Saturday, December 10, 2011

Namedpipe impersonation Attacks

Privilege escalation through namedpipe impersonation attack was a real issue back in 2000 when a flaw in the service control manager allowed any user logged onto a machine to steal the identify of SYSTEM. We haven't heard a lot about this topic since then, is it still an issue?

First of all, let's talk about the problem.

When a process creates a namedpipe server, and a client connects to it, the server can impersonate the client. This is not really a problem, and is really useful when dealing with IPC. The problem arises when the client has more rights than the server. This scenario would create a privilege escalation. It turns out that it was pretty easy to accomplish.
For example, let's assume that we have 3 processes: server.exe, client.exe and attacker.exe. Server.exe and client.exe have more privileges than attacker.exe. Client.exe communicates with server.exe using a namedpipe. If attacker.exe manages to create the pipe server before server.exe does, then, as soon as client.exe connects to the pipe, attacker.exe can impersonate it and the game is over.

First of all there are some flags buried in the CreateFile documentation to give control to the pipe client over what level of impersonation a server can perform. They are called the "Security Quality Of Service".

There are 4 flags to define the impersonation level allowed.

The server process cannot obtain identification information about the client, and it cannot impersonate the client.

The server process can obtain information about the client, such as security identifiers and privileges, but it cannot impersonate the client. ImpersonateNamedpipeClient will succeed, but no resources can be acquired while impersonating the client. The token can be opened and the information it contains can be read.

SECURITY_IMPERSONATION - This is the default
The server process can impersonate the client's security context on its local system. The server cannot impersonate the client on remote systems.

The server process can impersonate the client's security context on remote systems.

There are also 2 other flags:

Specifies that any changes a client makes to its security context is reflected in a server that is impersonating it. If this option isn't specified, the server adopts the context of the client at the time of the impersonation and doesn't receive any changes. This option is honored only when the client and server process are on the same system.

Prevents a server from enabling or disabling a client's privilege or group while the server is impersonating.

Every time you create a pipe in client mode, you need to find out what the server needs to know about you and pass the right flags to CreateFile. And if you do, don't forget to also pass SECURITY_SQOS_PRESENT, otherwise the other flags will be ignored.

Unfortunately, you don't have access to the source code of all the software running on your machine. I bet there are dozen of software running on my machine right now opening pipes without using the SQOS flags. To "fix" that, Microsoft implemented some restrictions about who a server can impersonate in order to minimize the chances of being exploited.

A server can impersonate a client only if one of the following is true.

  • The caller has the SeImpersonatePrivilege privilege.
  • The requested impersonation level is SecurityIdentification or SecurityAnonymous.
  • The identity of the client is the same as the server.
  • The token of the client was created using LogonUser from inside the same logon session as the server.

Only Administrators/System/SERVICES have the SeImpersonatePrivilege privilege. If the attacker is a member of these groups, you have much bigger problems.


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  2. plz explain more about impersonation..its a completely new thing for me

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