Universal Link / AASA Associated-Domain Confusion

← Improper Platform Usage

universal_link_aasa_confusion   HARD

Description

The app trusts an incoming Universal Link because it matched an associated domain, but weak AASA deployment (overly broad path patterns / wildcards, open redirects on the domain, environment confusion between staging and prod, or a compromised/parked subdomain) lets an attacker craft a URL that routes to a sensitive in-app handler with attacker-controlled parameters. The whole app->associated-domain->AASA->web-server chain is the boundary, not just link parsing. The secure path uses tight AASA path matching, re-validates every parameter, and never treats a matched link as an authorization decision (iOS Universal-Link/AASA class).

How it works

The app trusts an incoming Universal Link because it matched an associated domain, but the AASA deployment is weak: an over-broad /account/* and /* wildcard. The incoming URL is parsed and matched against the real bundled apple-app-site-association.json components, so a crafted URL routes to a sensitive in-app handler (resetPassword) with attacker-controlled parameters. The whole app to associated-domain to AASA to web-server chain is the boundary, not just link parsing. The secure path uses tight exact-path matching, refuses redirects, re-validates parameters, and never treats a matched link as an authorization decision, while the genuine link still works.

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 iOS simulator / a device you control. The demo needs no external tooling; for the optional on-device reproduction the relevant tools are: ipsw, Hopper.
2. Locate the target. From the home index, open Universal Link / AASA Associated-Domain Confusion (universal_link_aasa_confusion). 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 an attacker context (a crafted custom-URL-scheme / Universal Link, a malicious document provider / share extension, or a Shortcut), send the untrusted input to the handler and confirm the app performs the unsafe action (navigation, file/URL access, privileged operation) 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.
6. Map it back. Cross-reference the MASTG v2 test(s) MASTG-TEST-0395, MASTG-TEST-0070 for the canonical procedure and remediation.

Tools (optional)

• ipsw
• Hopper

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:

• crafted universal-link URL

Real-world references

Concrete public disclosures that match this vulnerability class:

• Apple: Supporting associated domains and AASA path matching