What are the common mistakes teams make when applying zero trust to APIs in a merger?

Where zero trust breaks down in a merger

The biggest mistakes are structural, not theoretical. In a merger, teams often try to bolt zero trust onto APIs after new systems, tokens, gateways, and ownership boundaries are already in motion. That leads to inconsistent policy enforcement, rushed trust decisions, and an access model that looks principled on paper but behaves differently across environments.

One common failure is treating every API the same. Merging organisations usually inherit different auth patterns, different scope conventions, and different assumptions about who can call what. If teams do not normalise those differences quickly, they end up with overlapping allow rules, stale integrations, and exceptions that quietly become the real policy.

Another mistake is confusing “authenticated” with “trusted.” zero trust for apis depends on more than a valid token or gateway check. It also requires strong claim validation, audience checking, least-privilege scopes, and controls that still hold when systems are rehomed, renamed, or exposed to new business units. A merger amplifies those failures because the target state keeps changing.

Teams also underinvest in visibility. If logs do not capture which identities called which APIs, with what scopes, from which environments, it becomes very difficult to spot abuse or prove that a control decision was correct. That gap is especially dangerous when integration work is happening faster than governance can catch up.

Finally, merger programmes often overestimate the safety of temporary access. Short-lived carve-outs, partner exceptions, and migration credentials are easy to create and hard to remove. In practice, those temporary paths become the longest-lived attack paths unless they are tracked with the same discipline as the permanent ones.

What actually goes wrong at policy, scope, and monitoring level

Zero trust succeeds when policy is explicit, identity context is strong, and every API decision is constrained to the minimum necessary access. The common merger mistakes are the opposite: broad scopes, weak claims, inconsistent policy translation, and monitoring that is delayed until after the consolidation is “done.”

Scope drift is a recurring issue. Teams often copy existing entitlements into the new environment instead of re-deriving them from business need, which creates APIs that can do far more than the merger use case requires. Claims drift follows when identity assertions, environment tags, or tenant boundaries are not revalidated after systems are connected.

Monitoring failures are just as important. API traffic may be visible in a gateway, but that does not mean the organisation can answer basic questions about which calls were approved, which were denied, and whether an exception was used outside its intended window. When control ownership is split across acquiring and acquired teams, missing telemetry can hide misuse long enough for lateral movement to take hold.

A useful reference point is OWASP API Security Top 10, because many merger failures map directly to broken authorisation, excessive exposure, and weak API-level trust decisions. For teams implementing the zero trust model itself, NIST SP 800-207 Zero Trust Architecture remains the cleanest baseline for thinking about policy enforcement, explicit verification, and least privilege.

Where merger teams need a more operational view of identity-bearing credentials and service access, NHIMG’s Ultimate Guide to NHIs is useful because API trust problems often ride on service accounts, keys, tokens, and other non-human access material that is easy to inherit but hard to govern.

Risk and Threat Considerations

In a merger, the risk is not just that one API is misconfigured. It is that two security models are being joined while both are still changing, which creates a larger attack surface, more exceptions, and weaker accountability for who can invoke what. That makes misuse, overreach, and lateral movement more likely if controls are not tightened before integration accelerates.

Failure mechanism: attackers, or simply unintended internal callers, can exploit broad scopes, stale tokens, copied entitlements, and incomplete logging to move through newly connected systems without tripping a clear policy boundary.

Impact: the organisation can lose confidence in its access decisions, expose sensitive data or transactions across the merged estate, and carry temporary migration access into a lasting compromise path.

Practitioner Guidance

What to prioritise: start by inventorying the APIs that bridge the two organisations, then rank them by business criticality, scope breadth, and whether they still rely on merger-only exceptions. The high-risk set is usually the one with the weakest ownership clarity and the broadest cross-environment access.

What to verify: confirm that each API decision is based on current claims, current audience, and current environment, not inherited trust from the pre-merger state. If a token, scope, or service principal can still reach production through a path nobody can explain quickly, treat that as an access governance problem, not just a documentation gap.

Common mistake: teams often focus on gateway placement and miss the policy lifecycle. Zero trust is not established by putting more traffic through a control point; it is established by making sure the decision logic, ownership, and revocation process keep pace with change.

Practitioner takeaway: In a merger, the safest API posture is the one that reduces temporary trust faster than the integration programme expands it.

Related resources from NHI Mgmt Group

• What are the common implementation mistakes teams make when applying policy driven filters to nested relations in Prisma?
• How should security teams implement zero trust in multi-cloud environments without relying on tunnels between clouds?
• Why do non-human identities complicate zero trust architecture?
• Why do non-human identities increase zero trust risk?