While performing incident response, Mandiant encounters attackers
actively using systems on a compromised network. This activity often
includes using interactive console programs via RDP such as the
command prompt, PowerShell, and sometimes custom command and control
(C2) console tools. Mandiant’s Innovation and Custom Engineering (ICE)
team researched how feasible it would be to capture this attacker
activity on an endpoint.
Depending on the target Windows version, capturing this data on a
live system can be difficult. The varying level of difficulty is
directly related to the evolving Windows implementation of virtual
consoles over the last decade.
This blog will discuss the implementation of the Windows console
architecture from years past, with a primary focus on the current
implementation present on modern versions of Windows.
A Review of Console Applications
The Windows PE loader determines if a file is a console application
when the "Subsystem" field in the PE optional header is set
to IMAGE_SUBSYSTEM_WINDOWS_CUI. If this flag is set, the loader
allocates a console server for the process. The details of how the
console server is implemented depends on the Windows version and has
undergone three major revisions since Windows XP. Regardless of the
implementation, when a client (e.g. cmd.exe, powershell.exe, etc.) is
executed, by default, a console server connection is established
typically via the AllocConsole Win32 API. The server process is what
the user typically interacts with when typing commands that are then
transferred to the client process through an Interprocess
communication (IPC) mechanism. A single console server can host one or
more clients simultaneously.
Evolution of the Windows Console
On Windows XP through Windows Vista, the Client/Server Runtime
Subsystem process (CSRSS)
was responsible for capturing user input and sending it to client
processes. The clients and CSRSS communicated using a Local
Procedure Call (LPC) Port to send captured input. Figure 1
illustrates the client-server console architecture implemented in
Windows XP and Vista.
Figure 1: Windows XP/Vista console architecture
This model suffered from privilege escalation vulnerabilities since
the client ran as the current user, but the CSRSS server process ran
as the Local System account. An attacker could exploit the exposed
attack surface of CSRSS by connecting as an unprivileged user and
triggering a vulnerable code path in CSRSS, giving them SYSTEM level access.
This architecture problem was addressed with the release of Windows
7 and Windows Server 2008 R2. Instead of CSRSS being the only console
server on the system, a new console host (conhost.exe) process was
introduced that hosted the console input thread. This process now ran
in the same context as the client, removing this attack scenario.
Figure 2 illustrates the updated Windows 7 console architecture.
Figure 2: Windows 7/Server 2008 R2
When a console
in Windows 7 is allocated, CSRSS executes a new instance of the
conhost.exe process. An Advanced Local Procedure Call (ALPC)
port is created with the following naming convention: \RPC
port is used along with a shared section object mapped into the client
and server processes so command line data can be easily shared. In
addition, an event object is created with the naming scheme \RPC
event object is used so the
client and server can notify each other when new data is
present. A single conhost.exe process can service multiple client
applications, as shown in the Windbg output in Figure 3.
Figure 3: Windbg output for analyzing
ALPC ports between a conhost process and multiple console
applications on Windows 7
The release of Windows 8 introduced the current console
implementation at the time of this writing. This new architecture
differs significantly from the previous ones in that there is now a
dedicated kernel driver to handle console I/O between client and
server processes. This new driver is named ConDrv.sys and brokers all
console communication on the system. This is exposed to user mode
applications by the driver with a device object named \Device\ConDrv.
This device object is opened from user mode using a list of supported
namespace parameters – Connect, Server, Input, Output,
Reference, CurrentIn, and CurrentOut – that are opened depending on
the needs of the application. Client applications will often have
multiple open handles to the console driver depending on the
functionality needed by the driver. This is shown in Figure 4.
Figure 4: Command line application with
multiple ConDrv handles open
When a console is allocated by a command line process,
kernelbase.dll will open a handle to \Device\ConDrv and request that a
new conhost.exe process be created. ConDrv will execute this process
from kernel mode and a memory descriptor list (MDL) chain is
allocated. This MDL chain is used to map memory pages between the
Conhost process and its clients so that data can be easily shared
between them. Instead of the LPC/ALPC ports used in the previous
versions, messages are now typically transferred using Fast I/O to the
console driver. Fast I/O allows an application to communicate with a
driver without the overhead of creating an I/O request packet (IRP)
for each request. IRPs are an operating system structure that are used
to deliver I/O data to device drivers. These fast I/O requests are
used to read and write to the console and are brokered by the ConDrv driver.
In Windows 10, conhost.exe is mostly a container process. The main
input thread along with all the server functionality executes in
either ConhostV2.dll or ConhostV1.dll. By default ConhostV2.dll is
loaded and provides new console functionality to Windows 10 users
(such as full screen console windows). ConhostV1.dll implements
“legacy mode” that can be enabled that will make consoles behave as
they did in Windows 7 and earlier. Regardless of the version being
used, ConDrv.sys is used to transfer messages between console clients
and servers. Figure 5 illustrates how all this fits together.
Figure 5: Console driver based
architecture used in Windows 10
Check out the follow-up post, "Monitoring
Windows Console Activity Part 2," for more.
This is a Security Bloggers Network syndicated blog post authored by Nick Harbour. Read the original post at: Threat Research Blog