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PaperCut’s 2023 zero-day let an unauthenticated attacker execute code remotely on affected PaperCut MF and NG Application Servers. The flaw, CVE-2023-27350, arose from an access-control failure in the Setup Wizard completion flow. PaperCut released fixes on March 8, 2023; reports of exploitation followed in mid-April. The incident shows how a routine internal service can have a large blast radius when it is network-reachable and runs with high privileges—not that internal apps are always attacked first.
What was the PaperCut zero-day?
CVE-2023-27350 was an improper-access-control vulnerability in PaperCut MF and NG that could allow an unauthenticated remote attacker to execute code on an affected Application Server. PaperCut rated it 9.8, Critical, on the CVSS scale. The flaw was distinct from CVE-2023-27351, a separate March 2023 vulnerability involving user account data.
PaperCut’s explanation centers on an HTTP session created during setup. The Setup Wizard’s final completion page was not correctly guarded after setup, allowing a crafted URL to expose a session variable and provide access to built-in administrator functionality. CISA and the FBI likewise described improper access controls in the SetupCompleted Java class that could enable administrator access.
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After gaining administrator access, an attacker could use existing PaperCut features to run code. PaperCut described disabling the print-scripting security sandbox and adding a script to download and execute code. CISA and the FBI also documented public proof-of-concept routes involving print scripting or User/Group Sync. Those are post-access execution paths; they do not change the initial flaw’s unauthenticated access-control failure.
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Why did a print-management server matter?
A print-management service can look like routine back-office infrastructure, but its privileges and network exposure determine its security impact. CISA and the FBI said PaperCut’s Application Server process, pc-app.exe, runs with SYSTEM privileges on Windows or root privileges on Linux, and that child processes inherit those privileges. Code execution through the service could therefore have consequences well beyond printer settings.
The lesson is about blast radius, not a proven rule that “boring” internal applications are attacked first. A service becomes consequential when an attacker can reach its management interface, exploit an unpatched weakness, and inherit a powerful process context.
When were the patches released, and when was exploitation reported?
| Date | What happened |
|---|---|
| January 10, 2023 | PaperCut says it received vulnerability reports through Trend Micro’s Zero Day Initiative. |
| March 8, 2023 | PaperCut released fixed versions 20.1.7, 21.2.11, and 22.0.9 and published its bulletin. |
| March 14, 2023 | PaperCut’s timeline says Trend Micro published additional vulnerability details. |
| April 14, 2023 | PaperCut later identified a potentially linked suspicious-activity log signature at 01:29 AEST. |
| April 18, 2023 | PaperCut received its first customer report of suspicious activity. The customer said security software alerted on an unpatched server. |
| April 21, 2023 | CISA added CVE-2023-27350 to its Known Exploited Vulnerabilities catalog. |
| May 11, 2023 | CISA and the FBI published a joint advisory describing exploitation beginning in mid-April. |
So, was PaperCut actively exploited? Yes: PaperCut said it had evidence that unpatched servers were being exploited in the wild, and CISA and the FBI described exploitation starting in mid-April 2023. The earliest date PaperCut found was a potentially linked log signature, not a confirmed first exploitation date.
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Which PaperCut versions were vulnerable?
CISA and the FBI identified these historical affected ranges for the Application Server:
- 8.0.0 through 19.2.7
- 20.0.0 through 20.1.6
- 21.0.0 through 21.2.10
- 22.0.0 through 22.0.8
PaperCut listed 20.1.7, 21.2.11, and 22.0.9 as fixed releases, with later releases also containing the fix. These are historical affected and fixed boundaries, not a recommendation to remain on an old release. Use PaperCut’s current supported-release and upgrade guidance for a present-day deployment.
PaperCut identified Application Servers as affected by these March 2023 flaws; its bulletin listed other PaperCut components as not affected by those two issues. That statement does not establish that those components are unaffected by later vulnerabilities.
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How can you assess exposure or possible compromise?
Check the server and its network reachability
- Identify the PaperCut product, Application Server version, and whether the server falls within an affected range.
- Determine whether the web management portal is reachable from the public internet or other untrusted networks. CISA and the FBI cite ports 9191 and 9192 as the defaults.
- Review server and security logs for the indicators and detection guidance in the CISA/FBI advisory. Evaluate indicators in your environment: the advisory warns that some are ephemeral or nondeterministic.
A vulnerable version is not by itself proof of compromise, and installing a patch now does not prove that no compromise occurred before patching. CISA and the FBI recommend that organizations that delayed patching assume compromise and investigate using the advisory’s detections.
Put the reported figures in context
CISA and the FBI reported that approximately 68% of exposed PaperCut servers in the United States were maintained by Education Facilities Subsector entities. That describes exposed U.S.-based servers, not the share that was vulnerable or compromised. PaperCut’s post-incident report said approximately 35% of its customers had patched as of April 20, 2023; that was PaperCut customer data at that point in time, not a global deployment rate. PaperCut also said it aimed to contact approximately 2,000 high-risk customers during proactive outreach.
What should you do if a PaperCut server is exposed?
Upgrade first
Prioritize upgrading to a currently supported PaperCut release that includes the relevant fixes. The 2023 fixed builds are useful for understanding the historical boundary, but they should not be treated as current release advice.
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Restrict access if an immediate upgrade is not possible
CISA and the FBI advised blocking inbound external traffic to the web management portal on ports 9191 and 9192 by default until patching is possible. They also described a stricter option: block all inbound traffic to the management portal. That can stop remote management, so account for the operational effect. Network restrictions reduce exposure; they do not replace patching.
If compromise is suspected, preserve evidence and rebuild
CISA and the FBI recommend preserving a backup of the current server, wiping and rebuilding the Application Server and/or Site Server, restoring the database from a known-safe backup point, and following broader incident-response procedures. Coordinate containment and investigation with your security team; do not treat a successful upgrade as a substitute for checking for earlier access.
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As of October 7, 2026, PaperCut has published separate security notices for later NG/MF vulnerabilities. Its urgent advisory, last updated September 10, 2026, reports active exploitation of other vulnerabilities and gives separate emergency and maintenance release guidance. It recommends restricting web access to trusted IP addresses for publicly reachable Application Servers, and says releases 26.0.5, 25.0.13, and 24.1.10 include fixes from the emergency releases and additional hardening.
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A separate PaperCut bulletin dated September 24, 2026 lists three further NG/MF issues, recommends 26.0.5 or 25.0.13 or later, and says PaperCut had no evidence that those listed issues were exploited. These 2026 notices concern different vulnerabilities; they are not a new exploitation report or patch notice for CVE-2023-27350. The current exploitation status of CVE-2023-27350 itself is not established by those notices.
For current deployment decisions, consult PaperCut’s security notices and its supported-release guidance, alongside the historical CISA/FBI advisory.
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