How Attackers Exploit Cloud File Sync Clients to Move Laterally Without Malware

Increasingly, it no longer applies.

That pattern has started to break down. In a growing number of cases, attackers move laterally without dropping anything new into the environment. Rather than relying on malware, they make use of applications that are already present and broadly trusted. Cloud file synchronization clients are a good example. Organizations deploy them on purpose, expect them to run all the time, and depend on them to move data between systems as part of everyday work.

When these clients are misused, they give attackers a way to expand access quietly. There is no exploit to catch and no suspicious execution to flag, which means many of the alerts teams rely on simply never fire.

Why Sync Clients Keep Appearing After Initial Access

Cloud file sync clients have several properties that make them particularly useful once an attacker is already inside an environment. They are persistent by design. They are signed and maintained by well-known vendors. They communicate externally as part of normal operation. Most importantly, they operate under the identity of a legitimate user and inherit whatever access that user already has.

Once an attacker controls an account or an endpoint, the sync client effectively becomes theirs as well. There is no need to modify the client, inject code, or exploit vulnerabilities in the application itself. The software continues to behave exactly as intended, following synchronization rules and access permissions that already exist.

From a defensive perspective, this creates an uncomfortable situation. The activity looks legitimate because, in many respects, it is. Files are being synced by an approved application using valid credentials. Nothing obvious breaks policy, even though the outcome may be harmful.

How Attackers Enter the Sync Context

Attackers rarely target synchronization software directly. Instead, they focus on whatever the client depends on to function. Most often, that means credentials or session material. Phishing remains a common entry point. So do session theft, reused passwords, token abuse, and OAuth misconfigurations. In other cases, attackers start with an already-compromised endpoint and inherit whatever authenticated sessions exist on that device.

Once a valid session is in place, the sync client does the rest. Files move because synchronization logic dictates they should, not because an attacker explicitly pushed them to each endpoint. At this stage, lateral movement does not involve execution. It involves replication.

In some investigations, the initial access is months old by the time sync abuse becomes visible. Credentials may have been harvested during a prior phishing campaign or leaked through unrelated incidents. Because the account continues to authenticate normally, no one revisits it until unusual data movement is noticed. This delay is part of what makes sync-based lateral movement effective. The attacker does not need persistence mechanisms or scheduled tasks. As long as the identity remains valid, the sync client continues to operate on their behalf.

This also complicates incident timelines. When defenders try to determine when lateral movement began, they often find that file replication occurred gradually, spread across weeks of routine synchronization rather than concentrated bursts. That slow pace blends neatly into background activity.

This distinction matters. Many detection strategies still hinge on identifying something “running” somewhere new. Sync-based movement bypasses that assumption entirely.

File Movement as a Lateral Technique

Traditional lateral movement relies on running tools or code on additional systems. Sync-based movement removes that requirement. Attackers introduce files into shared locations and allow synchronization to distribute them automatically.

Those files appear on other endpoints without scripts being launched, services being installed, or binaries being executed. Sometimes the files are inert and exist only to observe how data propagates. Sometimes they are designed to be opened later. In other cases, they serve as staging artifacts that reveal which systems participate in synchronization.

From a telemetry standpoint, all that is visible is file activity initiated by a trusted process. There is no clear execution chain to follow, and no obvious malicious payload to analyze.

In several real-world cases, attackers have used this approach to stage access rather than deliver payloads. Files placed into shared locations may contain credentials, internal documentation, or notes intended only for the attacker’s later reference. Because these artifacts sync naturally, they provide a map of which users and systems participate in shared workflows.

In environments with aggressive collaboration, this can unintentionally expose administrative endpoints, service accounts, or legacy systems that still synchronize data but are rarely monitored. The attacker does not need to interact with those systems directly. Synchronization reveals their existence automatically

The Role of Shared Access and Permission Inheritance

Shared folders and inherited permissions are what make this technique effective. In many organizations, collaboration spaces span teams, projects, and departments. Access accumulates over time as people join initiatives, change roles, or temporarily require visibility into shared resources. Once granted, access is rarely revisited unless something breaks or a formal review forces the issue.

Attackers who compromise a single account often gain visibility into far more data than expected. From there, they can introduce files into locations that sync across multiple machines. Each synced endpoint becomes another foothold, expanding the attacker’s reach without additional effort.

From those systems, attackers may observe workflows, harvest credentials, or pivot further using other mechanisms. None of this requires malware deployment, privilege escalation exploits, or unusual system behavior.

Blending Into Everyday Collaboration

Campaigns that rely on sync clients tend to move slowly and deliberately. Attackers pay attention to timing, volume, and appearance. Uploads occur during business hours. File sizes resemble existing content. Naming conventions match what teams already use. Synchronization frequency stays within expected ranges. Nothing spikes dramatically enough to stand out on its own.

This matters because many detection strategies are tuned to catch anomalies. In these scenarios, there often are none. The activity only becomes suspicious when viewed in context, and even then it can be easy to dismiss as normal collaboration noise.

What Network Telemetry Can Still Reveal

Even without malware, sync activity generates network data. There are outbound connections, TLS sessions, and predictable request patterns associated with synchronization. On their own, these signals are expected and rarely trigger concern. The value emerges when they are correlated with other events. An increase in sync activity shortly after an account compromise is one example. Synchronization originating from a device that has never participated in collaboration before is another.

Without context, these signals look benign. With context, they begin to tell a story.

Why Traditional Indicators Struggle Here

This technique avoids most of the indicators defenders are trained to look for. There are no suspicious file hashes to block. No unusual domains to sinkhole. No infrastructure clearly tied to known campaigns.

Often, there is not even a single “event” to investigate. Behavior-based rules struggle as well, because the observed behavior fits squarely within what the software is designed to do.

As a result, defenders are pushed away from signature-based detection and toward activity modeling and environmental baselining. That shift is not trivial, especially in environments where visibility is fragmented across teams.

Where Analysts Can Still Focus Their Efforts

Despite the low noise, opportunities for investigation remain. Sync behavior for a given user is often stable over time. Sudden changes can stand out when tracked longitudinally. New endpoints joining synchronization activity deserve attention, particularly when paired with recent identity changes.

File lineage analysis can also be useful. Understanding how content propagates across systems can reveal unexpected paths or over-permissive sharing. Identity events such as password resets, MFA changes, or new device registrations become far more informative when examined alongside synchronization data.

None of these signals works well in isolation. The signal emerges from their combination.

Structural Weaknesses That Make This Possible

This technique succeeds because of how environments are managed, not because of any novel attacker capability. Another contributing factor is ownership ambiguity. Shared folders often lack a clear owner responsible for reviewing access over time. When projects end or teams reorganize, permissions remain because no one feels accountable for pruning them. In regulated environments, reviews may exist on paper but focus on policy compliance rather than operational reality.

This fragmentation mirrors how security responsibilities are divided. Identity teams manage authentication. Endpoint teams manage devices. Cloud teams manage collaboration platforms. Sync abuse falls between those boundaries. Attackers benefit from that gap, because no single team sees enough of the picture to recognize what is happening early.

Permissions drift over time. Shared folders proliferate. Logs exist but are rarely reviewed in a coordinated way. Identity, endpoint, and cloud teams often operate independently, each with partial visibility.

Attackers do not need to bypass controls when those controls were never clearly defined or consistently enforced in the first place.

Reducing Risk Without Breaking Collaboration

Blocking synchronization clients outright is rarely practical. These tools exist because organizations rely on them.

More realistic mitigations focus on scope and visibility. Reducing unnecessary shared access helps limit blast radius. Monitoring first-time sync activity from new devices can surface early warning signs. Treating unusual synchronization behavior as something to investigate, rather than immediately alert on, often produces better results.

The goal is not to stop collaboration. It is to understand how collaboration behaves when something goes wrong.

Why This Technique Is Here to Stay

As environments become more identity-centric, this style of movement becomes easier rather than harder. When access equals trust and replication equals productivity, attackers will continue to exploit that relationship.

Malware is no longer required to move laterally. In many cases, permission alone is sufficient. Sync clients demonstrate how trusted systems can become unintended transport mechanisms when identity and access are misaligned with how data actually flows.

Defenders who assume that “nothing executed” means “nothing happened” will miss these intrusions. Those who take time to understand how trusted software behaves under compromised identities gain an advantage. The difference is not tooling, but perspective. Sync-based lateral movement does not announce itself as an attack. It looks like work continuing as usual, just under the wrong control.

Author bio

Ariel Perez is the founder of AKAVEIL TECHNOLOGIES, an IT company that builds secure cloud systems and automation for boutique law firms. He works closely with attorneys to design and secure cloud collaboration environments, with a focus on how file sharing, access controls, and identity impact real-world security.

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