building-c2-redirector-infrastructure

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• LICENSE
• SKILL.md
• api-reference.md · references/
• standards.md · references/
• agent.py · scripts/

SKILL.md (first lines)

# Building C2 Redirector Infrastructure

> **Authorized Use Only:** This skill is for authorized red-team engagements, adversary-emulation exercises, and defensive research only. Command-and-control infrastructure is dual-use; deploying redirectors to control malware on systems you are not explicitly authorized to test is illegal. Operate only inside an agreed scope with a signed rules-of-engagement document, and decommission infrastructure when the engagement ends.

## Overview

A C2 redirector is an intermediary host that sits between victim implants and the real team server. Beacons connect to the redirector's public domain/IP; the redirector inspects each request and either proxies legitimate C2 traffic back to the hidden team server or diverts everything else (scanners, blue-team analysts, sandboxes) to a benign decoy site. This protects the team server from discovery, takedown, and attribution, and lets operators rotate the public edge without rebuilding the backend. The technique maps to MITRE ATT&CK **T1090.002 (Proxy: External Proxy)** — adversaries route C2 through an intermediary node to obscure the true origin.

Redirectors come in two flavors. **Dumb pipes** (socat, iptables NAT) blindly forward a port and provide separation but no filtering. **Smart/filtering redirectors** (nginx `proxy_pass`, Apache `mod_rewrite` with `[P]`, or purpose-built tools like RedWarden) parse HTTP requests and only forward traffic that matches the implant's Malleable C2 profile — correct URI, User-Agent, headers — while sending everything else a `302` to a real website. The filtering logic is derived directly from the C2 framework's traffic profile, so the two must stay in lock-step. Tools such as `cs2modrewrite` automate generating Apache/nginx rules from a Cobalt Strike Malleable C2 profile.

This skill covers building both dumb and filtering redirectors with nginx and Apache, deriving filter rules from a malleable profile, layering TLS with Let's Encrypt, and applying OPSEC controls (categorized domains, domain fronting/CDN fronting, header validation, geo/UA filtering) for resilient, low-attribution infrastructure.

## When to Use

- Standing up red-team C2 that must survive blue-team triage and domain takedown requests.
- Separating a hidden team server from any internet-facing host during an engagement.
- Filtering implant traffic so only profile-matching requests reach the backend, diverting scanners.
- Adding TLS termination, domain categorization, and CDN/domain fronting to an HTTP(S) listener.
- Teaching defenders how external-proxy C2 (T1090.002) is constructed so they can detect it.

## Prerequisites

- One or more disposable cloud VPS instances (the redirector edge) and a separate, firewalled team-server host.
- A registered domain with controllable DNS, ideally aged/categorized.
- Root on the redirector host. Install the web server and TLS tooling:
  ```bash
  # Debian/Ubuntu redirector
  sudo apt update
  sudo apt install -y nginx apache2 socat certbot python3-certbot-nginx git
  # Enable Apache proxy modules if using mod_rewrite redirector
  sudo a2enmod rewrite proxy proxy_http ssl headers
  ```
- The C2 framework's Malleable C2 profile (Cobalt Strike `.profile`, Sliver/Havoc HTTP profile) defining URIs, User-Agent, and headers.
- `cs2modrewrite` to auto-generate rules from a Cobalt Strike profile:
  ```bash
  git clone https://github.com/threatexpress/cs2modrewrite
  ```
- Firewall the team server so it only accepts the redirector's source IP on the C2 port.

## Objectives

- Deploy a dumb-pipe redirector (socat/iptables) for fast port separation.
- Deploy a filtering nginx reverse-proxy redirector keyed to a malleable profile.
- Deploy an Apache `mod_rewrite` redirector with `[P]` proxying and `302` decoy fallback.
- Auto-generate redirector rules from a Cobalt Strike profile with `cs2modrewrite`.
- Terminate TLS with Let's Encrypt and harden the public edge.
- Apply OPSEC: header/UA validation, geo filtering, decoy diversion, and infra rotation.

## MITRE ATT&CK Mapping

| Technique ID | Official Name | Relevance |
|--------------|---------------|-----------|
| T1090.002 | Proxy: External Proxy | The redirector is an external intermediary that proxies C2 to hide the team server |
| T1090.004 | Proxy: Domain Fronting | CDN fronting routes beacon traffic through a trusted high-reputation domain |
| T1071.001 | Application Layer Protocol: Web Protocols | C2 is tunneled over HTTP/HTTPS shaped by the malleable profile |
| T1573.002 | Encrypted Channel: Asymmetric Cryptography | TLS termination at the redirector encrypts the beacon channel |
| T1583.006 | Acquire Infrastructure: Web Services | Disposable VPS/CDN edges are acquired for resilient C2 |

## Workflow

### 1. Lab and firewall the team server
Place the team server on a private host. Restrict its C2 port to the redirector's IP only.
```bash
# On the team server: only the redirector (203.0.113.10) may reach 443/tcp
sudo ufw default deny incoming
sudo ufw allow from 203.0.113.10 to any port 443 proto tcp
sudo ufw allow OpenSSH
sudo ufw enable
```

### 2. Dumb-pipe redirector (socat / iptables)
For quick separation with no filtering, forward the C2 port to the team server.
```bash
# socat foreground forward of 443 -> team server
socat TCP4-LISTEN:443,fork,reuseaddr TCP4:10.0.0.2:443

# Or iptables DNAT (persistent)
sysctl -w net.ipv4.ip_forward=1
iptables -t nat -A PREROUTING -p tcp --dport 443 -j DNAT --to-destination 10.0.0.2:443
iptables -t nat -A POSTROUTING -p tcp -d 10.0.0.2 --dport 443 -j MASQUERADE
```

### 3. Filtering nginx reverse-proxy redirector
Only proxy requests whose URI matches the malleable profile; send everything else a `302` to a decoy. Replace the location regex and User-Agent with values from your profile.
```nginx
# /etc/nginx/sites-available/redirector.conf
server {
    listen 443 ssl;
    server_name cdn.example.com;

    ssl_certificate     /etc/letsencrypt/live/cdn.example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/cdn.example.com/privkey.pem;

    # Proxy ONLY profile-matching C2 URIs to the hidden team server
    location ~ ^/(api/v2/jobs|cm/[a-z0-9]+|push) {
        # Require the implant's exact User-Agent
        if ($http_user_agent != "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36") {
            return 302 https://www.legitimate-decoy.com/;
        }
        proxy_pass https://10.0.0.2;
        proxy_ssl_verify off;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $remote_addr;
    }

    # Everything else -> benign decoy
    location / {
        return 302 https://www.legitimate-decoy.com/;
    }
}
```
```bash
sudo ln -s /etc/nginx/sites-available/redirector.conf /etc/nginx/sites-enabled/
sudo nginx -t && sudo systemctl reload nginx
```

### 4. Apache mod_rewrite redirector
Apache's `[P]` flag proxies matching requests to the team server; non-matches get a `302` redirect. This is the format `cs2modrewrite` produces.
```apache
# /etc/apache2/sites-available/redirector.conf  (inside <VirtualHost *:443>)

From mukul975/Anthropic-Cybersecurity-Skills, Apache-2.0.

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