File-Descriptor Capability Leakage

← Improper Platform Usage

file_descriptor_capability_leakage   HARD

Description

The app passes an open ParcelFileDescriptor (via Binder, openFile(), or detachFd()) to an untrusted caller for a file/socket the caller could not otherwise open, handing over a live capability that bypasses path-based permission checks - the recipient inherits access to whatever the FD points at, including a sensitive DB or a privileged socket. Conceptually different from path traversal: an already-open capability is transferred. The secure path opens FDs read-only to a narrowly-scoped, non-sensitive resource and validates the caller before returning any descriptor (Android FD-passing class).

How it works

The app opens a resource and passes the live ParcelFileDescriptor (via a Binder transaction, openFile(), or detachFd()) to an untrusted caller for a file or socket the caller could not otherwise open. An open FD is a capability: once handed over it bypasses path-based permission checks and the recipient inherits access to whatever it points at, here a sensitive database. This is conceptually different from path traversal, an already-open capability is transferred wholesale. This is an offline, deterministic simulation. The secure path verifies the caller, refuses sensitive resources, and returns only a read-only FD to a narrowly-scoped non-sensitive resource.

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, frida, jadx.
2. Locate the target. From the home index, open File-Descriptor Capability Leakage (file_descriptor_capability_leakage). 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
• frida
• 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:

• malicious companion app

Real-world references

Concrete public disclosures that match this vulnerability class:

• Android: ParcelFileDescriptor passing and detachFd capability transfer