Print Processors [T1547.011]
Name: Plist Modification
ID: T1547.011
Technique: T1547
Print Processors (T1547.011) is a sub-technique within the MITRE ATT&CK framework under the Persistence technique (T1547). Attackers exploit print processors, legitimate Windows components responsible for managing print jobs, to maintain persistence and execute malicious payloads. By registering malicious print processors, adversaries can silently execute arbitrary code during system boot or user logon, making detection challenging.
Print processors are Windows system components responsible for handling print jobs, formatting data, and sending it to printers. Attackers leverage these legitimate components by registering malicious print processor DLLs, enabling them to execute arbitrary code persistently.
Technical details include:
Print processors are registered in the Windows Registry under the following key:
The registry entry typically points to a DLL file:
Attackers place a malicious DLL into a trusted system directory (e.g., %SystemRoot%\System32\spool\prtprocs\[Environment]\) to avoid suspicion.
Upon system boot or print spooler restart, the malicious DLL is loaded, executing attacker-controlled code with SYSTEM privileges.
Real-world procedures:
Attackers may leverage social engineering, phishing, or initial compromise to gain administrative privileges.
Once administrative access is obtained, attackers register the malicious DLL as a print processor.
After registration, the print spooler service (spoolsv.exe) loads the malicious DLL automatically, providing persistence and privilege escalation opportunities.
Attackers typically employ Print Processors (T1547.011) in the following scenarios and attack stages:
Persistence Stage:
Ensuring long-term persistence after initial compromise to maintain access even after system reboots or user logoffs.
Avoiding traditional startup folders or scheduled tasks to evade detection.
Privilege Escalation:
Leveraging SYSTEM-level privileges granted by the print spooler service to escalate privileges from lower-level user accounts.
Advanced Persistent Threat (APT) Campaigns:
Used by sophisticated threat actors aiming for stealthy, persistent access in targeted environments.
Post-Exploitation Stage:
Maintaining backdoor access to compromised systems for future exploitation, lateral movement, or data exfiltration.
Detection of malicious print processors can be challenging due to their legitimate nature. However, several effective detection methods and indicators of compromise (IoCs) exist:
Registry Monitoring:
Monitor registry keys related to print processors for suspicious modifications:
Tools like Sysmon, Autoruns, or EDR solutions can provide visibility into registry changes.
File System Monitoring:
Monitor %SystemRoot%\System32\spool\prtprocs\[Environment]\ directory for unexpected or suspicious DLL files.
Check file hashes against known malicious samples.
Process Monitoring and Logging:
Observe unexpected DLL loads into the spooler service (spoolsv.exe) using tools like Sysinternals Process Monitor, Process Explorer, or advanced EDR solutions.
Event Logs Analysis:
Analyze Windows Event Logs, especially System and Security logs, for suspicious print spooler activities or errors related to print processor loading.
Threat Hunting:
Conduct proactive threat hunting for anomalous behavior associated with print spooler processes and DLL loading events.
Indicators of Compromise (IoCs):
Suspicious DLL filenames in print processor directories.
Unusual registry entries pointing to unknown or unsigned DLLs.
Anomalous process behavior involving spoolsv.exe or unexpected DLL loading.
Timely detection of malicious print processors is critical due to the potential severe impacts on system security and network integrity:
Stealthy Persistence:
Attackers can maintain long-term, undetected persistence, enabling continuous access to sensitive systems and data.
Elevated Privileges:
Malicious DLLs loaded by the spooler service typically execute with SYSTEM-level privileges, allowing attackers to escalate privileges and gain full control over compromised machines.
Lateral Movement:
Persistent access facilitates lateral movement across the network, further compromising additional systems.
Data Exfiltration and Espionage:
Attackers can silently exfiltrate sensitive data, intellectual property, or personally identifiable information (PII), causing severe financial and reputational damage.
Operational Disruption:
Malicious print processors can disrupt legitimate printing operations, causing service outages or productivity loss.
Early detection allows organizations to:
Rapidly contain and remediate the threat before significant damage occurs.
Limit lateral movement and reduce the scope of compromise.
Protect sensitive data and maintain operational continuity.
Real-world examples and attack scenarios involving Print Processors (T1547.011):
DePriMon Malware:
Discovered in targeted attacks, DePriMon malware utilized malicious print processors to achieve persistence.
Attackers created and registered malicious DLLs as print processors, enabling them to execute arbitrary code persistently and stealthily.
Impact included persistent backdoor access, data exfiltration, and espionage activities.
APT32 (OceanLotus) Campaigns:
APT32 threat group leveraged malicious print processors for persistence in targeted attacks against organizations in Southeast Asia.
Attackers registered malicious DLLs as print processors, enabling them to maintain stealthy, persistent access to compromised systems.
Impact included espionage, sensitive data theft, and prolonged undetected access.
FIN7 Threat Actor:
FIN7 used malicious print processors as part of their toolkit in financial sector attacks.
Attackers registered DLLs to maintain persistence across reboots, providing continuous access for financial data theft and lateral movement within compromised networks.
Impact included financial loss, data breaches, and reputational damage.
Tools used in these scenarios:
Custom-developed malicious DLLs specifically designed for print processor exploitation.
Standard Windows utilities and scripts for registry manipulation and DLL file placement.
Impacts observed:
Persistent, stealthy access to compromised systems.
Elevated privileges enabling full control of targeted machines.
Data exfiltration, espionage, and operational disruption.