The event property Process.DirectorySdSddl is a powerful tool: it makes the file system permissions of a process’ directory available for regex matching and rule evaluation.
Whenever an event occurs, uberAgent ESA checks if the event property Process.DirectorySdSddl is configured in at least one Threat Detection rule. If that is the case, uberAgent ESA does the following:
Security Descriptors are structures that contain multiple components, some of which are optional:
uberAgent ESA retrieves all of the SD components shown above.
Security descriptors are binary structures. In order for humans to read or regular expressions to match their contents, SDs must be converted to strings. Microsoft established a common format for that purpose, the Security Descriptor Definition Language.
uberAgent ESA converts all the security descriptor components to SDDL, but it does not stop there because SDDL has shortcomings.
SDDL strings are more or less 1:1 representations of the binary SD structure. This means that, with very few exceptions, users and groups are not shown by their names, but by their SIDs, for example S-1-5-21-3803133166-2955000686-238773884-1029 . Such a SID string is not very useful for regex matching, so uberAgent goes ahead and converts it to the well-known domain\user format before performing the regex matching.
The same is true for access masks, which store the actual permission in a 32-bit unsigned integer. In a raw SDDL string, an access mask might look like this: 0x1200a9 . That is not very useful for regex matching because multiple permissions can be combined in one access mask through bitwise OR. Again, uberAgent does the heavy lifting by converting access masks to a string format that is processed easily: SetACL’s. With this conversion, the cryptic access mask 0x1200a9 becomes the easily understandable string read_execute .
If an access mask contains a combination of multiple individual permissions, uberAgent’s SetACL string lists all the individual permission names separated by commas.
SDDL string for C:\Windows\System32 as obtained by the Windows API before uberAgent ESA’s simplifications:
O:S-1-5-80-956008885-3418522649-1831038044-1853292631-2271478464G:S-1-5-80-956008885-3418522649-1831038044-1853292631-2271478464D:PAI(A;;FA;;;S-1-5-80-956008885-3418522649-1831038044-1853292631-2271478464)(A;CIIO;GA;;;S-1-5-80-956008885-3418522649-1831038044-1853292631-2271478464)(A;;0x1301bf;;;SY)(A;OICIIO;GA;;;SY)(A;;0x1301bf;;;BA)(A;OICIIO;GA;;;BA)(A;;0x1200a9;;;BU)(A;OICIIO;GXGR;;;BU)(A;OICIIO;GA;;;CO)(A;;0x1200a9;;;AC)(A;OICIIO;GXGR;;;AC)(A;;0x1200a9;;;S-1-15-2-2)(A;OICIIO;GXGR;;;S-1-15-2-2)S:AINO_ACCESS_CONTROL
The same string after uberAgent ESA replaced SIDs with names:
O:NT SERVICE\TrustedInstallerG:NT SERVICE\TrustedInstallerD:PAI(A;;FA;;;NT SERVICE\TrustedInstaller)(A;CIIO;GA;;;NT SERVICE\TrustedInstaller)(A;;0x1301bf;;;SY)(A;OICIIO;GA;;;SY)(A;;0x1301bf;;;BA)(A;OICIIO;GA;;;BA)(A;;0x1200a9;;;BU)(A;OICIIO;GXGR;;;BU)(A;OICIIO;GA;;;CO)(A;;0x1200a9;;;AC)(A;OICIIO;GXGR;;;AC)(A;;0x1200a9;;;APPLICATION PACKAGE AUTHORITY\ALL RESTRICTED APPLICATION PACKAGES)(A;OICIIO;GXGR;;;APPLICATION PACKAGE AUTHORITY\ALL RESTRICTED APPLICATION PACKAGES)S:AINO_ACCESS_CONTROL
The same string after uberAgent ESA additionally replaced hex access masks with permission strings:
O:NT SERVICE\TrustedInstallerG:NT SERVICE\TrustedInstallerD:PAI(A;;full;;;NT SERVICE\TrustedInstaller)(A;CIIO;full;;;NT SERVICE\TrustedInstaller)(A;;change;;;SY)(A;OICIIO;full;;;SY)(A;;change;;;BA)(A;OICIIO;full;;;BA)(A;;read_execute;;;BU)(A;OICIIO;read_execute;;;BU)(A;OICIIO;full;;;CO)(A;;read_execute;;;AC)(A;OICIIO;read_execute;;;AC)(A;;read_execute;;;APPLICATION PACKAGE AUTHORITY\ALL RESTRICTED APPLICATION PACKAGES)(A;OICIIO;read_execute;;;APPLICATION PACKAGE AUTHORITY\ALL RESTRICTED APPLICATION PACKAGES)S:AINO_ACCESS_CONTROL
Here is a quick explanation of the security descriptor string format. For the full specifications please see Microsoft’s documentation.
Split into the SD’s components, the SDDL string from the example above is already much more readable:
O:NT SERVICE\TrustedInstaller G:NT SERVICE\TrustedInstaller D:PAI(A;;full;;;NT SERVICE\TrustedInstaller)(A;CIIO;full;;;NT SERVICE\TrustedInstaller)(A;;change;;;SY)(A;OICIIO;full;;;SY)(A;;change;;;BA)(A;OICIIO;full;;;BA)(A;;read_execute;;;BU)(A;OICIIO;read_execute;;;BU)(A;OICIIO;full;;;CO)(A;;read_execute;;;AC)(A;OICIIO;read_execute;;;AC)(A;;read_execute;;;APPLICATION PACKAGE AUTHORITY\ALL RESTRICTED APPLICATION PACKAGES)(A;OICIIO;read_execute;;;APPLICATION PACKAGE AUTHORITY\ALL RESTRICTED APPLICATION PACKAGES) S:AINO_ACCESS_CONTROL
As you can see, the four main components of a security descriptor are prepended by the following:
The DACL part of an SDDL string is a concatenation of access control entries (ACEs), each wrapped in parentheses. In this example, there are 13 ACEs in the ACL:
(A;;full;;;NT SERVICE\TrustedInstaller) (A;CIIO;full;;;NT SERVICE\TrustedInstaller) (A;;change;;;SY) (A;OICIIO;full;;;SY) (A;;change;;;BA) (A;OICIIO;full;;;BA) (A;;read_execute;;;BU) (A;OICIIO;read_execute;;;BU) (A;OICIIO;full;;;CO) (A;;read_execute;;;AC) (A;OICIIO;read_execute;;;AC) (A;;read_execute;;;APPLICATION PACKAGE AUTHORITY\ALL RESTRICTED APPLICATION PACKAGES) (A;OICIIO;read_execute;;;APPLICATION PACKAGE AUTHORITY\ALL RESTRICTED APPLICATION PACKAGES)
Please see Microsoft’s documentation for an explanation of the ACE properties.
As you saw above, SDDL strings can be complex. When writing regular expressions to match them, you need to know what you are dealing with. In other words, you need log samples.
Tip: while working on a Threat Detection rule involving the security descriptor, set the rule’s VerboseLogging config element to true . With verbose logging enabled, uberAgent’s write messages like the following to its log file:
GetPermissionsSddl,Read the SD of <\\?\C:\WINDOWS\System32>:
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