How do you patch or mitigate CVE-2023-27351 (PaperCut NG/MF Improper Authentication)?

By NHI Mgmt Group Editorial Team Updated October 19, 2026 Domain: Threats, Abuse & Incident Response

CISA's required action for CVE-2023-27351 is to apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. In practice, find every instance of PaperCut NG/MF in your estate, apply the vendor security update or documented mitigation, and if no fix can be applied, remove internet exposure or take the product out of service.

What this CVE means operationally

PaperCut NG/MF improper authentication is a patch-or-mitigate-now issue because the product sits on a trust boundary, often with broad internal reach. The practical question is not whether the flaw is interesting, but whether any instance is still reachable in your environment and whether the vendor’s security update or mitigation has been applied to it.

The first pass is inventory, because missed instances are the usual reason these vulnerabilities remain open after a remediation campaign. Find every PaperCut NG/MF deployment, confirm whether it is internet-facing or otherwise reachable from untrusted networks, and compare the live state against the vendor advisory and the CISA KEV entry. If you cannot verify a safe remedial path, remove exposure before treating the system as acceptable.

For prioritisation, use the vulnerability as a service-continuity and access-control problem, not just a software defect. If the product is still in service, the safest outcome is to move it onto the vendor-documented fix or mitigation path as quickly as possible, then verify that remote access, admin functions, and any exposed interfaces are no longer reachable from the public internet or other high-risk segments.

Why exposure persists after "patching"

The most common failure mode is partial remediation: one server is fixed while another PaperCut instance, test environment, or legacy node remains exposed. That leaves a live foothold even though the ticket is closed. This is especially dangerous when the vulnerable service is reachable by users or administrators from networks that are not tightly controlled.

A second failure mode is relying on compensating controls without proving they actually block the path the flaw uses. A firewall rule, reverse proxy, or network ACL can help, but only if it truly prevents access to the affected interface and is checked against the deployment’s real network path. If the product cannot be secured to an acceptable level, discontinuing use is the correct operational decision.

Because this CVE is on CISA’s Known Exploited Vulnerabilities catalogue, assume that delayed action increases the chance of opportunistic exploitation. Use the CISA Known Exploited Vulnerabilities Catalog and the NIST National Vulnerability Database as the authoritative references for tracking the issue, then apply the vendor’s instructions rather than improvising a local workaround.

How to verify remediation and watch for compromise

After applying the fix or mitigation, verify the observable state, not just the change request. Confirm the affected PaperCut service is at the expected version or configuration state, confirm the exposure path is closed, and confirm the product is no longer accessible in the way the vulnerable path required. For cloud-adjacent or externally reachable deployments, also confirm the environment matches the constraints in the vendor’s guidance and any applicable CISA direction.

When patching is delayed or impossible, assume the risk window remains open and move immediately to containment. That means removing internet exposure, restricting access to trusted administrative paths only, and considering temporary suspension if you cannot confidently reduce the blast radius. Zero-Day and Exploited Vulnerabilities Guide is useful here because the same discipline applies: emergency mitigation first, then verification, then post-fix hunting.

If the system was exposed before remediation, check for signs of earlier compromise before declaring victory. Review administrative logins, unusual configuration changes, unexpected account creation, service restarts, and any outbound activity that should not have occurred from a print management server. If the platform handled sensitive documents or credentials, rotate the credentials and sessions that may have been reachable through the vulnerable path.

Standards & Framework Alignment

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

CIS Controls v8, NIST CSF 2.0, NIST SP 800-53 Rev 5 and OWASP ASVS set the governance and control requirements practitioners need to meet.

Framework	Control / Reference	Relevance
CIS Controls v8	CIS-7 — Continuous Vulnerability Management	PaperCut KEV remediation depends on prompt identification and patching of exposed vulnerable systems.
Recommendation — Prioritise, remediate, and verify the vulnerable PaperCut instances before returning them to normal service.
NIST CSF 2.0	PR.IR-01 — Networks and environments are protected from unauthorized logical access and usage	Removing exposure and constraining access paths is central to mitigating an exploited authentication flaw.
Recommendation — Restrict or remove access paths to the affected service until the remediation is verified.
NIST SP 800-53 Rev 5	SI-2 — Flaw Remediation	The answer is about applying vendor fixes or mitigations for a known exploited vulnerability.
	IR-4 — Incident Handling	Delayed remediation requires compromise assessment and response actions if exposure preceded the fix.
Recommendation — Apply the vendor fix or approved mitigation and confirm the vulnerable condition is eliminated. Investigate for prior compromise when the vulnerable service was exposed before remediation.
OWASP ASVS	V6 — Authentication	Improper authentication vulnerabilities are directly about authentication failure and access control.
Recommendation — Validate that the affected authentication path is no longer reachable or exploitable after remediation.

Practitioner Guidance

What to prioritise: Treat asset discovery and exposure removal as the first two tasks, because a perfect patch on an unknown or still-public instance does not reduce risk. If you have multiple PaperCut deployments, validate each one independently rather than assuming a fleet-wide change landed everywhere.

What to verify: The right evidence is a combination of vendor-confirmed remediation, closed exposure, and post-change checks that show the vulnerable service is no longer reachable in the same way. If you cannot produce that evidence, the remediation should not be considered complete.

Decision rule: If the vendor-supported fix or mitigation can be applied cleanly, do it immediately and then hunt for compromise. If it cannot be applied safely, remove the service from exposure or discontinue use rather than leaving a known exploited flaw in production.

Practitioner takeaway: With exploited authentication flaws, the real control is not "we planned to patch", it is "we can prove the vulnerable path is gone and no earlier intrusion survived it."

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NHIMG Editorial Note
Reviewed and updated by the NHIMG editorial team on October 19, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org