iOS Custom URL Schemes and Deep Links are mechanisms that let an app be opened through a specially formatted link. If schemes are guessable or handlers are not validated, other apps or links can trigger sensitive actions, so developers should verify incoming data and avoid placing secrets in link parameters.
What iOS custom URL schemes and deep links are
These mechanisms let an app open from a specially formatted link. They are useful for navigation and app-to-app handoff, but they also create an input surface that other apps, browsers, or messages can reach.
On iOS, the security question is not whether the link opens the right screen, but whether the app can trust the source and the parameters it receives. If a handler accepts unvalidated input, a link can become a delivery path for unwanted actions.
How they are used in mobile workflows
Deep links commonly support password resets, account handoff, referral flows, content opening, and OAuth-style redirects. In each case, the link is acting like a tiny command channel, so the destination screen should treat it as untrusted data rather than as proof of user intent.
The same convenience that makes deep links attractive also makes them easy to abuse when several apps register similar schemes or when a scheme is predictable. The Mobile App Security Guide covers the broader controls that help teams harden these flows, including secure mobile input handling and deep link hijacking defense.
Why validation and secret handling matter
Deep link parameters can carry identifiers, tokens, and return destinations, which means the app must validate both structure and intent before taking action. A link that is syntactically valid can still be dangerous if it routes a user into an authenticated workflow without checking session state, origin assumptions, or whether the requested operation is appropriate.
Secrets should never be placed in link parameters because links are routinely logged, forwarded, previewed, and cached in places the app does not control. The same discipline applies to redirect targets, one-time codes, and other values that should not be exposed beyond the intended session.
When teams are testing for this class of weakness, it helps to look at how mobile apps store and expose sensitive values more generally. NHIMG’s iOS apps leaking hard-coded secrets is relevant because deep-link abuse and secret leakage often show up together in mobile applications.
Common failure modes and defensive design choices
The main failure modes are scheme squatting, link hijacking, over-trusted handlers, and unsafe parameter processing. If multiple apps can claim the same scheme, the user may not reach the intended app. If the destination app does not verify the link source or the request context, another app or message can trigger sensitive functions on the user’s behalf.
Good design keeps deep links narrow, state-aware, and input-hardened. That means validating the path, the parameter types, the allowed actions, and the expected application state before the app performs any privileged or sensitive operation.
The Mobile App Security Testing Guide is useful when you want to exercise these flows under runtime analysis, certificate pinning, app attestation, and app testing conditions rather than relying only on static review.
Risk and Threat Considerations
Deep links are risky because they can bridge untrusted external input directly into app behavior, including authenticated flows, account actions, or data retrieval. If the app assumes that opening a link is equivalent to user approval, an attacker can abuse that trust boundary to trigger actions the user never intended.
Failure mechanism: Predictable schemes, weak handler validation, and unsafe parameter parsing let another app, web page, or message cause the target app to open the wrong route or process sensitive data incorrectly.
Impact: The result can be unauthorized actions, token exposure, session abuse, account takeover support paths, or privacy leakage through logs, previews, and shared link handling.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP ASVS, CIS Controls v8 and NIST CSF 2.0 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP ASVS | V4 — API and Web Service | Deep links carry externally supplied parameters into app flows that need strict request validation. |
| V7 — Session Management | Sensitive deep links often rely on existing app sessions and must not bypass session checks. | |
| V8 — Authorization | Deep links can trigger privileged actions if authorization is not checked after route resolution. | |
| Recommendation — Validate link inputs and allowed actions before processing any state-changing request. Require a valid session and expected user state before honoring sensitive deep-link actions. Enforce authorization on the destination action, not just on link parsing. | ||
| CIS Controls v8 | CIS-16 — Application Software Security | Mobile deep-link handling is an application-security concern that needs secure input handling and testing. |
| Recommendation — Test deep-link handlers as attack surface and remediate unsafe parsing or action triggers. | ||
| NIST CSF 2.0 | PR.DS-01 — Data-at-rest is protected | Secrets placed in URLs can be exposed through logging, previews, and storage pathways. |
| Recommendation — Keep secrets out of URLs and protect any data that can be logged or forwarded. | ||
Practitioner Guidance
Why practitioners should care: Treat every deep link as untrusted input, even when it appears to come from your own ecosystem. The important control decision is whether the app validates the request before it changes state, not whether the link format is technically correct.
What to watch for: Pay close attention to flows that combine link parameters with authentication, redirects, or sensitive records. Those are the places where a harmless navigation feature can become a security-relevant action path.
Practitioner takeaway: Use deep links for navigation, not for implicit trust, and keep any sensitive operation behind explicit validation and session checks.