Is This Sid Taken? Varonis Hazard Labs Finds Synthetic Sid Shot Assault File

This attack involves threat actors with existing high privileges injecting "synthetic" into an Active Directory Access Control List (ACL) . This allows attackers to pre-assign permissions to a SID that does not yet exist in the environment, creating a silent "backdoor" that activates the moment a new account is created with that matching SID. Key Mechanics of the Attack

The vulnerability relies on the way Windows handles SID resolution. Because the system allows adding SIDs that aren't yet mapped to a user, the ACL essentially waits for its "missing half". This attack involves threat actors with existing high

These synthetic entries often appear as "Account Unknown" or long strings of numbers in the security tab, which administrators frequently ignore as remnants of deleted accounts rather than active threats. Because the system allows adding SIDs that aren't

A low-level account created later can suddenly "wake up" with Administrative or Domain Admin rights if those rights were pre-injected into the synthetic SID. Standard security tools often monitor for changes to

Standard security tools often monitor for changes to ACLs for existing users. Since the injection happens before the user exists, it can bypass traditional monitoring.

For more detailed technical analysis, you can view the original research on the Varonis Blog .

An attacker with high privileges (but perhaps needing to maintain long-term, hidden access) adds a non-existent SID to a resource's ACL.