[Android] Restrict AOT shared library engine flag to trusted paths #173359

camsim99 · 2025-08-06T17:27:51Z

Overview

Restricts the --aot-shared-library-name flag used for launching FlutterFragments to only accept paths that point to the application's internal storage.

Why restrict the path at all

Currently, when an application launches a FlutterFragment with an Intent that passes the --aot-shared-library-name flag, the Flutter engine will call dlopen on the native code that lives where the passed path points with no checks. So, if that path points to malicious code, we've got a problem. See go/flutter-fragment-flag-security-analysis for full context.

By restricting the path to a trusted location, we ensure that the application developer is either writing their own native code (that we can assume not to be malicious) or is importing third party code and manually packaging it with their application code, ensuring that it has been vetted by the application author (or at least indicating that it should be vetted).

Why restrict the path to the application's internal storage

The native code directory within internal storage is where native JNI libraries are stored by default when the AGP useLegacyPackaging option is set to true, so any compiled native code that's part of an Android app will live there.

If useLegacyPackaging is set to false or the application developer wishes to reference native code some other way, they can manually package it within internal storage, which is generally a secure place to store files for your application needs. Also, as far as I'm aware, this is the only secure place the application developers could reference native code by path in their application (for example, developers could put their native code in the app/src/main/assets directory, then copy it into internal storage for reference by path).

My primary sources:
https://developer.android.com/ndk/guides/concepts
https://developer.android.com/studio/projects/add-native-code
https://developer.android.com/training/data-storage

Prior art

Flutter has enforced a similar sort of path restriction for the file_selector_android package; see flutter/packages#8184. Just like that PR does, this PR makes sure to canonicalize paths passed to the flag and attempts to test sneaky partial internal storage paths to mitigate path traversal attacks.

Future work

As mentioned in my TODO, the pattern of willy-nilly adding any flags passed in an Intent to launch a FlutterFragment is dangerous. We plan to address this vulnerability in a future PR: #172553

P.S. This PR also renames `application_library_path` to `application_library_paths`

This is actually a vector of AOT shared library (plural) paths that can include the one passed with the --aot-shared-library-name flag as well as the fallbacks.

Pre-launch Checklist

I read the Contributor Guide and followed the process outlined there for submitting PRs.
I read the Tree Hygiene wiki page, which explains my responsibilities.
I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
I signed the CLA.
I listed at least one issue that this PR fixes in the description above.
I updated/added relevant documentation (doc comments with ///).
I added new tests to check the change I am making, or this PR is test-exempt.
I followed the breaking change policy and added Data Driven Fixes where supported.
All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

…_library_name_flag

engine/src/flutter/shell/platform/android/io/flutter/embedding/engine/loader/FlutterLoader.java

hellohuanlin

IOS PART LGTM

camsim99 · 2025-08-25T16:55:27Z

Ah I just realized I have a real failure on Android to address here. I'll update the PR when I've gotten the chance to take a look.

camsim99 · 2025-08-25T23:32:02Z

Ah I just realized I have a real failure on Android to address here. I'll update the PR when I've gotten the chance to take a look.

Fixed if you can take a second look! @reidbaker

reidbaker · 2025-08-26T15:46:35Z

engine/src/flutter/shell/platform/android/io/flutter/embedding/engine/loader/FlutterLoader.java

+
+    // Canocalize path for safety analysis.
+    File aotSharedLibraryFile = getFileFromPath(aotSharedLibraryPath);
+    String aotSharedLibraryPathCanonicalPath = aotSharedLibraryFile.getCanonicalPath();


what happens if I pass "--aot-shared-library-name=<>?/\1234-some-invalid-string"

Maybe it results in an empty pathname? https://docs.oracle.com/javase/7/docs/api/java/io/File.html#File(java.lang.String) similar to an empty string

I'm glad you pointed this out because it looks like an IOException is thrown in the path is invalid. I added a try/catch so we can return null early in this case + a test.

I didn't do the same for the internal storage directory because I think it's safe to assume that it is a valid path.

…paths (flutter/flutter#173359)

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…paths (flutter/flutter#173359)

…lutter#173359) ### Overview Restricts the `--aot-shared-library-name` flag used for launching `FlutterFragment`s to only accept paths that point to the application's internal storage. ### Why restrict the path at all Currently, when an application launches a `FlutterFragment` with an `Intent` that passes the `--aot-shared-library-name` flag, the Flutter engine [will call `dlopen`](https://github.com/flutter/flutter/blob/f45a8f8b719e9c0648d0ac563023522e72a80a00/engine/src/flutter/fml/platform/posix/native_library_posix.cc#L14) on the native code that lives where the passed path points with no checks. So, if that path points to malicious code, we've got a problem. See go/flutter-fragment-flag-security-analysis for full context. By restricting the path to a trusted location, we ensure that the application developer is either writing their own native code (that we can assume not to be malicious) or is importing third party code and manually packaging it with their application code, ensuring that it has been vetted by the application author (or at least indicating that it should be vetted). ### Why restrict the path to the application's internal storage The [native code directory within internal storage](https://developer.android.com/reference/android/content/pm/ApplicationInfo#nativeLibraryDir) is where native JNI libraries are stored by default when the AGP [`useLegacyPackaging` option](https://developer.android.com/build/releases/past-releases/agp-4-2-0-release-notes#compress-native-libs-dsl) is set to true, so any compiled native code that's part of an Android app will live there. If `useLegacyPackaging` is set to false or the application developer wishes to reference native code some other way, they can manually package it within internal storage, which is generally a secure place to store files for your application needs. Also, as far as I'm aware, this is the only secure place the application developers could reference native code *by path* in their application (for example, developers could put their native code in the `app/src/main/assets` directory, then copy it into internal storage for reference by path). _My primary sources:_ https://developer.android.com/ndk/guides/concepts https://developer.android.com/studio/projects/add-native-code https://developer.android.com/training/data-storage ### Prior art Flutter has enforced a similar sort of path restriction for the `file_selector_android` package; see flutter/packages#8184. Just like that PR does, this PR makes sure to canonicalize paths passed to the flag and attempts to test sneaky partial internal storage paths to mitigate path traversal attacks. ### Future work As mentioned in my TODO, the pattern of willy-nilly adding any flags passed in an `Intent` to launch a `FlutterFragment` is dangerous. We plan to address this vulnerability in a future PR: flutter#172553 ### P.S. This PR also renames `application_library_path` to `application_library_paths` This is actually a vector of AOT shared library (plural) paths that can include the one passed with the `--aot-shared-library-name` flag as well as the fallbacks. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [ ] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord]. **Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

…paths (flutter/flutter#173359)

…lutter#173359) ### Overview Restricts the `--aot-shared-library-name` flag used for launching `FlutterFragment`s to only accept paths that point to the application's internal storage. ### Why restrict the path at all Currently, when an application launches a `FlutterFragment` with an `Intent` that passes the `--aot-shared-library-name` flag, the Flutter engine [will call `dlopen`](https://github.com/flutter/flutter/blob/f45a8f8b719e9c0648d0ac563023522e72a80a00/engine/src/flutter/fml/platform/posix/native_library_posix.cc#L14) on the native code that lives where the passed path points with no checks. So, if that path points to malicious code, we've got a problem. See go/flutter-fragment-flag-security-analysis for full context. By restricting the path to a trusted location, we ensure that the application developer is either writing their own native code (that we can assume not to be malicious) or is importing third party code and manually packaging it with their application code, ensuring that it has been vetted by the application author (or at least indicating that it should be vetted). ### Why restrict the path to the application's internal storage The [native code directory within internal storage](https://developer.android.com/reference/android/content/pm/ApplicationInfo#nativeLibraryDir) is where native JNI libraries are stored by default when the AGP [`useLegacyPackaging` option](https://developer.android.com/build/releases/past-releases/agp-4-2-0-release-notes#compress-native-libs-dsl) is set to true, so any compiled native code that's part of an Android app will live there. If `useLegacyPackaging` is set to false or the application developer wishes to reference native code some other way, they can manually package it within internal storage, which is generally a secure place to store files for your application needs. Also, as far as I'm aware, this is the only secure place the application developers could reference native code *by path* in their application (for example, developers could put their native code in the `app/src/main/assets` directory, then copy it into internal storage for reference by path). _My primary sources:_ https://developer.android.com/ndk/guides/concepts https://developer.android.com/studio/projects/add-native-code https://developer.android.com/training/data-storage ### Prior art Flutter has enforced a similar sort of path restriction for the `file_selector_android` package; see flutter/packages#8184. Just like that PR does, this PR makes sure to canonicalize paths passed to the flag and attempts to test sneaky partial internal storage paths to mitigate path traversal attacks. ### Future work As mentioned in my TODO, the pattern of willy-nilly adding any flags passed in an `Intent` to launch a `FlutterFragment` is dangerous. We plan to address this vulnerability in a future PR: flutter#172553 ### P.S. This PR also renames `application_library_path` to `application_library_paths` This is actually a vector of AOT shared library (plural) paths that can include the one passed with the `--aot-shared-library-name` flag as well as the fallbacks. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [ ] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord]. **Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Add draft fix code, rename c++ vars, add todos

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camsim99 added 12 commits August 6, 2025 10:41

Gemini makes some good points

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working state for apk native dir and internal storage generally

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Start adding tests

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finish adding tests, start fixes

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undo refactor

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get rid of slashes

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formatting

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remove todo and api cant use

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undo my formatting and add log

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Merge remote-tracking branch 'upstream/master' into harden_aot_shared…

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undo deletion

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github-actions bot removed the tool Affects the "flutter" command-line tool. See also t: labels. label Aug 12, 2025

camsim99 added 4 commits August 13, 2025 09:49

some minor wording improvements

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just check for internal storage

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Merge remote-tracking branch 'upstream/master' into harden_aot_shared…

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corrections

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address reviews

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nit

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check if symlinks are supported

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add try/catch for invalid path + add test

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Merge branch 'master' into harden_aot_shared_library_name_flag

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Overview

Why restrict the path at all

Why restrict the path to the application's internal storage

Prior art

Future work

P.S. This PR also renames application_library_path to application_library_paths

Pre-launch Checklist

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P.S. This PR also renames `application_library_path` to `application_library_paths`