Custom / Signature Permission Squatting

← Improper Platform Usage

custom_signature_permission_squatting   MEDIUM

Description

An IPC component is ‘protected’ by a custom permission whose protectionLevel is normal/dangerous (not signature), or by a permission name a malicious app can DEFINE FIRST (install-order squatting), so the guard is trivially obtained or owned by the attacker - the interface the developer believes is protected is externally reachable. Manifest-level and deterministic. The secure path uses a signature protection level and verifies the caller’s signing identity, not merely permission possession (Android permission-squatting class).

How it works

An IPC component is “protected” by a custom permission whose protectionLevel is normal or dangerous (not signature), or by a permission name a malicious app can define first (install-order squatting). A normal permission is auto-granted to any app that requests it, and a squatted permission is attacker-owned, so the guard is trivially obtained or attacker-controlled and the interface the developer believes is protected is externally reachable. The secure path requires a signature protection level and verifies the caller’s signing identity matches the app’s, refusing the squatter.

How to exercise it. DVMA is the harness - open this module from the home index and tap the demo action. The screen ships the malicious input and simulates the attacker (e.g. the companion app, crafted intent, or scanned payload) in-process, and the evidence panel prints the proof. The Tools (optional) and Attack inputs below are only needed to reproduce the exploit end-to-end on a real device.

Exploit steps

1. Set up. Build DVMA with a flavor that enables the Improper Platform Usage category (e.g. --dart-define-from-file=config/flavors/dev.json) and run on an Android emulator you control. The demo needs no external tooling; for the optional on-device reproduction the relevant tools are: adb, drozer, apktool, jadx.
2. Locate the target. From the home index, open Custom / Signature Permission Squatting (custom_signature_permission_squatting). The How it works section above describes this module’s specific weakness; the screen states the intended-secure behavior and exposes the vulnerable action.
3. Exploit. From a co-resident/attacker context (adb shell am start/drozer, or a crafted deep link / QR), send the untrusted input to the exported component or WebView and confirm the app performs the unsafe action (navigation, JS execution, file/URI access) with no validation.
4. Observe the evidence. Trigger the vulnerable action and read the evidence panel - it prints the concrete proof (leaked value, accepted replay, executed payload, or unauthorized result).
5. Contrast with the secure path. Run the module’s secure/hardened action (where provided) and confirm the same attack is rejected - this is what a correct implementation should do.

Tools (optional)

• adb
• drozer
• apktool
• jadx

Attack inputs

Payloads/artifacts you author for the on-device attack. The demo already ships and simulates these in-process (e.g. the malicious companion app / crafted intent is emulated inside the screen), so you only need to craft them to reproduce the exploit on a real device:

• co-resident app that defines/holds the custom permission

Real-world references

Concrete public disclosures that match this vulnerability class:

• OWASP MASTG: custom permission protectionLevel and permission squatting