CVE-2023-27351

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.