Pentesting

Authentication, Credentials, Token privileges, UAC and EFS

Security Support Provider Interface (SSPI)

Is the API that can be use to authenticate users.

The SSPI will be in charge of finding the adequate protocol for two machines that want to communicate. The preferred method for this is Kerberos. Then the SSPI will negotiate which authentication protocol will be used, these authentication protocols are called Security Support Provider (SSP), are located inside each Windows machine in the form of a DLL and both machines must support the same to be able to communicate.

Main SSPs

  • Kerberos: The preferred one

    • %windir%\Windows\System32\kerberos.dll

  • NTLMv1 and NTLMv2: Compatibility reasons

    • %windir%\Windows\System32\msv1_0.dll

  • Digest: Web servers and LDAP, password in form of a MD5 hash

    • %windir%\Windows\System32\Wdigest.dll

  • Schannel: SSL and TLS

    • %windir%\Windows\System32\Schannel.dll

  • Negotiate: It is used to negotiate the protocol to use (Kerberos or NTLM being Kerberos the default one)

    • %windir%\Windows\System32\lsasrv.dll

The negotiation could offer several methods or only one.

Local Security Authority (LSA)

The credentials (hashed) are saved in the memory of this subsystem for Single Sign-On reasons. LSA administrates the local security policy (password policy, users permissions...), authentication, access tokens... LSA will be the one that will check for provided credentials inside the SAM file (for a local login) and talk with the domain controller to authenticate a domain user.

The credentials are saved inside the process LSASS: Kerberos tickets, hashes NT and LM, easily decrypted passwords.

Credentials Storage

Security Accounts Manager (SAM)

Local credentials are present in this file, the passwords are hashed.

LSASS

We have talk about this. Different credentials are saved in the memory of this process.

LSA secrets

LSA could save in disk some credentials:

  • Password of the computer account of the Active Directory (unreachable domain controller).

  • Passwords of the accounts of Windows services

  • Passwords for scheduled tasks

  • More (password of IIS applications...)

NTDS.dit

It is the database of the Active Directory. It is only present in Domain Controllers.

Credential Manager store

Allows browsers and other Windows applications to save credentials.

Access Tokens

Learn more about tokens in this tutorials: https://medium.com/@seemant.bisht24/understanding-and-abusing-process-tokens-part-i-ee51671f2cfa and https://medium.com/@seemant.bisht24/understanding-and-abusing-access-tokens-part-ii-b9069f432962

It is used to describe the security context of a process or a thread.

When a user logins, he is given an access token, and when the user creates a process, an access token derivated from the one of the user is assigned. The token contains information like the SID of the uses, the groups of the user, the privileges assigned to the user and its groups...

You can see this information executing whoami /all or using Process Explorer from Sysinternals.

When a local administrator logins, two access tokens are created: One with admin rights and other one with normal rights (Default one), and to execute a process as administrator the UAC is involved.

The access token has also a reference of the logon sessions inside the LSASS, this is useful if the process needs to access some objects of the network.

You can see the current logon sessions executing as administrator the binary logonsessions of Sysinternals. You can create a new logon session with new credentials that will be used only in the network:

runas /user:domain\username /netonly cmd.exe

Impersonate Tokens

Using the incognito module of metasploit you can easily list and impersonate other tokens. This could be useful to perform actions as if you where the other user. You could also escalate privileges with this technique.

UAC

UAC is used to allow an administrator user to not give administrator privileges to each process executed. This is achieved using default the low privileged token of the user. When, the administrator executes some process as administrator, a UAC elevation is performed and if it is successfully completed, the privileged token is used to create the process.

To differentiate which process is executed with low or high privileges Mandatory Integrity Controls (MIC) are used. There are 5 levels of integrity:

  • Untrusted(0): Processes launched by members of the Guest group. Writing operations are mostly blocked.

  • Low(1): Used by Internet Explorer. File and registry writing is blocked.

  • Medium(2): When UAC is enabled, this is the default level of integrity. A process in this level can request to elevate his integrity level.

  • High(3): Processes running with administrator privileges.

  • System(4): Services and other applications (Wininit, Winlogon, Smss...)

Some programs are autoelevated automatically if the user belongs to the administrator group. These binaries have inside their Manifests the autoElevate option with value True. The binary has to be signed by Microsoft also.

Then, to bypass the UAC (elevate from medium integrity level to high) some attackers use this kind of binaries to execute arbitrary code because it will be executed from a High level integrity process.

You can check the Manifest of a binary using the tool sigcheck.exe from Sysinternals. And you can see the integrity level of the processes using Process Explorer or Process Monitor (of Sysinternals).

Check UAC

First you need to check the value of the key EnableLUA, if it's 1 then UAC is activated, if its 0 or it doesn't exist, then UAC is inactive.

reg query HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\System\

Then you have to check the value of the key ConsentPromptBehaviorAdminin the same entry of the registry as before (info from here):

  • If 0 then, UAC won't prompt (like disabled)

  • If 1 the admin is asked for username and password to execute the binary with high rights (on Secure Desktop)

  • If 2 (Always notify me) UAC will always ask for confirmation to the administrator when he tries to execute something with high privileges (on Secure Desktop)

  • If 3 like 1 but not necessary on Secure Desktop

  • If 4 like 2 but not necessary on Secure Desktop

  • if 5(default) it will ask the administrator to confirm to run non Windows binaries with high privileges

Then, you have to take a look at the value of LocalAccountTokenFilterPolicy If the value is 0, then, only the RID 500 user (built-in Administrator) is able to perform admin tasks without UAC, and if its 1, all accounts inside "Administrators" group can do them.

And, finally take a look at the value of the key FilterAdministratorToken If 0(default), the built-in Administrator account can do remote administration tasks and if 1 the built-in account Administrator cannot do remote administration tasks, unless LocalAccountTokenFilterPolicy is set to 1.

Summary

  • If EnableLUA=0or doesn't exist, no UAC for anyone

  • If EnableLua=1 and LocalAccountTokenFilterPolicy=1 , No UAC for anyone

  • If EnableLua=1 and LocalAccountTokenFilterPolicy=0 and FilterAdministratorToken=0, No UAC for RID 500 (Built-in Administrator)

  • If EnableLua=1 and LocalAccountTokenFilterPolicy=0 and FilterAdministratorToken=1, UAC for everyone

UAC bypass

It is important to mention that it is much harder to bypass the UAC if it is in the higest security level (Always) than if it is in any of the other levels (Default).

The UAC bypass is needed in the following situation: the UAC is activated, your process is running in a medium integrity context, and your user belongs to the administrators group. All this information can be gathered using the metasploit module: post/windows/gather/win_privs

You can also check the groups of your user and get the integrity level:

net user %username%
whoami /groups | findstr Level

Very Basic UAC "bypass" (full file system access)

If you have a shell with a user that is inside the Administrators group you can mount the C$ shared via SMB (file system) local in a new disk and you will have access to everything inside the file system (even Administrator home folder)

net use Z: \\127.0.0.1\c$
cd C$

Or you could just access it:

dir \\127.0.0.1\c$\Users\Administrator\Desktop

More UAC bypass

All the techniques used here to bypass AUC require a full interactive shell with the victim (a common nc.exe shell is not enough).

You can get using a meterpreter session. Migrate to a process that has the Session value equals to 1:

(explorer.exe should works)

Empire and Metasploit have several modules to bypass the UAC. You should also check the tool: UACME

Basic UAC bypass methodology

  1. Find a binary that will autoelevate (check that when it is executed it runs in a high integrity level).

  2. With procmon find "NAME NOT FOUND" events that can be vulnerable to DLL Hijacking.

  3. You probably will need to write the DLL inside some protected paths (like C:\Windows\System32) were you don't have writing permissions. You can bypass this using:

    1. wusa.exe: Windows 7,8 and 8.1. It allows to extract the content of a CAB file inside protected paths (because this tool is executed from a high integrity level).

    2. IFileOperation: Windows 10.

  4. Prepare a script to copy your DLL inside the protected path and execute the vulnerable and autoelevated binary.

Another UAC bypass technique

Consists on watching if an autoElevated binary tries to read from the registry the name/path of a binary or command to be executed (this is more interesting if the binary searches this information inside the HKCU).

EFS (Encrypted File System)

EFS works by encrypting a file with a bulk symmetric key, also known as the File Encryption Key, or FEK. The FEK is then encrypted with a public key that is associated with the user who encrypted the file, and this encrypted FEK is stored in the $EFS alternative data stream of the encrypted file. To decrypt the file, the EFS component driver uses the private key that matches the EFS digital certificate (used to encrypt the file) to decrypt the symmetric key that is stored in the $EFS stream. From here.

Examples of files being decrypted without the user asking for it:

  • Files and folders are decrypted before being copied to a volume formatted with another file system, like FAT32.

  • Encrypted files are copied over the network using the SMB/CIFS protocol, the files are decrypted before they are sent over the network.

The encrypted files using this method can be tansparently access by the owner user (the one who has encrypted them), so if you can become that user you can decrypt the files (changing the password of the user and logins as him won't work).

Check EFS info

Check if a user has used this service checking if this path exists:C:\users\<username>\appdata\roaming\Microsoft\Protect

Check who has access to the file using cipher /c <file> You can also use cipher /e and cipher /d inside a folder to encrypt and decrypt all the files

Decrypting EFS files

Being Authority System

This way requires the victim user to be running a process inside the host. If that is the case, using a meterpreter sessions you can impersonate the token of the process of the user (impersonate_token from incognito). Or you could just migrate to process of the user.

Knowing the users password