How should security teams handle legacy applications and privileged accounts that sit outside single sign-on coverage?

Why This Matters for Security Teams

When a business relies on SSO as the primary access plane, legacy applications and privileged accounts outside that boundary become the easiest place for control drift to accumulate. SSH keys, API keys, certificates, shared admin logins, and embedded secrets often sit in systems with weaker lifecycle controls, which means they can outlive the user, the project, or the vendor relationship. NHI Management Group’s Ultimate Guide to NHIs — Key Challenges and Risks notes that 96% of organisations store secrets outside secrets managers in vulnerable locations, and 97% of NHIs carry excessive privileges.

The practical issue is not just where access is granted, but where it can be revoked, rotated, and audited. SSO does not automatically govern an SSH private key on a build server or a certificate embedded in an aging application. Current guidance suggests treating these assets as non-human identities that require their own control model, not as exceptions that can be handled manually forever. The OWASP Non-Human Identity Top 10 is especially relevant here because it frames secrets, rotation, and privilege sprawl as systemic risks rather than isolated hygiene issues. In practice, many security teams discover the real blast radius only after a legacy credential has already been reused, copied, or forgotten.

How It Works in Practice

Handling out-of-SSO access starts with inventory and classification. Security teams need to separate human interactive access from machine and privileged access, then identify which legacy systems cannot speak modern federation protocols. From there, the goal is to move each asset into a governed lifecycle: unique ownership, approved storage, time-bounded use, rotation, revocation, and logging. That applies to service accounts, database credentials, API keys, certificates, scheduled job tokens, and admin SSH keys.

A workable pattern is to put privileged non-SSO credentials behind a vault or secrets platform, then broker access at request time rather than distributing static values broadly. This mirrors the same direction seen in NHI governance: short-lived access, strong audit trails, and explicit offboarding. NIST SP 800-53 Rev. 5 helps here because it reinforces access enforcement, least privilege, auditability, and credential management as control objectives, not optional enhancements. For many teams, the operational question is less “Can SSO cover this?” and more “What compensating controls make this credential survivable until the system is modernised?”

• Assign a named owner to every legacy credential and privileged account.
• Store secrets in a controlled system, not in code, shared documents, or ad hoc password stores.
• Enforce rotation and revocation on a fixed schedule, with emergency revoke paths.
• Limit use by scope, host, environment, and time window.
• Log every retrieval and every authentication event for review.

Where possible, replace shared admin accounts with per-operator elevation, and replace long-lived keys with short-lived credentials issued only for the approved task. These controls tend to break down when legacy applications require hard-coded credentials or when operational teams lack a maintenance window to update dependent jobs and integrations.

Common Variations and Edge Cases

Tighter credential controls often increase operational overhead, requiring organisations to balance security gains against application fragility and support load. That tradeoff is most visible in mainframe integrations, vendor-managed systems, embedded devices, and batch jobs that cannot easily accept modern federation. In those cases, current guidance suggests compensating controls rather than pretending the app is SSO-enabled when it is not.

One common edge case is a privileged account shared by multiple administrators. If a replacement is not immediately feasible, the account should still be wrapped with PAM, monitored sessions, and rapid rotation so its use is attributable and time-bounded. Another is certificate-based trust for internal services: certificates may not be “interactive” like a user login, but they are still identities and should be treated as such, with expiry tracking and renewal ownership. NHI Management Group’s research on the Ultimate Guide to NHIs — Key Challenges and Risks highlights how often organisations lose visibility once secrets move outside central control, while the Microsoft SAS Key Breach shows how exposed non-SSO credentials can create broad downstream exposure.

The hard rule is simple: if SSO cannot enforce lifecycle and access policy, a separate governance path is required. There is no universal standard for every legacy stack yet, so teams should document compensating controls, exception expiry dates, and a modernization plan rather than allowing permanent carve-outs.

Related resources from NHI Mgmt Group

• How should security teams handle disconnected applications that sit outside identity tooling?
• How should security teams handle legacy Group Policy Preferences password exposure in Active Directory environments?
• How should security teams defend identity providers that sit between users and SaaS applications?
• How should security teams govern non-human identities alongside human accounts?