Unmasking the Ghost: Forensic Analysis of Passkey Synchronization Hijacking

Introduction

The global transition toward a passwordless future has been accelerated by the widespread adoption of FIDO2 and WebAuthn standards. Passkeys—cryptographic key pairs stored on user devices—have successfully eliminated traditional phishing vectors. By binding a credential to a specific web domain, passkeys prevent users from inadvertently handing over their credentials to spoofed login portals. However, as the security community has learned time and again, shifting the authentication paradigm does not eliminate risk; it simply relocates it.

To solve the user-experience hurdle of losing a physical device, major operating system vendors (Apple, Google, and Microsoft) introduced synced passkeys. These credentials are end-to-end encrypted (E2EE) and automatically synchronized across a user’s cloud-linked devices via services like iCloud Keychain, Google Password Manager, and Windows Hello. But what happens when an attacker compromises the underlying cloud account itself? By enrolling a rogue device into the victim’s synchronization fabric, the attacker silently inherits their entire cryptographic identity. In this post, we will dissect the mechanics of Passkey Synchronization Hijacking, walk through an ethical hacking simulation of this attack vector, and detail the forensic artifacts left behind for incident responders.

The Attack Vector: Sync Hijacking Explained

In a traditional WebAuthn flow, a hardware-bound key (such as a YubiKey) cannot be duplicated. The private key remains securely sealed inside the authenticator’s hardware security module (HSM). Synced passkeys relax this constraint for the sake of usability. The private key is wrapped in an encryption key derived from the user’s account credentials and recovery factors, then uploaded to the cloud provider’s synchronization network.

Passkey Sync Hijacking occurs when an adversary bypasses or compromises the primary identity provider (IdP) account—for example, via session hijacking, cookie theft, or sophisticated social engineering of recovery mechanisms. Once inside the cloud account, the attacker registers a new device (such as a clean macOS virtual machine or a secondary Android device) into the victim’s device trust circle.

“By compromising the synchronization fabric, an attacker bypasses the domain-binding protection of WebAuthn. They do not need to phish the passkey; they simply wait for the platform provider to deliver it directly to their machine.”

Once the rogue device is trusted, the cloud provider’s background services automatically sync the private keys. The attacker can now authenticate to any high-value target (e.g., corporate portals, banking apps, or personal email) as if they were the legitimate user, completely bypassing multi-factor authentication (MFA) prompts that rely on WebAuthn.

Simulating the Attack: An Ethical Hacker’s Perspective

To understand the forensic footprint of this attack, we must first look at how it executes. In a controlled lab environment, we simulated an attacker who has acquired a victim’s active Google session token. The objective is to extract the synced passkeys onto an attacker-controlled machine.

1. Session Importation: The attacker imports the stolen session cookies into a clean browser instance on an attacker-controlled platform.
2. Device Enrollment: The attacker accesses the victim’s Google Account security settings and registers a new device to the account. Depending on the provider’s trust model, this may require solving a prompt on the victim’s primary device, which can be accomplished via session pinning or prompt fatigue (MFA bombing).
3. Key Syncing: Once enrolled, the Google Play Services or Chrome synchronization agent on the attacker’s machine initiates a sync. The local keychain receives the synced passkeys.
4. Assertion Generation: The attacker navigates to the target enterprise application. The browser prompts the local authenticator, which silently signs the WebAuthn challenge using the synced private key. The login is successful.

Importantly, the enterprise application sees a perfectly valid, cryptographically signed WebAuthn assertion. To the application’s authentication server, this login is indistinguishable from a legitimate user logging in from their primary device.

Forensic Investigation: Tracing the Digital Footprints

Because the authentication server validates a cryptographically correct signature, network logs alone will not reveal the compromise. Incident responders must look at the perimeter of the cloud synchronization fabric and client-side operating system artifacts.

1. Auditing the Cloud Provider’s Enrollment Logs

The first line of inquiry is identifying when and how a new device joined the sync circle. Security teams should query their IdP and unified log solutions for device registration events.

• Google Workspace / Cloud Identity: Search Admin SDK Directory API logs for device_enrollment events. Look for anomalies in the userAgent, ipAddress, and registration timestamp relative to the user’s historical location patterns.
• Apple Enterprise / iCloud: Look for Unified Log events generated by the Cloud Device Provisioning daemon (com.apple.CoreCDP). Specifically, look for changes in the iCloud account’s trusted device list.

2. Analyzing Local Client-Side Artifacts

When investigating a suspected compromised endpoint, or when performing a forensic sweep of an attacker’s seized machine, several local databases store critical metadata about passkey usage.

Windows Artifacts

On Windows 11 systems, Windows Hello handles the storage and execution of WebAuthn operations. Forensic analysts should focus on the Windows Event Log ecosystem:

• Event Log Path: %SystemRoot%System32WinevtLogsMicrosoft-Windows-WebAuthn-Operational.evtx
• Event ID 1000 (Credential Creation): This event fires when a passkey is registered. Inspect the RelyingPartyId and UserEntityId to see which accounts were established.
• Event ID 2000 (Assertion/Authentication): This event logs successful authentication attempts. Analyze the timestamp, process ID calling the WebAuthn API (typically browser processes like chrome.exe or msedge.exe), and the target domain.

By comparing the timestamps of Event ID 2000 on the legitimate user’s machine with the target application’s access logs, you can identify discrepancies where logins occurred, but no local WebAuthn event was recorded—a clear indicator of sync hijacking.

macOS and iOS Artifacts

On macOS, passkeys are managed by the securityd daemon and stored securely within the Keychain database. While the private keys are inaccessible, the Unified Log contains valuable diagnostic data. Run the following log query to isolate cloud keychain synchronization events:

log show --predicate 'process == "securityd" AND eventMessage CONTAINS "KeychainSync"' --info

Look closely for entries detailing “peer-to-peer” trust establishment or key syncing transactions. A sudden spike in synchronization activity coupled with the registration of a new peer ID (representing the attacker’s device) indicates key exfiltration.

Furthermore, the local SQLite database tracking Safari and Chrome WebAuthn metadata (often found in user library directories under ~/Library/Containers/com.apple.Safari/Data/Library/WebAuthn/) can be analyzed for credential creation dates that do not align with known user actions.

Defending Against Passkey Sync Hijacking

Relying solely on the cryptographic strength of WebAuthn is no longer sufficient when the storage plane is fluid. Organizations must implement defensive layers to mitigate the risk of sync hijacking.

Enforce Hardware-Bound Credentials

For high-privilege accounts (such as domain administrators, security analysts, and executives), organizations should configure their Identity Providers to reject synced passkeys. This is achieved by setting the WebAuthn authenticatorAttachment parameter to cross-platform and requiring residentKey options that enforce hardware-backed attestation (e.g., requiring FIPS-compliant physical security keys).

Implement Device Posture Checking

Zero Trust Network Access (ZTNA) policies must validate the health and identity of the device *before* accepting the passkey authentication. If an attacker syncs a passkey to a non-compliant, unmanaged virtual machine, the authentication request should be blocked at the gateway, regardless of the validity of the cryptographic signature.

Continuous Session Monitoring

Since sync hijacking relies on the initial compromise of a cloud account, detecting session hijacking remains paramount. Implement robust session management controls, such as:

• Continuous Access Evaluation (CAE): Instantly revoke active sessions if a user’s IP address changes radically or if an anomalous device enrollment occurs.
• Strict Device Trust Circles: Require MFA confirmation on an *existing* trusted device before allowing any new device to join the cloud synchronization circle.

Conclusion

Passkeys represent a monumental leap forward in authentication security, effectively neutralising classic credential harvesting attacks. However, forensic investigators and security architects must recognize that passkeys have shifted the security perimeter from the login form to the cloud synchronization fabric. By understanding how sync hijacking works and knowing where to hunt for trace artifacts in local event logs and cloud audit trails, digital forensics professionals can successfully detect, trace, and remediate these sophisticated ghost credentials before they cause catastrophic damage.