Microsoft Windows Security

  • 3/15/2012

Application Identification (AppID)

Historically, security decisions in Windows have been based upon a user’s identity (in the form of the user’s SID and group membership), but a growing number of security components (AppLocker, firewall, antivirus, antimalware, Rights Management Services, and others) need to make security decisions based upon what code is to be run. In the past, each of these security components used their own proprietary method for identifying applications, which led to inconsistent and overly-complicated policy authoring. The purpose of AppID is to bring consistency to how the security components recognize applications by providing a single set of APIs and data structures.

Just as a user is identified when she logs in, an application is identified just before it is started by generating the main program’s AppID. An AppID can be generated from any of the following attributes of the application: Fields within a code-signing certificate embedded within the file allow for different combinations of publisher name, product name, file name, and version. APPID://FQBN is a Fully Qualified Binary Name, and it is a string in the following form: {Publisher\Product\Filename,Version}. The Publisher name is the Subject field of the x.509 certificate used to sign the code, using the following fields: O = Organization, L = Locality, S = State or Province, and C = Country.

File hash. There are several methods that can be used for hashing. The default is APPID://SHA256HASH. However, for backward compatibility with SRP and most x.509 certificates, SHA-1 (APPID://SHA1HASH) is still supported. APPID://SHA256HASH specifies the SHA-256 hash of the file.

The partial or complete path to the file. APPID://Path specifies a path with optional wildcard characters (“*”).

The AppID is stored in the process’s access token, allowing any security component to make authorization decisions based upon a single, consistent identification. AppLocker uses conditional ACEs (described earlier) for specifying whether a particular program is allowed to be run by the user.

When an AppID is created for a signed file, the certificate from the file is cached and verified to a trusted root certificate. The certificate path is re-verified daily to ensure the certificate path remains valid. Certificate caching and verification are recorded in the system event log. See Figure 6-24.

Figure 6-24

Figure 6-24 Event Viewer showing AppID service verifying signature of a program.