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Process Access

Sysmon Process Access events will monitor of one process opening another. The access with higher permissions allows for also reading the content of memory, patching memory, process hollowing, creations of threads and other tasks that are abused by attackers. This technique has been used for access to credentials, keys and data that are in the process memory.

This task is common for processes that query information on another process, such as Task Manager, tasklist.exe and others, this requires that a baseline be stablished and filter out at a SIEM level taking into consideration other factors like image fullpath, parent process and account used so as to prevent any whitelisted processes to be used as staging for attacks.

Sysmon generates this event using ObRegisterCallbacks levering its driver. The main 2 filtering fields recommended are:

  • TargetImage - File path of the executable being accessed by another process.

  • GrantedAccess - The access flags (bitmask) associated with the process rights requested for the target process

As a minimum it is recommended to filter including critical processes, as a minimum:

  • C:\Windows\system32\lsass.exe

  • C:\Windows\system32\csrss.exe

  • C:\Windows\system32\wininit.exe

  • C:\Windows\system32\winlogon.exe

  • C:\Windows\system32\services.exe

Check for masks of known tools for credential dumping, process injection and process hollowing. Great care should be taken when setting masks since Sysmon does a literal comparison of the mask string provided against the one returned. It is not a bitwise operation, care should be taken to track the proper combinations.

Access Mask
PROCESS_CREATE_PROCESS 0x0080
PROCESS_CREATE_THREAD 0x0002
PROCESS_DUP_HANDLE 0x0040
PROCESS_SET_INFORMATION 0x0200
PROCESS_SET_QUOTA 0x0100
PROCESS_QUERY_LIMITED_INFORMATION 0x1000
SYNCHRONIZE 0x00100000
PROCESS_QUERY_INFORMATION 0x0400
PROCESS_SUSPEND_RESUME 0x0800
PROCESS_TERMINATE 0x0001
PROCESS_VM_OPERATION 0x0008
PROCESS_VM_READ 0x0010
PROCESS_VM_WRITE 0x0020

The PSGumshoe PowerShell module has a function for creating and parsing mask strings. https://github.com/PSGumshoe/PSGumshoe/blob/sysmon_events/EventLog/Get-SysmonAccessMask.ps1

The fields for the even are:

  • RuleName: Rule that triggered the event

  • UtcTime: Time in UTC when event was created

  • SourceProcessGUID: Process Guid of the source process that opened another process.

  • SourceProcessId: Process ID used by the OS to identify the source process that opened another process.

  • SourceThreadId: ID of the specific thread inside of the source process that opened another process

  • SourceImage: File path of the source process that created a thread in another process

  • TargetProcessGUID: Process Guid of the target process

  • TargetProcessId: Process ID used by the OS to identify the target process

  • TargetImage: File path of the executable of the target process

  • GrantedAccess: The access flags (bitmask) associated with the process rights requested for the target process

  • CallTrace: Stack trace of where open process is called. Included is the DLL and the relative virtual address of the functions in the call stack right before the open process call

Example:

<Sysmon schemaversion="4.22">
   <EventFiltering>
 <RuleGroup name="" groupRelation="or">
      <ProcessAccess onmatch="include">
        <!-- Detect Access to LSASS-->
         <Rule groupRelation="and">
          <TargetImage name="technique_id=T1003,technique_name=Credential Dumping" condition="is">C:\Windows\system32\lsass.exe</TargetImage>
          <GrantedAccess>0x1FFFFF</GrantedAccess>
         </Rule>
         <Rule groupRelation="and">
          <TargetImage name="technique_id=T1003,technique_name=Credential Dumping" condition="is">C:\Windows\system32\lsass.exe</TargetImage>
          <GrantedAccess>0x1F1FFF</GrantedAccess>
         </Rule>
         <Rule groupRelation="and">
          <TargetImage name="technique_id=T1003,technique_name=Credential Dumping" condition="is">C:\Windows\system32\lsass.exe</TargetImage>
          <GrantedAccess>0x1010</GrantedAccess>
         </Rule>
         <Rule groupRelation="and">
          <TargetImage name="technique_id=T1003,technique_name=Credential Dumping" condition="is">C:\Windows\system32\lsass.exe</TargetImage>
          <GrantedAccess>0x143A</GrantedAccess>
         </Rule>

        <!--Dumping credentials from services or setting up a keylogger-->
         <Rule groupRelation="and">
          <TargetImage name="technique_id=T1003,technique_name=Credential Dumping" condition="is">C:\Windows\system32\csrss.exe</TargetImage> <!--Mitre T1098--> <!--Mitre T1075--> <!--Mitre T1003--><!-- depending on what you're running on your host, this might be noisy-->
          <GrantedAccess>0x1F1FFF</GrantedAccess>
         </Rule>
         <Rule groupRelation="and">
          <TargetImage name="technique_id=T1003,technique_name=Credential Dumping" condition="is">C:\Windows\system32\wininit.exe</TargetImage> <!--Mitre T1098--> <!--Mitre T1075--> <!--Mitre T1003--><!-- depending on what you're running on your host, this might be noisy-->
          <GrantedAccess>0x1F1FFF</GrantedAccess>
         </Rule>
         <Rule groupRelation="and">
          <TargetImage name="technique_id=T1003,technique_name=Credential Dumping" condition="is">C:\Windows\system32\winlogon.exe</TargetImage> <!--Mitre T1098--> <!--Mitre T1075--> <!--Mitre T1003--><!-- depending on what you're running on your host, this might be noisy-->
          <GrantedAccess>0x1F1FFF</GrantedAccess>
         </Rule>
         <Rule groupRelation="and">
          <TargetImage name="technique_id=T1003,technique_name=Credential Dumping" condition="is">C:\Windows\system32\services.exe</TargetImage> <!--Mitre T1098--> <!--Mitre T1075--> <!--Mitre T1003--><!-- depending on what you're running on your host, this might be noisy-->
          <GrantedAccess>0x1F1FFF</GrantedAccess>
         </Rule>
         <Rule groupRelation="or">
            <GrantedAccess name="technique_id=T1003,technique_name=Credential Dumping">0x0810</GrantedAccess>
         </Rule>

         <!-- Detect process hollowing-->
         <Rule groupRelation="or">
            <GrantedAccess name="technique_id=T1093,technique_name=Process Hollowing">0x0800</GrantedAccess>
            <GrantedAccess name="technique_id=T1093,technique_name=Process Hollowing">0x800</GrantedAccess>
         </Rule>
         <!-- Detect process process injection-->
         <Rule groupRelation="or">
            <GrantedAccess name="technique_id=T1055,technique_name=Process Injection">0x0820</GrantedAccess>
            <GrantedAccess name="technique_id=T1055,technique_name=Process Injection">0x820</GrantedAccess>
         </Rule>
      </ProcessAccess>
</RuleGroup>
</EventFiltering>
</Sysmon>

Some examples of actions from security tools like Mimikatz and their access masks

Command Sysmon 10 Security 4663 Kernel Object
lsadump::lsa /patch GrantedAccess 0x1438 AccessMask 0x10
lsadump::lsa /inject rantedAccess 0x143a AccessMask 0x10
lsadump::trust /patch GrantedAccess 0x1438 AccessMask 0x10
misc:memssp GrantedAccess 0x1438 AccessMask 0x10
Procdump mimidump GrantedAccess 0x1fffff AccessMask 0x10
Task Manage minidump GrantedAccess 0x1400, 0x1000, 0x1410 and 0x1fffff AccessMask 0x10
sekurlsa::* GrantedAccess 0x1010 AccessMask 0x10