CVE-2026-5327 Overview
A command injection vulnerability has been discovered in efforthye fast-filesystem-mcp versions up to 3.5.1. The vulnerability exists in the handleGetDiskUsage function within the src/index.ts file, allowing attackers to inject and execute arbitrary commands through manipulated input. This vulnerability can be exploited remotely, and a public exploit has been released, increasing the risk of active exploitation.
Critical Impact
Remote attackers can inject arbitrary commands through the handleGetDiskUsage function, potentially leading to unauthorized system access, data exfiltration, or complete system compromise.
Affected Products
- efforthye fast-filesystem-mcp versions up to 3.5.1
- Applications integrating fast-filesystem-mcp for filesystem operations
- Systems using the vulnerable handleGetDiskUsage function
Discovery Timeline
- April 2, 2026 - CVE-2026-5327 published to NVD
- April 2, 2026 - Last updated in NVD database
Technical Details for CVE-2026-5327
Vulnerability Analysis
This command injection vulnerability (CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component) affects the handleGetDiskUsage function in the fast-filesystem-mcp package. The vulnerability stems from insufficient input validation when processing user-supplied data, allowing malicious actors to inject shell commands that are executed by the underlying system.
The fast-filesystem-mcp package is designed to provide filesystem operations through an MCP (Model Context Protocol) interface. When the handleGetDiskUsage function receives input, it fails to properly sanitize special characters or command separators, enabling attackers to append arbitrary commands to legitimate requests.
Root Cause
The root cause of this vulnerability is improper input validation in the handleGetDiskUsage function located in src/index.ts. The function appears to construct system commands using user-supplied input without adequate sanitization, escaping, or parameterization. This allows specially crafted input containing shell metacharacters (such as ;, |, &&, or backticks) to break out of the intended command context and execute additional commands.
Attack Vector
The attack can be performed remotely over the network. An attacker can exploit this vulnerability by sending a crafted request to the handleGetDiskUsage endpoint with malicious command sequences embedded in the input parameters. Since the exploit has been publicly released, attackers can leverage existing proof-of-concept code to target vulnerable installations.
The vulnerability allows for injection attacks that bypass normal input restrictions, potentially enabling:
- Arbitrary command execution on the host system
- Data exfiltration through reverse shells or command output
- System enumeration and lateral movement
- Denial of service through resource-intensive commands
Technical details about the exploitation methodology can be found in the GitHub Issue #15 and the associated Bug Report PDF.
Detection Methods for CVE-2026-5327
Indicators of Compromise
- Unusual command sequences in web server or application logs containing shell metacharacters (;, |, &&, $(, backticks)
- Unexpected child processes spawned by the Node.js application running fast-filesystem-mcp
- Network connections initiated from the application server to unknown external hosts
- Filesystem access patterns indicative of reconnaissance or data exfiltration
Detection Strategies
- Implement application-layer monitoring to detect command injection patterns in requests to the handleGetDiskUsage endpoint
- Deploy web application firewalls (WAF) with rules to block common command injection payloads
- Monitor process creation events for suspicious child processes spawned by Node.js applications
- Analyze application logs for requests containing shell metacharacters or encoded command sequences
Monitoring Recommendations
- Enable verbose logging for the fast-filesystem-mcp application to capture all requests to the disk usage functionality
- Configure SIEM rules to alert on command injection indicators in application and web server logs
- Implement network monitoring to detect unusual outbound connections from application servers
- Set up file integrity monitoring on critical system files and directories
How to Mitigate CVE-2026-5327
Immediate Actions Required
- Identify all instances of fast-filesystem-mcp version 3.5.1 or earlier in your environment
- Implement input validation at the application layer to reject requests containing shell metacharacters
- Consider disabling or restricting access to the handleGetDiskUsage functionality until a patch is available
- Apply network-level controls to limit access to affected endpoints
Patch Information
As of the last update, the project maintainers have not responded to the vulnerability report submitted via GitHub Issue #15. No official patch is currently available. Organizations should monitor the fast-filesystem-mcp GitHub repository for updates and consider alternative mitigations in the interim.
Workarounds
- Implement a reverse proxy or WAF in front of the application to filter malicious input before it reaches the vulnerable function
- Modify the application code locally to add input sanitization that escapes or rejects shell metacharacters
- Restrict network access to the affected service to trusted IP addresses only
- Consider replacing the vulnerable package with an alternative that provides similar functionality with proper input validation
The recommended approach for input sanitization involves escaping shell metacharacters and validating that disk usage queries contain only expected path characters. For detailed vulnerability information, refer to the VulDB entry for this vulnerability.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
