CVE-2023-27351 is an improper authentication flaw (CWE-287) in the SecurityRequestFilter class of PaperCut NG and MF print management software, where the authentication algorithm is improperly implemented. A remote, unauthenticated attacker can trigger it over the network with no user interaction or special privileges to bypass authentication on the affected server (CVSS 3.1: 7.5). Once authentication is bypassed, the attacker gains access to the PaperCut system; in observed campaigns this access was leveraged to deliver Cl0p and LockBit ransomware, as confirmed by Microsoft. Organizations running PaperCut NG (version 22.0.5, Build 63914, is cited in the advisory) or PaperCut MF are affected. The flaw was exploited as a zero-day, is CISA KEV-listed (added 2026-04-20) with known ransomware use, and EPSS places the 30-day exploitation probability at 78.1%.
What to do: Upgrade PaperCut NG/MF to the fixed release per the vendor's emergency patch advisory, first confirming the running build (NG 22.0.5, Build 63914, is cited as affected). Restrict internet-facing access to PaperCut servers and hunt for signs of post-exploitation, given confirmed use to deliver Cl0p and LockBit ransomware. US federal agencies must apply mitigations per CISA BOD 22-01 (or vendor instructions) or discontinue use of the product if mitigations are unavailable.
Affected
PaperCut NG
22.0.5 (Build 63914) explicitly cited as affected; CISA lists PaperCut NG broadly without a full version range
PaperCut MF
affected per CISA listing; no specific version range provided in the data
Estimated exposure
large≈75,000+ sites/organizations (PaperCut NG/MF is deployed at tens of thousands of organizations; public scans have found thousands of servers directly… — PaperCut NG/MF is a widely deployed print-management platform used by tens of thousands of schools, universities and businesses (the vendor cites roughly 75,000+ organizations), while internet-facing scans have identified only thousands of…
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
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.
CISA Known Exploited Vulnerability
Affected
PaperCut NG/MF
Required action
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
PaperCut warns that a zero-day in NG and MF print management products is being actively exploited, shipping emergency patches with no CVE assigned yet.
PaperCut Software confirmed active exploitation of a zero-day affecting PaperCut NG and MF, with confirmed customer incidents, and released emergency patches on Friday ahead of technical details or a CVE assignment. The company urges customers to restrict web access to trusted IP addresses, disconnect application servers from the internet, and watch for indicators including tampered or truncated server.log files, database error strings, and suspicious activity involving pc-app.exe. The article recalls the May 2023 exploitation of CVE-2023-27350 and CVE-2023-27351 by the financially motivated Lace Tempest group, a Clop ransomware RaaS affiliate, though no attacker attribution has been given for the current campaign.
PaperCut warns of active zero-day exploitation chaining CVE-2026-81578 and CVE-2026-82078 for pre-auth remote code execution in NG/MF print management.
PaperCut Software confirmed attackers are chaining two vulnerabilities in PaperCut NG and MF: CVE-2026-81578, an improper access control flaw in the web management interface allowing unauthenticated configuration changes, and CVE-2026-82078, unsafe dynamic class loading in database connection utilities enabling arbitrary Java bytecode execution. Huntress reproduced a pre-authentication remote configuration takeover and full RCE chain against PaperCut NG 25.0.11.75758 and observed limited exploitation at two customers, including post-exploitation whoami and ver commands. The vendor released Emergency Patch Release 2 with additional hardening and urged restricting Application Server web access to trusted IPs. In 2023, Clop and LockBit affiliates abused CVE-2023-27350 and CVE-2023-27351 in the same software.
Microsoft reports China-linked ransomware group Storm-1175 switched from Medusa to a new C++ strain, StormEncryptor, likely exploiting N-able flaw CVE-2026-18577.
Microsoft Threat Intelligence reports that the financially motivated, China-linked group Storm-1175 began deploying a new ransomware strain called StormEncryptor on August 2, 2026, replacing its previous Medusa ransomware. StormEncryptor is written in C++, appends the .encrypted extension to files, and drops a !!!README_FIRST!!!.txt ransom note in each scanned directory. Microsoft assesses the group is likely exploiting CVE-2026-18577, an authentication bypass in N-able disclosed on August 2, 2026 and added to CISA's Known Exploited Vulnerabilities catalog the next day. Since 2023, Storm-1175 has exploited more than 16 vulnerabilities in products including Microsoft Exchange, Ivanti, ConnectWise ScreenConnect, JetBrains TeamCity, SimpleHelp, CrushFTP, and GoAnywhere MFT, often moving from initial access to data theft and ransomware deployment within days.