Most companies will use a certificate to sign their applications before they release their software to the world. This helps the user know to that the software they are running actually came from the software vendor, and hasn’t been altered or changed by someone.
Certificates are based on key pairs. There is a public key, and a private key. In terms of digitally signing an application, the public key is often just referred to as the Certificate.
How it works, in simpified terms… The software vendor holds a private key, and they guard it, keeping it safe in their organization. You can also think of is their fingerprint that they’ll use when signing something as it is unique. The public key is what we can see. Using a hash in the digitally signed application, we can use their public key, to see is if the hash value can be verified. If it checks out then we know that the digital signature is valid. If it doesn’t, well then we know the signature has been altered.
The I’ll show you below how you can pull the public half of the Certificate from an application. In this example we’ll pull Adobe’s certificate from Adobe Reader DC.
Right click on the application you want the signature of and select “Properties”
Click the “Digital Signature” tab, select the signature, then click the “Details” button.
Note: If you do not see the “Digital Signature” tab, then the file is not digitally signed.
Click the “View Certificate” button.
Click the “Details” tab and then select the “Copy to File” button.
Follow the “Certificate Export Wizard”.
After completing the export wizard, you’ll have the digital signature certificate of the digitally signed application.
Using GPOs is a great way to allow or block programs from running on your corporate network. Just be careful and limit yourself to only blocking the applications which you actually have a need to block. Don’t go too crazy locking down programs
Microsoft first made the introduction of “Software Restriction Policies” in Windows Server 2008 and they’ve continued to evolve. Today I will show you four ways which Microsoft allows us to restrict programs from running.
File Path / File Name Rule
Network Zone Rule
Hash Rule
Certificate Rule
To begin, fire up the Group Policy Management Editor. Click on the start menu and type “gpmc.msc”. If you are on a Domain Controller it should work. If you’re on a workstation you’ll likely have to run Server Manager as a Domain Admin (or other user with the correct administrative privileges), choose “Group Policy Management” from the ‘Tools’ dropdown.
Once it’s open, scroll down to the folder “Group Policy Objects” and right-click on it to create a “New ” policy object. Give it an appropriate name, something like “Software Restrictions – Test”. Now find and right-click on your new policy and select “Edit…”.
The software restriction policy exists under both “Computer Configuration” and “User Configuration”. So depending on your needs, you can lock down either the user or the computer.
Drill down into the policy… “Policies” -> “Windows Settings” -> “Security Settings” -> “Software Restriction Policies”.
Right-click on “Software Restriction Policies” and click “New Software Restriction Policies”
Select and open the “Additional Rules” folder.
Right-click under the two pre-existing default entries, and then from that drop-down menu select the type of rule you want to create. I’ll expand on the four methods below…
There are three security levels used in all of these rules:
DISALLOWED: Software will not run, regardless of the access rights of the user.
BASIC USER: Allows programs to run only as standard user. Removes the ability to “Run as Administrator”.
UNRESTRICTED: No changes made by this policy – Software access rights are determined by the file access rights of the user.
My examples below all show how to block software with ‘dissallowed’ rules. But just remember that you can just as easily allow for software by using ‘basic user’ and ‘unrestricted’ rules. Use them wisely!
1. Block by File Path / File Name Rule
In this example I will show you how to lock down the computer from running WordPad.
Select “New Path Rule”.
Type, or use the “Browse…” button, to enter the file path or file name you wish to block. Make sure that the ‘Security level’ is set to “Dissallow”. Then click ‘OK’.
Note: System variables will all function in the rule, variables such as %windir%, %ProgramFiles(x86)%, %AppData%, %userprofile%, and others.
It is important to note that many applications launch in more than just one way. So you may have to block multiple executables to fully block the application, just fyi.
You also need to take note of where/how software get launched from, as some applications have multiple ways they can be launched. Just FYI, in case you start banging your head as to why some block rule doesn’t seem to be working.
Also be careful using just the file name itself to try to block a program from running. If you were to block just the file name ‘update.exe’ for example, hundreds of applications all ship with an ‘update’ executable and they would all be hindered and unable run.
My rule of thumb is to always use the full path unless it’s truly a unique file name, and even then I still prefer to use the full path.
2. Block by Network Zone Rule
Select “New Network Rule”.
Select the Network zone you want to block. Make sure that the ‘Security level’ is set to “Dissallow”. Then click ‘OK’.
These rules allow you to block programs if they come from sites you’ve designated into a zone, like your Restricted sites. Or in the case that you were to be creating an allow rule, your local Intranet. While this option exists, it seems unlikely to me that most SMBs ever use it.
3. Block by Hash Rule
In this example I will show you how to lock down the computer from running WordPad.
Select “New Hash Rule”.
Use the “Browse…” button to navigate to the file which you are wanting to block. Select the file and click ‘Open’. It will automatically pull the needed file information and the “hash” it needs from the file you selected. Make sure that the ‘Security level’ is set to “Dissallow”. Then click ‘OK’.
The only problem this method has is that file hashes change any time there is ANY change to file. It doesn’t matter how small of a change is made, it will always create a new hash. That means that hash rules are best applied to older software that you are trying to kill, and not for programs that get updated often.
4. Block by Certificate Rule
In this example we will be blocking applications signed by Adobe Inc.
Select “New Certificate Rule”.
Use the “Browse…” button to navigate to the certificate file which you are wanting to use to block signed software. Select the file and click ‘Open’. Make sure that the ‘Security level’ is set to “Dissallow”. Then click ‘OK’.
Certificate rules are by far one of the most secure rules as they rely on certificates from trusted publishers. Because of this but they require more work on the PC’s part as it goes out and tries to verify the validity of the certificate, so they may significantly effect performance. I can’t tell you how much of an impact they’ll create, but it’s enough that MS warns us. Also, if the certificate ever expires, you’ll need create a new rule.
