CVE-2026-39363: Vite Information Disclosure Vulnerability

CVE-2026-39363 Overview

CVE-2026-39363 is a high-severity information disclosure vulnerability affecting Vite, a popular frontend tooling framework for JavaScript. The vulnerability allows attackers who can connect to the Vite development server's WebSocket without an Origin header to retrieve the contents of arbitrary files on the server. By invoking fetchModule via the custom WebSocket event vite:invoke and combining file://... paths with ?raw or ?inline query parameters, an attacker can exfiltrate sensitive file contents as JavaScript strings.

The core issue lies in the access control bypass where security restrictions enforced via server.fs.allow in the HTTP request path are not applied to the WebSocket-based execution path, creating a dangerous inconsistency in the security model.

Critical Impact

Attackers can read arbitrary files from the Vite development server, potentially exposing sensitive configuration files, environment variables, source code, and credentials.

Affected Products

• Vite versions 6.0.0 to before 6.4.2
• Vite versions 7.0.0 to before 7.3.2
• Vite versions 8.0.0 to before 8.0.5

Discovery Timeline

• 2026-04-07 - CVE-2026-39363 published to NVD
• 2026-04-08 - Last updated in NVD database

Technical Details for CVE-2026-39363

Vulnerability Analysis

This vulnerability is classified under CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor). The flaw enables unauthenticated remote attackers to bypass file system access controls when connecting through the WebSocket interface.

The Vite development server implements access control via server.fs.allow to restrict which files can be served through HTTP requests. However, this same protection was not applied to the WebSocket communication channel. When an attacker connects to the WebSocket endpoint without providing an Origin header, they can leverage the vite:invoke event to call internal functions like fetchModule that would normally be protected.

The attack works by requesting files using the file:// protocol scheme combined with ?raw or ?inline query parameters, which instructs Vite to return the file contents as a JavaScript export string (e.g., export default "..."). This effectively transforms any accessible file into retrievable data.

Root Cause

The root cause is an incomplete security implementation where the server.fs.allow access control mechanism was only enforced on HTTP request paths but not on the WebSocket-based execution path. This created a security boundary inconsistency that allowed the file system restrictions to be completely bypassed via WebSocket communication.

Attack Vector

The attack is network-based and can be executed without any user interaction or prior authentication. An attacker must be able to establish a WebSocket connection to the Vite development server. The attack involves:

1. Connecting to the Vite dev server's WebSocket endpoint without an Origin header
2. Sending a vite:invoke event with fetchModule as the target function
3. Specifying a file:// path to the target file with ?raw or ?inline parameters
4. Receiving the file contents as a JavaScript string export

For detailed technical information regarding the vulnerability mechanism and exploitation, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-39363

Indicators of Compromise

• Unusual WebSocket connections to the Vite development server without Origin headers
• WebSocket messages containing vite:invoke events with fetchModule calls
• Requests for file paths outside the normal project structure using file:// protocol
• Log entries showing access to sensitive files like .env, configuration files, or system files

Detection Strategies

• Monitor WebSocket connections for missing or suspicious Origin headers
• Implement logging for all vite:invoke WebSocket events and analyze for fetchModule calls with file:// paths
• Use network intrusion detection systems (IDS) to identify patterns of arbitrary file path requests
• Review server logs for unusual file access patterns through the development server

Monitoring Recommendations

• Enable verbose logging on development servers to capture WebSocket communication details
• Configure alerting for WebSocket connections originating from unexpected IP addresses or lacking proper headers
• Implement file integrity monitoring on sensitive configuration files that could be targeted
• Regularly audit development server access logs for anomalous activity patterns

How to Mitigate CVE-2026-39363

Immediate Actions Required

• Upgrade Vite to patched versions: 6.4.2, 7.3.2, or 8.0.5 depending on your major version
• Restrict network access to Vite development servers to trusted networks only
• Avoid exposing Vite development servers to the public internet
• Review development server logs for any signs of exploitation attempts

Patch Information

The vulnerability has been fixed in the following Vite versions:

Update your Vite installation using your package manager to the appropriate fixed version. For more details, see the GitHub Security Advisory.

Workarounds

• Ensure development servers are only accessible from localhost or trusted internal networks
• Use a firewall or network segmentation to prevent external access to development server ports
• Consider using a reverse proxy with Origin header validation in front of the development server
• Disable WebSocket functionality if not required for your development workflow

# Restrict Vite dev server to localhost only
npm run dev -- --host localhost

# Or configure in vite.config.js
# server: { host: 'localhost' }