Secure Boot, an Unsigned Vendor Module, and a PCI DSS Audit
You run 400 Ubuntu 22.04 virtual machines and 40 physical database hosts under PCI DSS. Secure Boot is enabled, kernel lockdown is set to integrity, AIDE runs nightly, and auditd forwards to a central SIEM. A vendor backup agent ships an unsigned DKMS kernel module; after the 5.15.0-105 kernel update it fails to load, and backups are incomplete. The vendor can provide a signed module only for RHEL, not Ubuntu. Operations proposes disabling Secure Boot on all 40 database hosts to restore backups before a three-week audit deadline. Separately, AIDE reports that /etc/audit/audit.rules changed on three hosts, and the local audit log shows a gap of 11 minutes. The database hosts are segmented, but the backup agent has root and outbound HTTPS access to the vendor cloud.
1. Recommend how to restore backups on Ubuntu while keeping Secure Boot and PCI evidence. Include key-management and rollout steps.
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Do not disable Secure Boot across all 40 database hosts. That weakens boot integrity for the most sensitive systems and creates a visible exception just before an audit. Prefer signing the module with an organization-controlled key and enrolling that key in the Machine Owner Key database. Generate a dedicated module-signing key, store it in an HSM or offline system with strict access, sign the DKMS output with kmodsign, and enroll the public key with mokutil. Enrolling MOK requires console access and a reboot, so schedule it per host and verify with mokutil --list-enrolled and dmesg after loading.
Kernel lockdown integrity should still permit a module signed by a trusted MOK key; test one host before scaling. If the vendor will not support this, evaluate a user-space backup agent or a supported RHEL host for the agent rather than weakening all Ubuntu hosts. Document the signing key's owner, rotation, and enrollment procedure as a change-control record. If a temporary exception is unavoidable, scope it to the minimum hosts, add compensating controls such as restricted egress, AppArmor confinement, and enhanced monitoring, and set an expiry before the audit.
2. AIDE and auditd show tampering on three hosts. What investigation and containment sequence do you follow?
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Treat the three hosts as potentially compromised. First isolate them from the network except for a management VLAN, preserve volatile evidence, and stop automated remediation from overwriting logs. Compare the current audit rules with the configuration-management baseline and with auditctl -l; check the SIEM for CONFIG_CHANGE, service restarts, and the 11-minute gap. Use rpm -V audit and AIDE to identify other modified files, and review authentication, sudo, and cron activity around the gap. A change to audit rules plus a logging gap suggests deliberate defense evasion, not a benign edit.
Contain by rotating credentials and keys that were present on those hosts, removing them from the backup rotation, and rebuilding from a known-good image after forensic capture. Do not simply restore the audit rules and continue. After rebuild, enforce immutable audit configuration, verify central log forwarding, and add an alert for auditd stops or rule changes. For PCI evidence, record the timeline, scope, containment actions, and validation that the rebuilt hosts match the baseline.
3. If the vendor cannot sign the module and operations demands the backup agent, define a compensating control and its residual risk.
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A defensible compensating control would run the backup agent only on hosts with a dedicated backup NIC or VLAN, restrict egress to the vendor's documented endpoints, confine the agent with an AppArmor profile, remove interactive shell access from the agent account, and monitor process execution and file integrity continuously. The agent should not have general root capabilities beyond what the module needs, and the signing key or module should be distributed only through a controlled repository. Backups should be verified restorable and encrypted, with separate credentials from production administration.
The residual risk is significant: an unsigned kernel module runs in kernel space, can bypass AppArmor and many file-integrity controls, and can persist across updates. Network restrictions reduce data exfiltration but do not prevent local privilege escalation or kernel compromise. This should be a time-bound exception with an owner, audit approval, expiry date, and a migration plan to a signed or user-space agent. If the exception cannot be approved, the correct decision is to delay the agent rollout, not to disable Secure Boot on all database hosts.