GhostLock: Exploiting a Legitimate Windows API to Deny File Access

Overview

A security researcher has unveiled a proof-of-concept tool called GhostLock that demonstrates a troubling twist on file access denial. By abusing a legitimate Windows file API, the tool can effectively block access to files stored either locally or on SMB network shares—without needing elevated privileges or installing any persistent hooks. This technique highlights how even trusted system components can be repurposed for malicious ends, raising critical questions about endpoint security and data integrity.

How GhostLock Works

GhostLock leverages the Windows LockFile API, which is designed to allow applications to lock portions of a file to prevent concurrent write operations. The tool opens a handle to a target file and acquires a lock over the entire byte range. Once locked, any other process—including the legitimate user or system services—cannot read or write to that file until the lock is released. The attacker can maintain this lock indefinitely, effectively rendering the file inaccessible.

The attack works on both local NTFS volumes and remote SMB shares because the LockFile API operates at the file system level. On SMB, the lock is enforced server-side, so even the file owner cannot bypass it without closing the malicious handle from the attacking machine. This makes GhostLock a potent tool for denial-of-service attacks on shared files or documents critical to business operations.

Implications for Security

Attack Scenarios

• Ransomware without encryption: Instead of encrypting files and demanding a ransom, an attacker could lock critical files and demand payment for unlocking them. This sidesteps many detection mechanisms that look for encryption patterns.
• Sabotage of collaborative work: In environments relying on SMB shares (e.g., file servers), an attacker can lock shared documents, halting productivity until the lock is removed.
• Data exfiltration cover: While files are locked, legitimate access is denied, masking unauthorized copying or movement of data.

Detection Challenges

Because GhostLock uses a legitimate API, traditional antivirus or endpoint detection may not flag the behavior as malicious. The process that holds the lock appears as a normal application using LockFile, which is common for database software, editors, and backup tools. This low visibility makes the attack difficult to spot without specialized monitoring of file locks.

Mitigation Strategies

1. Audit file lock activity: Use Windows security event logging (e.g., Sysmon) to monitor for unexpected or prolonged file locks, especially from unfamiliar processes.
2. Implement least privilege: Restrict which users and applications can run that have network access to SMB shares; lock down permissions so only trusted processes can hold locks.
3. Use File Server Resource Manager (FSRM): Configure file screens or quota warnings to alert on excessive file locking.
4. Network segmentation: Isolate sensitive file servers from general workstations to limit the blast radius if a machine is compromised.
5. Employ behavioral analysis: Tools that detect unusual file access patterns—like a single process locking many files across different shares—can help identify a GhostLock-type attack.

While GhostLock is a proof of concept, its underlying technique is already usable by adversaries. Organizations should treat this as a wake-up call to harden file access controls and monitor for anomalous locking behavior.