App Group Shared-Container Privilege Amplification

← Privileged System-Provider Activation

app_group_shared_container_amplification   MEDIUM

Description

The app stores secrets (tokens, credentials, keys) in an App Group shared container / shared UserDefaults that every member of the group can read/write with no per-item access control, so a less-trusted extension (or an over-broadly-scoped sibling) reads or tampers with data it should never see. Shared-container membership amplifies privilege - ‘App Group used’ is not the finding, over-sharing is (Apple App Group shared-container class, OWASP MASTG-BEST-0068).

How it works

The main app persists secrets, here an auth/session token, in an Apple App Group shared container (shared UserDefaults or a container obtained via containerURL(forSecurityApplicationGroupIdentifier:)) keyed by an app-group id. Every member of that group, including a low-trust custom keyboard extension, can read and write every item with no per-item access control. Membership amplifies privilege: the keyboard extension reads a token it should never see. “App Group used” is not the finding; over-sharing is. On an App-Group-entitled iOS build (or on Android) this runs a real cross-member file write/read through the shared container; otherwise it falls back to a deterministic simulation. The secure path enforces per-item access scoping so sensitive items are served only to the entitled main app.

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 Privileged System-Provider Activation category (e.g. --dart-define-from-file=config/flavors/dev.json) and run on an iOS simulator / a device you control. The demo needs no external tooling; for the optional on-device reproduction the relevant tools are: frida, objection, idb, plutil.
2. Locate the target. From the home index, open App Group Shared-Container Privilege Amplification (app_group_shared_container_amplification). 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. Drive the provider-activation / enablement workflow (accessibility, notification-listener, VPN, IME, phone-account, MediaProjection, credential-provider) and confirm the granted capability is invoked on an untrusted caller’s behalf - the enablement flow is the boundary.
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.
6. Map it back. Cross-reference the MASTG v2 test(s) MASTG-TEST-0388 for the canonical procedure and remediation.

Tools (optional)

• frida
• objection
• idb
• plutil

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:

• low-trust extension in the same App Group

Real-world references

Concrete public disclosures that match this vulnerability class:

• OWASP MASTG-BEST-0068: secure data sharing between app extensions and containing apps