing app AND on the reading app, which is once again very visible.
I'll add more to your list: an app can request to write a file inside your downloads folder, and another one can show a popup asking you to open it! And if someone shows up with a hammer at your door, he can also hit your fingers really hard to make you tell him the data.
Android apps are, by default, very well isolated. INTERNET is a permission like any other, just not surfaced as a dangerous one (like permanent access to your background location would be), or a runtime one (like access to your current location while the app is in foreground)
I'm assuming that all works even with application sandboxing (1) or am I mis-reading how that is applied to applications.
Man I need to move my movement to GrapheneOS up to be sooner.
(1) https://source.android.com/docs/security/app-sandbox
Apps get to choose what permissions are needed to access their intents, so under Android's security model, really it's Chrome's fault (or whatever browser the user has installed) for exposing an intent that allows apps that don't have the INTERNET permission to exfoltrate data.
Similarly, apps are also allowed to collude to share data with each other if they want; that's how stuff like Google Play Services works.
You might as well blame the phone for not being encased in concrete and thrown into a well. You might not want your text message application to have the internet permission, but you'd certainly want to be able to open links from it.
The problem here is a different one: The user granting an app access to their files and then having no way to prevent that app from sending that data to the developer of the app.
Good grief. What a huge vulnerability. Is there some benefit that justifies this?
You need something like that for plugin / extension like system.
Third, it is being deprecated. and people are screaming "bad google" "android is no longer free!"
sharedUserIds cannot be applied blindly, they need the app to be signed with the same key, both need to declare their sharedUserId to be the same, etc.
It's not a guarantee then, it's just a lot less to check. There's no silent route out so all that's left is what the app chooses to send via intents and Gander sends two, both when you tap something ("Share" and "Show in file manager")
Nothing anywhere builds a URL out of the file contents. No browser launch, no Custom Tabs and no shared user ID. The only content provider is closed to other apps. The one time a file goes anywhere is when you hit Share and pick the app, and what it gets is read access to that one file.
In case you grep it there's an https:// in ViewerActivity.kt. That's the local virtual host WebViewAssetLoader serves the bundled renderers from, it never leaves the process.
5B:5C:F6:4A:94:23:7C:D5:F0:E0:85:76:00:38:BC:1C:EB:DF:18:DA:BA:5C:B3:EA:CA:7C:15:9F:22:A7:E2:4B
Fixed in the manifest just now, it'll be in the next build.
odt and odp dont work at all. odt is a fair ask.. its the same shape of problem as docx so it's doable. Less sure about odp though.. the pptx renderer is already the weakest part of this.
And yeah, fewer apps on the phone was pretty much the whole reason I started building this.
docx was annoying in a different way. Bullets came out as junk characters for a while and I couldnt work out why. I then found that Word writes them as private use area codepoints in the Symbol font so unless you have that exact font you get some junk. I just map them back to unicode after the render.
xlsx was the easy one, SheetJS does the hard part.
On faithful though none of this is faithful the way LibreOffice is. Its more that "you can read the document and it looks roughly right" which is what I wanted out of an app of 14MB.
I mostly stopped using these types of apps and switched to F-Droid alternatives.
> Requirements: JDK 17+ and the Android SDK (platform 35).
Bummer. I am stuck with Android 14 for the moment.
Android 14 is well past the floor, you're good to go!
1. Markdown loads as preview
2. Plain text files wrap instead of scrolling to the right
Good job and thank you for sharing
How is accessibility? Do PDFs work with screenreaders for example?
> A tiny, open source, fully offline file viewer for Android that opens PDF, Word,
Perhaps correct Word to DOCX. Word is not just DOCX and DOCX is not just Word. This user needs DOC viewing and would be disappointed having installed this app.
I wish you every success with this.