PaperCut NG/MF Critical Zero-Day Exploited in the Wild
Overview
On August 27, 2026, PaperCut Software published an urgent security advisory stating that it is investigating active exploitation of a vulnerability affecting PaperCut NG and PaperCut MF. PaperCut has confirmed customer incidents and is treating the issue as a security emergency. At the initial time of disclosure, the vulnerability had not been assigned a CVE identifier, and PaperCut had not publicly disclosed a CVSS score, vulnerability class, authentication requirements, or the technical details of the exploit path. However on August 28, the vendor assigned CVE-2026-81578 and CVE-2026-82078 for the two vulnerabilities that make up the exploit chain.
|
CVE ID |
Description |
CWE |
CVSSv4 |
|
CVE-2026-81578 |
Authentication Bypass |
CWE-306 Missing authentication for critical function. |
|
|
CVE-2026-82078 |
Unsafe Dynamic Class Loading in Database Connector |
CWE-470 Use of Externally-Controlled input to select classes or code (‘unsafe reflection’). |
PaperCut NG and PaperCut MF are print management platforms commonly deployed within enterprise, education, and other organizational environments. Because the PaperCut Application Server provides web-accessible administrative and application functionality, organizations with servers exposed to the public internet should prioritize remediation and access restriction.
PaperCut stated in its advisory that information supplied by a university customer’s security team and digital forensics and incident response team enabled its security response team to reproduce the vulnerability in PaperCut NG and PaperCut MF. On August 28, 2026 at 02:10 AEST, PaperCut released emergency patches for PaperCut NG and PaperCut MF versions 25 and 26, followed later the same day with patches for version 24.
PaperCut has been targeted in the past; in 2023, CVE-2023-27350 was broadly exploited in the wild by multiple threat-actor groups, including ransomware operators. This prior history increases the urgency organizations should address this new zero-day with.
PaperCut currently considers all versions of PaperCut NG and PaperCut MF potentially impacted. Customers operating internet-accessible PaperCut Application Servers should take immediate action even if no suspicious activity has been observed.
On August 31, 2026, both CVE-2026-81578 and CVE-2026-82078 were added to the U.S. Cybersecurity and Infrastructure Security Agency’s (CISA) list of known exploited vulnerabilities (KEV), based on evidence of active exploitation. A Metasploit module is now available to validate exposure to the exploit chain.
Technical overview
The vulnerability is an authentication bypass that lets attackers invoke privileged PaperCut components. This can be leveraged to reconfigure an external database lookup. When this lookup is triggered, malicious SQL can be executed, resulting in remote code execution.
PaperCut uses the Apache Tapestry framework, whose “complex direct” request format can identify one page to display and a different page containing the component to execute. PaperCut validates access only to the displayed page. By selecting either the public Error page or Exception page for display, an attacker can bypass authentication while invoking administrative components belonging to ConfigEditor or UserList. Additionally, the first emergency patch could be bypassed by using the Home page for display, however the newest version of the vendor patch correctly remediates this bypass.
The attack uses HTTP POST requests to the following URIs (Note that the path segment with the value 1 shown below can be any value for this path segment, and the Error path segment may also be the Exception or Home path segment):
/app?service=direct/1/Error/ConfigEditor/quickFindForm
/app?service=direct/1/Error/ConfigEditor/$Form
/app?service=direct/1/Error/UserList/$QuickFind.$FormThe first two URIs provide unauthenticated access to PaperCut’s configuration editor. The third can invoke a user or card search that triggers the configured external database lookup.
An attacker first uses the ConfigEditor requests to modify four external user-lookup settings:
user-lookup.db-driver
user-lookup.db-url
user-lookup.id-to-username-sql
user-lookup.enabledThese settings normally allow administrators to connect PaperCut to an external card database. After bypassing authentication, however, the attacker can configure them with a malicious JDBC connection and a malicious SQL statement.
By leveraging PaperCut’s bundled Apache Derby database driver and supplying a Derby CALL statement that activates its foreignViews feature, Derby opens an attacker-controlled H2 JDBC URL. H2 processes an inline INIT statement that creates a JavaScript-backed database trigger. PaperCut includes the Nashorn JavaScript engine, allowing that trigger to start an operating-system process. However it is expected that other mechanisms to execute an arbitrary command can also be used instead of Nashorn. Finally, the attacker submits a search through the forged UserList request. This activates the external lookup and executes the malicious SQL.
Mitigation guidance
Organizations running PaperCut NG or PaperCut MF should prioritize patching on an emergency basis, particularly where the PaperCut Application Server is accessible from the public internet.
PaperCut has released emergency patches for PaperCut NG and PaperCut MF versions 24, 25 and 26.
Updated on September 1, 2026: Note that the vendor has released a third version of the emergency patch. Any organization that has applied either the original first version or the second version of the emergency patch is not fully protected, and must apply the third emergency patch immediately.
The vendor notes that these builds have not undergone their normal release process and are intended as emergency fixes for customers with public-facing servers that cannot otherwise sufficiently mitigate exposure.
PaperCut recommends that administrators immediately restrict web access to trusted IP addresses only, such as internal corporate network ranges. Firewall rules, network access controls, reverse-proxy restrictions, or equivalent measures should be used to prevent untrusted internet hosts from reaching PaperCut web interfaces.
Please read the PaperCut security advisory for the latest remediation guidance, updated indicators of compromise, and additional release information.
Artifacts/Evidence Sources and IOCs
For detection and forensic analysis, PaperCut has identified several preliminary artifacts and evidence sources that may indicate compromise.
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Application activity: Alerts from intrusion-detection, endpoint-security, or network-monitoring products involving the PaperCut Application Server, particularly suspicious post-exploitation activity associated with pc-app.exe.
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Log integrity: Missing, unexpectedly truncated, or deleted PaperCut server.log files.
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PaperCut server.log entries:
PaperCut has not yet published validated network-based indicators such as malicious IP addresses, domains, or URLs.
The vendor specifically warns that the absence of these indicators should not be interpreted as evidence that a system has not been affected.
Rapid7 customers
Exposure Command, InsightVM, and Nexpose
Exposure Command, InsightVM, and Nexpose customers can assess exposure to this new PaperCut zero-day, with an authenticated vulnerability check expected to be available in the August 28 (today’s) content release.
