How should security teams handle public certificate renewal when SAN entries contain invalid underscore characters?

Security teams should inventory every publicly rooted certificate, identify SAN entries with underscores, and replace them before revocation deadlines take effect. The practical fix is to centralise certificate discovery, group non-compliant certificates, and reissue them through approved public CA workflows. Manual cleanup is risky because one missed certificate can trigger outages and leave a public-facing service non-compliant.

Why SAN Underscores Turn Renewal Into a Certificate Lifecycle Problem

Underscores in Subject Alternative Name fields are not a cosmetic issue, they create a renewal and compliance problem because public certificate issuance has to align with baseline browser and CA rules. When a renewal window opens, teams need to know which certificates will be rejected, which services still depend on them, and which replacements must be issued before the old ones age out.

The practical implication is that certificate handling must move from ad hoc renewals to certificate lifecycle management. In practice, the SAN value itself can force a replacement rather than a routine reissue, so discovery and inventory matter as much as the renewal transaction.

Publicly trusted certificates are governed by issuance and revocation expectations that are enforced through ecosystem rules, not only internal policy. Teams that treat underscore remediation as a later cleanup step risk discovering too late that the replacement path, not the old certificate, is the only compliant route.

What a Safe Renewal Workflow Looks Like

A safe workflow starts by centralising discovery across all publicly rooted certificates, then grouping the certificates that contain invalid SAN characters so they can be handled consistently. That matters because the operational unit is usually the service, not the individual certificate, and one missed endpoint can still break traffic after the deadline.

The most reliable fix is to reissue through approved public CA workflows and retire the non-compliant certificate before revocation or expiry becomes the forcing event. This is less about manual certificate editing and more about making the replacement path repeatable, traceable, and owned by a single process. For renewal-heavy environments, credential rotation at scale is the right mental model, even when the artifact is a certificate rather than a password or token.

Teams should also map certificate owners, dependent hosts, and deployment windows before they start replacement. If the SAN is embedded in a production service name, the real work is coordinating cutover, validation, and rollback, not just obtaining a fresh certificate.

Where certificate handling is already tied to workload identity or mTLS, this is a lifecycle and trust-boundary issue, not a paperwork exercise. Workload identity and trust bundle management can help teams think more rigorously about certificate replacement, validation, and service continuity.

What Usually Goes Wrong During Underscore Remediation

The main failure mode is partial coverage. One certificate may be fixed while another duplicate, embedded, or rarely used certificate remains in circulation, and the service fails later when the non-compliant instance expires or is revoked. That is why inventory quality is the control, not just the renewal action.

Another common problem is assuming every certificate can be renewed in place. With public CAs, a SAN containing an invalid underscore can require a new certificate profile, updated service config, and sometimes a changed DNS or application naming convention. Teams that skip that dependency check often end up with a last-minute outage during a planned change window.

Operationally, the risk increases when certificate sprawl is tied to manual ownership. Secret and certificate sprawl usually hides the same root cause: assets are easier to issue than to find, classify, and retire. That is why renewal readiness depends on discovery, not just on a calendar reminder.

Public certificate governance also benefits from using the same discipline applied to key lifecycle and cryptoperiod management. The closer teams are to disciplined lifecycle control, the less likely they are to miss a certificate that is technically valid today but operationally doomed tomorrow. NIST SP 800-57 Key Management is useful here because it reinforces lifecycle thinking around cryptoperiods, rotation, and retirement.

Risk and Threat Considerations

Underscore-related renewal failures create a real exposure window: a service can remain reachable until the old certificate expires or is revoked, then fail abruptly when the non-compliant replacement is not ready. The issue is not only standards conformance, it is service continuity, because public-facing dependencies often fail at the exact point the business expects a seamless renewal.

Failure mechanism: A certificate with an invalid SAN entry is left in service, or a replacement certificate is issued too late, so revocation or expiry removes a trust anchor before the new certificate is deployed.

Impact: Public endpoints can break, users may lose access, and teams may be forced into emergency remediation with limited rollback options and reduced confidence in certificate governance.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

OWASP Non-Human Identity Top 10 addresses the attack surface, NIST SP 800-57, NIST CSF 2.0 and CIS Controls v8 set the technical controls, and ISO/IEC 27001:2022 defines the regulatory obligations.

Framework	Control / Reference	Relevance
NIST SP 800-57	Key Management Recommendations	Certificate renewal is a key and lifecycle management problem.
Recommendation — Apply lifecycle controls to track, rotate, and retire certificates before expiry.
NIST CSF 2.0	PR.DS-01 — Data-at-rest is protected	Public certificates must be managed as protected trust material with controlled replacement.
Recommendation — Protect certificate assets and ensure replacements are deployed before trust is withdrawn.
ISO/IEC 27001:2022	A.8.24 — Use of cryptography	Public certificate handling is part of cryptographic asset governance and renewal control.
Recommendation — Control certificate issuance, renewal, and retirement under cryptographic governance.
CIS Controls v8	CIS-5 — Account Management	Strong asset and ownership management is needed to find every certificate that must be replaced.
Recommendation — Inventory certificate owners and remove non-compliant certificates before deadlines.
OWASP Non-Human Identity Top 10	NHI-07 — Long-Lived Secrets	Certificate renewal failures often arise when long-lived trust material persists beyond its safe window.
Recommendation — Replace expiring certificate material before it becomes a brittle long-lived dependency.

Practitioner Guidance

What to verify: Confirm the inventory is complete before scheduling any renewal work. The minimum check is whether every publicly rooted certificate has an owner, a service dependency, and a status flag for underscore-related non-compliance.

Decision rule: If the certificate is publicly trusted and the SAN contains an invalid underscore, treat it as a replacement case, not a routine renew-in-place case. That decision should be made before the revocation deadline, because waiting for the CA to force the issue increases outage risk.

Common mistake: Teams often fix the visible certificate but leave duplicates in load balancers, sidecars, or old deployment paths. The right test is whether the bad SAN can still be presented anywhere in the path, not whether one dashboard shows a successful renewal.

Practitioner takeaway: The safest approach is to run certificate renewal as a controlled inventory and replacement exercise, because compliance failure and outage risk come from missed dependencies, not from the SAN typo alone.