Custom URL schemes are app-specific link handlers that let a mobile device open an installed application from a URI. Attackers abuse poorly validated deep links to trigger unwanted actions, pass malicious parameters or redirect users into phishing flows. Defenders should validate inputs in the app, restrict sensitive actions and prefer safer app-link mechanisms where possible.
How Custom URL Schemes Work
Custom URL schemes are mobile app handlers that let a device route a URI directly into an installed application. They are useful for fast app-to-app navigation, but they are less trustworthy than web-originated links because the operating system often cannot verify who created the link or what data it carries.
That trust gap is why custom schemes can become a security boundary inside the app itself. A scheme may open the right app, yet still carry a dangerous path, state change, or parameter that the app accepts without sufficient validation.
Why Deep Links Become Dangerous
The main hazard is not the link mechanism alone, but the action it can trigger. Poorly designed deep links can launch privileged screens, prefill sensitive fields, or move a user into a flow the app did not intend to expose outside its normal navigation path.
Attackers also abuse parameter handling. If an app parses query values, return URLs, or identifiers without strict validation, a crafted link can redirect the user, alter application state, or send them into a phishing page that looks like a legitimate in-app journey. The Mobile App Security Guide covers the adjacent mobile controls that matter here, including deep link hijacking, intent spoofing, and safer mobile authentication patterns.
Safer Design Patterns for Mobile Links
Safer handling starts by treating deep-link data as untrusted input. Apps should accept only the exact parameters they need, enforce allowlists for destinations and actions, and refuse any link that tries to reach an administrative, payment, or account-recovery flow without an additional in-app check.
Where possible, app links and universal links are preferable because they rely on verified web domains and reduce the chance that another app can impersonate the same scheme. Even then, the app still has to validate state, because a verified launcher does not guarantee a safe action.
Deep-link hardening is closely related to iOS apps leaking hard-coded secrets, because a weak link flow becomes much more dangerous when the target app also exposes keys, tokens, or other sensitive material that attackers can reach through that flow.
How Teams Should Test and Govern Deep Links
Teams should inventory every custom scheme and every route it opens, including hidden or legacy paths that are no longer obvious in the UI. That inventory should be reviewed alongside normal mobile testing, because deep-link issues often sit in the gap between navigation testing and authentication testing.
Security review should verify that link handlers do not bypass login checks, session checks, or step-up requirements. The most important question is whether a link can cause a meaningful action by itself, or whether the app forces the same trust checks it would use if the user navigated there manually.
In practice, custom URL schemes should be treated as an input and routing control, not as a convenience feature. The more sensitive the action, the less acceptable it is for a deep link to reach it without a deliberate authorization decision inside the app.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP ASVS, NIST SP 800-53 Rev 5 and CIS Controls v8 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP ASVS | V4 — API and Web Service | Deep links carry untrusted inputs into app routing and action logic. |
| V8 — Authorization | Sensitive in-app actions triggered by links need authorization checks, not just routing. | |
| Recommendation — Validate deep-link parameters and reject unexpected destinations or actions. Require authorization checks before a deep link can reach privileged workflows. | ||
| NIST SP 800-53 Rev 5 | AC-3 — Access Enforcement | Deep links can attempt to invoke protected app functions or state changes. |
| SI-10 — Information Input Validation | Custom schemes are untrusted input that must be validated before use. | |
| Recommendation — Enforce access rules before allowing link-driven access to protected functions. Validate all deep-link inputs and reject malformed or unexpected parameters. | ||
| CIS Controls v8 | CIS-16 — Application Software Security | Mobile deep-link handling is an application security design and testing concern. |
| Recommendation — Test app link handlers for unsafe routing, parameter abuse, and bypass paths. | ||
Related resources from NHI Mgmt Group
- Why do deep links matter to identity governance in mobile apps?
- What happens when a browser allows custom URL schemes to be probed without strong user protection?
- Why do insecure deep links increase the risk of OAuth token theft in mobile apps?
- How should security teams handle URL obfuscation in phishing links?
Deepen Your Knowledge
Free weekly newsletter
Subscribe to the NHI & AI Identity Journal
The latest on NHI and Agentic AI security – articles, research, breaches, news and events every week.
Bonus 33% off our NHI Course when you subscribe.
Reviewed and updated by the NHIMG editorial team on October 20, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org