CVE-2023-27351 is an improper authentication vulnerability in PaperCut NG/MF. CISA added it to the Known Exploited Vulnerabilities catalogue on 20 April 2026, which means attackers have used it in real attacks. PaperCut NG/MF contains an improper authentication vulnerability that could allow remote attackers to bypass authentication on affected installations via the SecurityRequestFilter class.
What this vulnerability means in practice
Improper authentication in a product like PaperCut NG/MF means the normal trust check that separates unauthorised requests from administrative or privileged actions can be bypassed. In this case, the danger is not just initial access, but the potential to reach application functions that were intended to be protected by the login boundary.
Because the weakness sits in authentication logic, it can affect the product before any user-level monitoring or downstream access policy has a chance to intervene. For practitioners, that makes the authentication layer a high-value control point, and one that should be treated as exposed until version status and vendor guidance are confirmed through the CVE Program and the NIST National Vulnerability Database.
Why this class of flaw is dangerous
Authentication bypasses are dangerous because they can convert a remote, unauthenticated request into trusted application access. In a print management platform, that can expose administrative workflows, sensitive configuration, user data, or operational controls that were never meant to be reachable without valid credentials.
The exploitability profile is especially concerning when the vulnerable component is internet-facing or reachable from broad internal networks. If an attacker can reach the affected service, they may not need stolen credentials, phishing, or a prior foothold to begin interacting with the application as though they were an authorised user.
For this reason, organisations should assume that public exposure, weak segmentation, and delayed patching all increase practical risk. Even when exploitation is not yet confirmed in a specific environment, vulnerability management should prioritise asset scope, KEV status, and internet reachability together rather than treating patching as a standalone task.
How organisations check exposure and reduce blast radius
Exposure review starts with a complete inventory of where PaperCut NG/MF is deployed, which instances are reachable, and whether those instances sit behind compensating controls. That includes checking whether the vulnerable service is exposed to the internet, whether it is segmented from general user traffic, and whether administrative interfaces are restricted to trusted networks.
Vendor advisories remain the authoritative source for affected versions, remediation status, and product-specific guidance, while the KEV catalogue indicates whether real-world exploitation has been observed. Those two references should be used together so that remediation decisions are based on both product facts and active exploitation pressure.
Where immediate patching is not possible, compensating controls should narrow reachability, reduce privilege on adjacent systems, and increase scrutiny of any authentication-related traffic. The goal is to limit how far an attacker could move if the authentication boundary fails.
Detection, recovery, and post-exploitation review
After remediation, organisations should review logs and administrative actions for signs that the vulnerability was used before the fix was applied. For an authentication bypass, that review should focus on suspicious login patterns, unexpected administrative changes, unusual requests to protected endpoints, and any behaviour that suggests a session was established without a legitimate sign-in.
Post-patch validation is important because fixing the code does not by itself prove the environment was never abused. If an attacker gained access through the bypass, the incident response question becomes whether they altered settings, created accounts, accessed documents, or used the platform as a stepping stone to other systems.
When compromise is plausible, rotate any credentials, API keys, or service account material that may have been exposed through the affected application path, and verify that privileged actions were not performed during the exposure window.
Risk and Threat Considerations
This vulnerability creates a direct remote-access risk because it weakens the trust boundary that stands between an external request and a protected application function. If the service is reachable, an attacker may be able to bypass authentication entirely and operate as an authenticated user or administrator.
Failure mechanism: The vulnerable authentication path can accept a request without performing the intended identity check, which lets malicious traffic reach protected functions and potentially establish unauthorised access.
Impact: Successful abuse can lead to administrative takeover, exposure of sensitive data or configuration, and follow-on movement into adjacent systems that trust the compromised application.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
CIS Controls v8 and NIST SP 800-53 Rev 5 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| CIS Controls v8 | CIS-7 — Continuous Vulnerability Management | This CVE requires exposure review, prioritisation, and remediation of an actively exploited flaw. |
| CIS-4 — Secure Configuration of Enterprise Assets and Software | Exposure is reduced by hardening reachability and limiting the service's attack surface. | |
| Recommendation — Prioritise the vulnerable PaperCut asset, verify exposure, and remediate it through continuous vulnerability management. Restrict network exposure and harden the PaperCut deployment to reduce attack surface. | ||
| NIST SP 800-53 Rev 5 | SI-2 — Flaw Remediation | An exploitable authentication flaw requires tracked remediation and verification. |
| AC-2 — Account Management | Post-compromise review must check whether unauthorised access created or altered accounts. | |
| AU-6 — Audit Record Review, Analysis, and Reporting | Detection and post-patch review depend on analysing authentication and admin activity logs. | |
| Recommendation — Remediate the authentication flaw quickly and confirm the affected hosts are corrected. Review account creation and changes for signs of abuse after exploitation. Review audit records for unauthorised authentication and privileged actions. | ||
Practitioner Guidance
Why practitioners should care: Treat this as more than a patch-and-forget issue. If the product was exposed while vulnerable, assume the possibility of silent access and validate whether any protected actions were performed during the exposure window.
What to watch for: Look for anomalous administrative activity, new or unexpected accounts, unusual configuration changes, and requests that appear to have reached sensitive application routes without a corresponding legitimate authentication event.
Practitioner takeaway: For KEV-listed authentication flaws, remediation and compromise review should happen together, because restoring code integrity does not automatically restore trust in prior activity.
Related resources from NHI Mgmt Group
- CVE-2023-49105 (ownCloud Improper Authentication)
- CVE-2023-35082 (Ivanti Endpoint Manager Mobile (EPMM) and MobileIron Core Authentication Bypass)
- CVE-2023-35078 (Ivanti Endpoint Manager Mobile Authentication Bypass)
- CVE-2023-28461 (Array Networks AG and vxAG ArrayOS Missing Authentication for Critical Function)