CriticalCVE-2023-27351

    PaperCut NG/MF Improper Authentication Vulnerability (CVE-2023-27351)

    Published Last modified In CISA KEV since Sources: CVE.org

    What is affected, what to do

    This vulnerability in PaperCut MF/NG allows an unauthenticated remote attacker to bypass authentication controls, exposing sensitive data and enabling limited write operations without any prior access or credentials. The flaw stems from improper authentication handling (CWE-287) and is exploitable over the network with low complexity, requiring no user interaction. Active exploitation in the wild has been confirmed, with threat actors leveraging this vulnerability in ransomware campaigns, prompting CISA to add it to the Known Exploited Vulnerabilities catalog. Organisations running affected versions of PaperCut MF or NG must treat this as a priority remediation — a patch is available and should be applied immediately.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update papercut_mf to version 20.1.7, papercut_ng to version 21.2.11. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • PaperCut NG eq 22.0.5 (Build 63914)..22.0.5 (Build 63914)

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    8.2

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v3

    8.2

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: CVE.org

    78.1 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.57

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    247.86 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploited
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    8.2 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    8.2
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    78.1 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.57
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-287

    Response and mitigation

    Why act now?

    Prioritisation rationale

    The flaw is network-reachable without authentication and leaks user records, card numbers and password hashes. In the field it is chained with the sibling PaperCut authentication bypass to gain admin access and then abuse the scripting feature for code execution on the server. CISA flags known ransomware campaign use, and the bug sits among the CVEs most likely to be exploited. Typical victims are education, public administration, healthcare and mid-market firms running centralised print management, which is usually wired into Active Directory. Treat any instance reachable from untrusted networks as critical with a sub-24-hour patch deadline. Internal-only instances remain high priority because of lateral-movement risk and the AD integration.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Patch: Upgrade PaperCut NG/MF on the Application Server. Affected branches are 15.0–20.1.7, 21.0.0–21.2.11 and 22.0.0–22.0.9. The vendor lists 20.1.7, 21.2.11 and 22.0.9 or later as the fixed builds. Legacy 15.x–19.x installs get no fix and must be upgraded. Confirm the exact fixed build in the PaperCut vendor advisory.
      • Kill exposure now: Firewall the Application Server web interface (default TCP 9191/HTTP, 9192/HTTPS) from the internet and from all non-admin networks. Allow access only from the admin jump-host segment.
      • Hunt before you patch: Take forensic copies of server.log and the application log. Then review them for unexpected admin logins, changes to print or device scripting, and modified user-sync settings. Any hit means the host is treated as compromised and full IR starts, including an assessment of NIS2 reporting duties.
      • Rotate secrets: Change the PaperCut admin password, all internal PaperCut user passwords (their hashes are retrievable) and the LDAP/AD sync bind account. Treat user card numbers and PINs as disclosed and reissue them.
      • Inventory: Find every PaperCut instance, including forgotten test or branch servers. Check for external visibility of 9191/9192 with your ASM tooling or a Shodan sweep of your IP ranges.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Place the Application Server in a dedicated print-management VLAN. Inbound, allow only print clients, release stations and MFDs. Outbound, allow only the required destinations (directory, SMTP, licensing) and deny all other egress to break payload staging and C2.
      • PaperCut IP allowlisting: Under Options → Advanced → Security, restrict "Allowed IP addresses" for admin access, web services and devices to known subnets. This is a vendor-recommended hardening step.
      • WAF/IPS: Block or alert on unauthenticated external requests to /app?service=page/SetupCompleted and to admin API paths. Enable the current ET "PaperCut" exploit signatures in Suricata or Snort and confirm the rules are up to date.
      • Endpoint hardening: Use an EDR or ASR rule to stop the PaperCut service process (pc-app.exe, or the Java process under /papercut/) from spawning shells (cmd.exe, powershell.exe, /bin/sh). Run the service under a least-privilege account rather than SYSTEM/root where the install supports it.
      • Configuration: Keep script sandboxing enforced (print.script.sandboxed=Y, device.script.sandboxed=Y). Minimise internal PaperCut accounts and prefer directory-backed authentication with MFA in front of the admin console.

    3. Runbook step 3

      Detection rules

      • Web/proxy logs: Look for requests to SetupCompleted from non-admin sources. Example SPL: index=proxy uri_path="/app" uri_query="*SetupCompleted*" dest_port IN (9191,9192).
      • PaperCut application/audit log (server.log): Watch for "Admin user … modified the config key print.script.sandboxed" or "device.script.sandboxed", unexpected "User/Group sync settings changed" entries, and admin logins from unfamiliar IPs. Forward these logs to the SIEM via syslog.
      • EDR/Sysmon EID 1 – process ancestry: Parent pc-app.exe or a PaperCut-path java.exe spawning cmd.exe, powershell.exe, rundll32.exe, certutil.exe or bitsadmin.exe. Sigma shape: ParentImage|endswith: '\pc-app.exe' AND Image|endswith: ['\cmd.exe','\powershell.exe'].
      • Network telemetry (Zeek/Suricata): Flag outbound connections from the Application Server to internet hosts that are not on the allowlist. Pay particular attention to .exe, .ps1 or .dll downloads and to RMM tooling such as AnyDesk or Atera. Also watch for unusually large admin-API responses, which can indicate bulk user-data exfiltration.
      • Windows Security/Sysmon: Correlate EID 4720/4732 (new local accounts or group additions) and Sysmon EID 11 (new files under \PaperCut MF\server\ or in temp directories) with PaperCut admin activity.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    This vulnerability allows remote attackers to bypass authentication on affected installations of PaperCut NG 22.0.5 (Build 63914). Authentication is not required to exploit this vulnerability. The specific flaw exists within the SecurityRequestFilter class. The issue results from improper implementation of the authentication algorithm. An attacker can leverage this vulnerability to bypass authentication on the system. Was ZDI-CAN-19226.

    Source: cvelistv5, NVD (NIST), CISA KEV Catalog, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    PaperCutNG
    • 22.0.5 (Build 63914)
    • —
    papercutpapercut_mf
    • 15.0 – < 20.1.7
    • 22.0.0 – < 22.0.9
    • 21.0.0 – < 21.2.11
    • 20.1.7
    • 22.0.9
    • 21.2.11
    papercutpapercut_ng
    • 21.0.0 – < 21.2.11
    • 15.0 – < 20.1.7
    • 22.0.0 – < 22.0.9
    • 21.2.11
    • 20.1.7
    • 22.0.9

    Vendors and products

    Weakness classes (CWE)

    • CWE-287Improper Authentication

      Class

      When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.

      Entry at MITRE CWE-287

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited

    References & sources

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedtrendmicro.com

      Show 6 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://www.papercut.com/kb/Main/PO-1216-and-PO-1219
      Referenceadded
      —
      https://www.zerodayinitiative.com/advisories/ZDI-23-232/
      Referenceremoved
      https://www.papercut.com/kb/Main/PO-1216-and-PO-1219
      —
      Referenceremoved
      https://www.zerodayinitiative.com/advisories/ZDI-23-232/
      —
      Reference typeremoved
      https://www.papercut.com/kb/Main/PO-1216-and-PO-1219 Types: Vendor Advisory
      —
      Reference typeremoved
      https://www.zerodayinitiative.com/advisories/ZDI-23-232/ Types: Third Party Advisory, VDB Entry
      —

      Source identifier: zdi-disclosures@trendmicro.com

    2. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: active, automatable: yes

      Show 5 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-27351
      Referenceremoved
      https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-27351
      —
      Reference typeremoved
      https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-27351 Types: US Government Resource
      —
      SSVC assessmentremoved
      {"id":"CVE-2023-27351","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-04-20T00:00:00+00:00"}
      —
      SSVC assessmentadded
      —
      {"id":"CVE-2023-27351","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-04-21T03:55:38.095423Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    3. Record modifiedCVE Program

      Show 6 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://www.papercut.com/kb/Main/PO-1216-and-PO-1219
      Referenceadded
      —
      https://www.zerodayinitiative.com/advisories/ZDI-23-232/
      Referenceremoved
      https://www.papercut.com/kb/Main/PO-1216-and-PO-1219
      —
      Referenceremoved
      https://www.zerodayinitiative.com/advisories/ZDI-23-232/
      —
      Reference typeremoved
      https://www.papercut.com/kb/Main/PO-1216-and-PO-1219 Types: Vendor Advisory
      —
      Reference typeremoved
      https://www.zerodayinitiative.com/advisories/ZDI-23-232/ Types: Third Party Advisory, VDB Entry
      —

      Source identifier: af854a3a-2127-422b-91ae-364da2661108

    Linked advisories

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