PaperCut NG/MF: authentication bypass flaw under attack

What is affected

PaperCut NG/MF is mostly run by companies, schools and public bodies, not on home computers. You are unlikely to have it yourself, but organisations that hold your data may. Breaches that start with flaws like this one are how personal data ends up leaked.

PaperCut NG/MF contains an improper authentication vulnerability that could allow remote attackers to bypass authentication on affected installations via the SecurityRequestFilter class.

What an attacker can do

This is a authentication bypass flaw. It lets an attacker get in without a valid username and password and act as a legitimate user, often as an administrator.

How it is reached

Over the internet or network

Access the attacker needs

No account needed

Does the victim have to do something?

No, works without the victim doing anything

CISA records this vulnerability as known to be used in ransomware campaigns: attackers use it to get in, then encrypt files and demand payment.

What to do

1. At home: nothing to install. Keep your own devices updated and use a different password for every service, so that a breach at one organisation does not open your other accounts.
2. If you administer PaperCut NG/MF at work: apply the vendor's fix or mitigation now and review logs for signs of compromise, because the flaw was being exploited before it was listed.
3. If the product is past its support date and no fix exists, take it off the internet or retire it.

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Vendor advisories and fixes

• papercut.comwww.papercut.com/kb/Main/PO-1216-and-PO-1219

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on April 20, 2026, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by May 4, 2026, a deadline that has already passed. That deadline does not bind anyone else, but it shows how seriously the agency rates it.

The EPSS model estimates a 78% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 99.6% of all scored vulnerabilities.

Its CVSS severity score is 7.5 out of 10 (high), as recorded in the US National Vulnerability Database.

Technical 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.