Why do legacy auth paths and local accounts increase risk for agentic systems?

Why legacy auth paths become the preferred route for agents

Agentic systems tend to take the path of least resistance. If a legacy login flow is already working, the agent will often use it because it is faster, more reliable, and less likely to trigger extra prompts or failures than a modern centralised flow. That matters because convenience can quietly become policy when the system optimises for execution over governance.

Legacy auth also tends to sit outside the strongest control plane. When a workflow bypasses SSO, MFA, or a shared identity provider, it usually also bypasses the normal points where security teams enforce session policy, conditional access, logging, and step-up verification. The result is not just a different login method, but a different security boundary.

For agentic systems, that boundary shift is especially important because the agent is not a human operator making a one-off choice. It may repeatedly reuse the same route, inherit whatever is easiest to access, and keep doing so at scale. A local account or old service login can therefore become the de facto operating path even if it was never intended to support autonomous action.

Why local accounts increase blast radius and reduce visibility

Local accounts increase risk because they fragment control. Instead of one centrally governed identity with shared policy and audit coverage, you get credentials that are attached to a single host, application, or workflow and may be invisible to the wider identity program. That makes inventory, ownership, and review harder, especially when the account was created for convenience or troubleshooting.

They also increase blast radius when an agent can act with that account across multiple systems or long-lived sessions. If the account is overused, poorly rotated, or shared between humans and automation, compromise becomes harder to contain. The issue is not only authentication strength, but the fact that the account can persist outside normal lifecycle controls and outlast the reason it was created.

Local accounts are often weaker from a detection standpoint as well. Without central logging, correlated access review, and consistent identity naming, it becomes much harder to answer basic questions such as who used the account, from where, for what action, and whether the use was authorised. For an agentic system, that lack of attribution is a practical security gap, not just an audit inconvenience.

What changes when legacy auth meets autonomous decision-making

The risk is amplified because agents optimise under friction. If one path requires fewer steps, fewer failures, or fewer policy prompts, the agent may prefer it even when it is less governed. That means legacy auth does not just add exposure at the edge, it can shape system behaviour by rewarding the least controlled route.

This creates a dangerous combination: an autonomous process with execution authority, a login path that avoids modern guardrails, and a credential that may be difficult to notice or revoke quickly. In practice, that can turn a tactical workaround into a durable trust relationship.

For that reason, legacy auth and local accounts should be treated as control exceptions, not harmless compatibility features. When they are tolerated for convenience, they often become the easiest route for both benign overuse and malicious abuse.

Risk and Threat Considerations

Legacy auth paths and local accounts create a security exposure because they weaken central policy enforcement and reduce the chance that abnormal access will be seen quickly. In an agentic environment, the problem is not only misuse, but repeated machine selection of the easiest available path, which can institutionalise weak controls across many actions.

Failure mechanism: The agent discovers that a local account or old auth flow works with less friction than governed access, then keeps using it because the environment rewards success, not compliance. That bypasses stronger identity controls and leaves fewer central signals for monitoring, review, or revocation.

Impact: A compromised or over-permissioned legacy path can provide persistent access, reduce attribution, and expand blast radius across systems the account can reach. If the account is shared, long-lived, or poorly inventoried, incident response becomes slower and containment becomes less reliable.

Practitioner Guidance

What to prioritise: Start by identifying every auth path an agent can use, then rank them by governance quality, not by convenience. Any local account, fallback credential, or non-central login route that can reach production should be treated as a high-priority review item.

What to verify: Confirm that the account has an owner, a documented purpose, a revocation path, and central logging that is actually queryable. If you cannot quickly answer who can use it, how it is rotated, and whether it is still needed, it is already too risky for autonomous use.

Common mistake: Teams often secure the primary SSO path while leaving old accounts in place for edge cases. For agentic systems, those edge cases are exactly where the control gap becomes operationalised, because the agent will keep returning to the path that works fastest.

Practitioner takeaway: The real issue is not legacy access in isolation, it is legacy access that an autonomous system can repeatedly choose at scale. If a path is easier than the governed one, assume it will be used, then either bring it under the control plane or remove it.

Related resources from NHI Mgmt Group

• Why do service accounts and legacy authentication paths increase ransomware risk?
• Why do AI agents increase non-human identity risk in existing IAM programmes?
• When does just-in-time access reduce risk for agentic AI, and when does it fall short?
• How should security teams limit the risk from AI agents that have access to production systems?