Suspicious Java Class File Created in PaperCut Server Library

Last updated a day ago on 2026-08-28

Created a day ago on 2026-08-28

About

Detects creation of Java .class files within the PaperCut NG/MF Application Server library directory. During active exploitation of CVE-2026-82078 (chained with CVE-2026-81578), attackers deliver hex-encoded malicious .class payloads into the PaperCut server/lib path (observed examples include Udydn.class and Moo97.class) so arbitrary bytecode executes inside the PaperCut JVM / Application Server process.

Tags

Domain: EndpointOS: WindowsOS: LinuxOS: macOSUse Case: Threat DetectionUse Case: VulnerabilityTactic: Initial AccessTactic: ExecutionTactic: Defense EvasionData Source: Elastic DefendLanguage: eql

Severity

critical

Risk Score

MITRE ATT&CK™

Initial Access (TA0001)(external, opens in a new tab or window)

↳ Exploit Public-Facing Application (T1190)(external, opens in a new tab or window)

Execution (TA0002)(external, opens in a new tab or window)

↳ Command and Scripting Interpreter (T1059)(external, opens in a new tab or window)

Defense Evasion (TA0005)(external, opens in a new tab or window)

↳ Reflective Code Loading (T1620)(external, opens in a new tab or window)

License

Elastic License v2(external, opens in a new tab or window)

Definition

Rule Type: Event Correlation Rule
Integration Pack: Prebuilt Security Detection Rules
Index Patterns: logs-endpoint.events.file-*
Related Integrations: endpoint(external, opens in a new tab or window)
Query

✄𐘗text code block:
✄𐘗file where host.os.type in ("windows", "linux", "macos") and
  event.action in ("creation", "overwrite") and
  file.extension : "class" and
  file.path : (
    "?:\\Program Files\\PaperCut*\\server\\lib\\*",
    "?:\\Program Files (x86)\\PaperCut*\\server\\lib\\*",
    "/opt/papercut/server/lib/*",
    "/usr/local/papercut/server/lib/*",
    "/Applications/PaperCut*/server/lib/*"
  )

Install detection rules in Elastic Security

Detect Suspicious Java Class File Created in PaperCut Server Library in the Elastic Security detection engine by installing this rule into your Elastic Stack.

To setup this rule, check out the installation guide for Prebuilt Security Detection Rules(external, opens in a new tab or window).