CriticalCVE-2026-81578

    PaperCut MF/NG: Authentication Bypass (CVE-2026-81578)

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

    What is affected, what to do

    An improper access control vulnerability exists in the web management interface of PaperCut MF and PaperCut NG. Under specific conditions, unauthenticated remote requests targeting administrative functions can trigger backend actions prior to the completion of access validation checks.

    The vulnerability is being actively exploited; treat it as a priority. Update PaperCut MF/NG to version 24.1.10, PaperCut MF/NG to version 25.0.13, PaperCut MF/NG to version 26.0.5. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • PaperCut MF/NG lt *..24.1.10
    • PaperCut MF/NG lt 25.0.0..25.0.13
    • PaperCut MF/NG lt 26.0.0..26.0.5
    Fixed in
    • PaperCut MF/NG 24.1.10
    • PaperCut MF/NG 25.0.13
    • PaperCut MF/NG 26.0.5

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

    Metrics

    9.8

    Priority

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

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: CVE.org

    83.7 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.69

    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
    253.41 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 exploitedSource: VulnCheck KEV (opens in a new tab), CIRCL Sightings How we classify
    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
    9.8 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
    9.8
    CVSS v4

    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
    8.8
    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
    83.7 %
    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
    99.69
    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-305

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable with no authentication and no user interaction, and lets an attacker modify administrative configuration — chained with CVE-2026-82078 the realistic outcome is remote code execution on a server that is typically domain-joined. The EPSS score places it among the vulnerabilities most likely to be exploited right now, and the KEV listing confirms observed in-the-wild activity. For NIS2-scoped organisations running PaperCut as print infrastructure (common in public sector, healthcare, and education), treat internet-exposed instances as P1 incidents until proven clean, and internal instances as P2 with a 72-hour patch SLA. No ransomware campaign is currently flagged for this CVE, but the predecessor CVE-2023-27350 was heavily abused by ransomware operators — plan defensively on that precedent.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory PaperCut footprint: locate every PaperCut NG and PaperCut MF instance running 24.1.9, 25.0.2–25.0.12, or 26.0.2–26.0.4 via asset DB, software deployment tool, or nmap sweeps on TCP 9191/9192.
      • Apply vendor patch: upgrade to the fixed release named in the PaperCut security bulletin — exact build number is not confirmed here, consult the vendor advisory. Internet-exposed servers first.
      • Pull the admin interface (TCP 9191 HTTP, 9192 HTTPS) off the public internet immediately: perimeter rule "deny any → PaperCut:9191,9192", restrict access to admin VLAN or VPN/jump host only.
      • Audit admin activity: in the PaperCut console under "Logs → Application Log" review every config change over the last 30 days, focusing on print-script edits, device registrations, user/group sync changes, and SMTP/auth settings.
      • Assume compromise on exposed hosts: rotate admin passwords, invalidate API tokens, reissue credentials for linked service accounts (LDAP bind, SMTP, database) — chaining with CVE-2026-82078 is effectively RCE.
      • Snapshot the PaperCut configuration and [app-path]/server/data/ before patching to preserve forensic evidence of any config tampering.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: place PaperCut in a dedicated management segment; inbound limited to printer ports (IPP 631, 9100) and client access (9191 from client LAN), admin interface reachable only from jump-host range.
      • Reverse-proxy or WAF in front: nginx or HAProxy with path allowlist — restrict /app and /admin to admin source IPs; block anonymous POSTs against /rpc/ endpoints until the patch is verified.
      • IPS/Suricata signatures: enable vendor rules for this CVE where available, or write a custom alert on HTTP POST to /app?service=direct/ and /rpc/api/rpc/* without a valid session cookie.
      • Least-privilege hardening: run the PaperCut service under a dedicated non-admin service account; disable the "print script" feature in configuration if not operationally required — it is the known chaining vector to code execution.
      • OS-level hardening: AppLocker/WDAC on Windows or SELinux policy on Linux to prevent the PaperCut JVM from spawning cmd.exe, powershell.exe, or /bin/sh child processes.
      • Egress filtering: restrict outbound traffic from the PaperCut host to required destinations only (LDAP, SMTP, printer return channels) to break C2 callbacks.

    3. Runbook step 3

      Detection rules

      • Web/reverse-proxy logs: unauthenticated POSTs to admin paths. KQL sketch: WebLogs | where UrlPath has_any ("/app?service=direct", "/rpc/api/rpc", "SetupCompleted") and StatusCode in (200,302) and (CookieHeader !has "JSESSIONID" or CookieHeader == "").
      • Sysmon EID 1 on the PaperCut host: process ancestry pc-app.exe or java.exe → cmd.exe/powershell.exe/wscript.exe/bash is a strong indicator of chaining with CVE-2026-82078. Sigma: parent_image endswith 'pc-app.exe' and image endswith ('cmd.exe','powershell.exe').
      • File-integrity monitoring on [app-path]/server/data/config.properties, print-scripts/, and server/custom/ — alert on writes outside approved change windows (auditd -w watch or Windows EID 4663).
      • PaperCut Application Log: audit events for admin.config.change, print.script.updated, device.created without a preceding successful admin login from the same user/IP context.
      • Zeek/Suricata: HTTP requests from scanning user agents (curl, python-requests, Go-http-client) hitting port 9191/9192, correlated with subsequent configuration-change events.

    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

    An improper access control vulnerability exists in the web management interface of PaperCut MF and PaperCut NG. Under specific conditions, unauthenticated remote requests targeting administrative functions can trigger backend actions prior to the completion of access validation checks. This allows an unauthenticated remote attacker to modify certain system configurations.

    Source: cvelistv5, NVD (NIST), CISA KEV Catalog

    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
    PaperCutPaperCut MF/NG
    • 25.0.0 – < 25.0.13
    • 26.0.0 – < 26.0.5
    • < 24.1.10
    • 25.0.13
    • 26.0.5
    • 24.1.10
    papercutpapercut_mf
    • 25.0.2 – < 25.0.12
    • < 24.1.9
    • 26.0.2 – < 26.0.4
    • 25.0.12
    • 24.1.9
    • 26.0.4
    papercutpapercut_ng
    • 26.0.2 – < 26.0.4
    • < 24.1.9
    • 25.0.2 – < 25.0.12
    • 26.0.4
    • 24.1.9
    • 25.0.12

    Vendors and products

    Weakness classes (CWE)

    • CWE-305Authentication Bypass by Primary Weakness

      Base

      The authentication algorithm is sound, but the implemented mechanism can be bypassed as the result of a separate weakness that is primary to the authentication error.

      Entry at MITRE CWE-305

    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
    Metasploit Framework (Pull Request)Metasploit PR #21842 (opens in a new tab)github.comPoC (publicly reported)reported, not verified—
    PoC-in-GitHub (nomi-sec)papercut-toolkit (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)PaperCut-CVE-2026-81578-82078 (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • papercut.comkb/Main/security-bulletin-27-aug-2026-urgent-security-advisory
    • github.comrapid7/metasploit-framework/pull/21842Exploit
    • cisa.govCISA KEVgovernment-resource

    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 modifiedeb41dac7…

      1 individual change: Affected versions (publisher)

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)added
      —
      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/81xxx/CVE-2026-81578.json">CVE-2026-81578</a>

      Source identifier: eb41dac7-0af8-4f84-9f6d-0272772514f4

    2. Record modifiedCISA ADP

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

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentchanged
      {"id":"CVE-2026-81578","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-31T15:25:24.506491Z"}
      {"id":"CVE-2026-81578","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-28T00:00:00+00:00"}

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

    3. Initial analysis by NVDNVD

      Newly listed as affected:
      • PaperCut MF (before 24.1.9; 25.0.2 – before 25.0.12; 26.0.2 – before 26.0.4)
      • PaperCut NG (before 24.1.9; 25.0.2 – before 25.0.12; 26.0.2 – before 26.0.4)

      CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

      Show 5 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      CVSSadded
      —
      AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
      Product configurationadded
      —
      OR *cpe:2.3:a:papercut:papercut_mf:*:*:*:*:*:*:*:* versions up to (excluding) 24.1.9 *cpe:2.3:a:papercut:papercut_ng:*:*:*:*:*:*:*:* versions up to (excluding) 24.1.9 *cpe:2.3:a:papercut:papercut_mf:*:*:*:*:*:*:*:* versions from (including) 25.0.2 up to (excluding) 25.0.12 *cpe:2.3:a:papercut:papercut_mf:*:*:*:*:*:*:*:* versions from (including) 26.0.2 up to (excluding) 26.0.4 *cpe:2.3:a:papercut:papercut_ng:*:*:*:*:*:*:*:* versions from (including) 25.0.2 up to (excluding) 25.0.12 *cpe:2.3:a:papercut:papercut_ng:*:*:*:*:*:*:*:* versions from (including) 26.0.2 up to (excluding) 26.0.4
      Reference typeadded
      —
      PaperCut: https://www.papercut.com/kb/Main/security-bulletin-27-aug-2026-urgent-security-advisory/ Types: Patch, Vendor Advisory
      Reference typeadded
      —
      CISA-ADP: https://github.com/rapid7/metasploit-framework/pull/21842 Types: Issue Tracking, Patch
      Reference typeadded
      —
      CISA-ADP: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-81578 Types: Patch, Third Party Advisory, US Government Resource

      Source identifier: nvd@nist.gov

    4. Record modifiedCISA ADP

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

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentchanged
      {"id":"CVE-2026-81578","role":"CISA Coordinator","options":[{"exploitation":"poc"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-31T12:34:44.713666Z"}
      {"id":"CVE-2026-81578","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-31T15:25:24.506491Z"}

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

    5. CISA KEV entry updated9119a7d8…

      4 individual changes: Date Added, Due Date, Required Action, Vulnerability Name

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Date Addedadded
      —
      2026-08-31
      Due Dateadded
      —
      2026-08-31
      Required Actionadded
      —
      2026-08-31
      Vulnerability Nameadded
      —
      2026-08-31

      Source identifier: 9119a7d8-5eab-497f-8521-727c672e3725

    Show 4 older changes

    1. Record modifiedCISA ADP

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-81578

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

    2. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: proof of concept, automatable: yes

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://github.com/rapid7/metasploit-framework/pull/21842
      SSVC assessmentchanged
      {"id":"CVE-2026-81578","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-28T00:00:00+00:00"}
      {"id":"CVE-2026-81578","role":"CISA Coordinator","options":[{"exploitation":"poc"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-31T12:34:44.713666Z"}

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

    3. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: none known, automatable: yes

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentchanged
      {"id":"CVE-2026-81578","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-28T15:34:59.094633Z"}
      {"id":"CVE-2026-81578","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-28T00:00:00+00:00"}

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

    4. Receivedeb41dac7…

      CVSS V4.0: AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:H/VA:L/SC:N/SI:N/SA:N

      Weakness type: CWE-305

      Description added or changed

      Show 5 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionadded
      —
      An improper access control vulnerability exists in the web management interface of PaperCut MF and PaperCut NG. Under specific conditions, unauthenticated remote requests targeting administrative functions can trigger backend actions prior to the completion of access validation checks. This allows an unauthenticated remote attacker to modify certain system configurations.
      CVSSadded
      —
      AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:H/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
      Weakness type (CWE)added
      —
      CWE-305
      Affected versions (publisher)added
      —
      New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/81xxx/CVE-2026-81578.json">CVE-2026-81578</a>
      Referenceadded
      —
      https://www.papercut.com/kb/Main/security-bulletin-27-aug-2026-urgent-security-advisory/

      Source identifier: eb41dac7-0af8-4f84-9f6d-0272772514f4

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