Add MASTG-TEST for Sensitive Data Stored Unencrypted via Java File APIs in the App Sandbox

[Copilot]

Closes #3795

No test existed specifically targeting Java File APIs (openFileOutput, FileOutputStream, FileWriter) for unencrypted sensitive data writes to the Android app sandbox — only the generic filesystem-snapshot test (MASTG-TEST-0207) covered this area indirectly.

New artifacts (fake IDs, to be assigned at merge)

• MASTG-TEST-0x01 (tests-beta/android/MASVS-STORAGE/) — Static test for MASWE-0006; uses semgrep to find openFileOutput/FileOutputStream/FileWriter calls in decompiled code; references MASTG-TEST-0x02 as its dynamic counterpart
• MASTG-TEST-0x02 (tests-beta/android/MASVS-STORAGE/) — Dynamic test for MASWE-0006; uses runtime method hooking (Frooky) to monitor File API calls and correlate with Cipher/KeyStore calls; mirrors the iOS MASTG-TEST-0300/0301 pattern adapted for Android File APIs
• MASTG-BEST-0x01 (best-practices/) — Prescribes EncryptedFile (AES-256-GCM-HKDF-4KB, KeyStore-backed) as the mitigation
• rules/mastg-android-unencrypted-internal-file-storage.yml — Three semgrep rules flagging openFileOutput, new FileOutputStream(new File(ctx.getFilesDir(), …)), and new FileWriter(…) in decompiled Java
• MASTG-DEMO-0x01 (demos/android/MASVS-STORAGE/) — Static semgrep demo for MASTG-TEST-0x01; includes MastgTest.kt, MastgTest_reversed.java, run.sh, and output.txt (2 findings); references MASTG-DEMO-0x02 as its dynamic counterpart
• MASTG-DEMO-0x02 (demos/android/MASVS-STORAGE/) — Dynamic Frooky demo for MASTG-TEST-0x02; reuses the code sample from MASTG-DEMO-0x01 via {{ ../MASTG-DEMO-0x01/MastgTest.kt }}; includes hooks.json, run.sh, output.json, evaluate.sh, and evaluation.txt

Demo sample highlights

MastgTest.kt (in MASTG-DEMO-0x01, shared by MASTG-DEMO-0x02) writes two files to internal storage without encryption:

// FAIL — plaintext password via openFileOutput
context.openFileOutput("secret_token.txt", Context.MODE_PRIVATE).use { it.write(password.toByteArray()) }

// FAIL — plaintext API key via FileOutputStream
FileOutputStream(File(context.filesDir, "api_key.txt")).use { it.write(apiKey.toByteArray()) }

The static demo (MASTG-DEMO-0x01) runs semgrep and surfaces 2 findings pointing to the exact lines. The dynamic demo (MASTG-DEMO-0x02) hooks openFileOutput and FileOutputStream.write at runtime; evaluation.txt (produced by evaluate.sh) shows that neither write is preceded by any Cipher calls, confirming the data is stored unencrypted.

[cpholguera]

@copilot ensure you follow the relevant .github/instructions and keep the static and dynamic tests separate, check other examples in the repo. Sometimes we create one and then in the other we say that it's the dynamic counterpart (and shortly highlight the special thing about the static or the dynamic as we do in some tests already). Do the same for the demos, you can say that one's the static and the other one is the dynamic counterpart but will use the same code sample with ../MASTG-DEMO-...

[Copilot]

@copilot ensure you follow the relevant .github/instructions and keep the static and dynamic tests separate, check other examples in the repo. Sometimes we create one and then in the...

Done in commit 707a114. The test and demo are now split:

Tests:

• MASTG-TEST-0x01 — type: [static] — uses semgrep to find openFileOutput/FileOutputStream/FileWriter calls in decompiled code; references MASTG-TEST-0x02 as its dynamic counterpart
• MASTG-TEST-0x02 — type: [dynamic] — overview says "This test is the dynamic counterpart to @MASTG-TEST-0x01"

Demos:

• MASTG-DEMO-0x01 — static semgrep demo for MASTG-TEST-0x01: includes MastgTest.kt/MastgTest_reversed.java, the semgrep rule, run.sh, and output.txt (2 findings); references MASTG-DEMO-0x02 as the dynamic counterpart
• MASTG-DEMO-0x02 — dynamic Frooky demo for MASTG-TEST-0x02: uses {{ ../MASTG-DEMO-0x01/MastgTest.kt }} (no own code sample), includes hooks.json, run.sh, output.json, evaluate.sh, and evaluation.txt; references MASTG-DEMO-0x01 as the static counterpart

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• semgrep.dev
  • Triggering command: /home/REDACTED/.local/bin/pysemgrep osemgrep -c ../../../../rules/mastg-android-unencrypted-internal-file-storage.yml ./MastgTest_reversed.java (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pysemgrep osemgrep -c /tmp/test_rule.yml ./MastgTest_reversed.java (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pysemgrep osemgrep --config /tmp/test_rule.yml ./MastgTest_reversed.java --no-git-ignore (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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