building-c2-redirector-infrastructure skill

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 proxypass, Apache modrewrite 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:

# 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:

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

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.

# 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.

# 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.

# /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/;
    }
}

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.

# /etc/apache2/sites-available/redirector.conf  (inside <VirtualHost *:443>)
RewriteEngine On
SSLProxyEngine On
# Require the implant User-Agent
RewriteCond %{HTTP_USER_AGENT} "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36" [NC]
# Match valid C2 URIs (GET/POST/stager) from the malleable profile
RewriteCond %{REQUEST_URI} ^/(api/v2/jobs|cm/[a-z0-9]+|push)/?$
# Proxy to the hidden team server, preserving the URI
RewriteRule ^.*$ https://10.0.0.2%{REQUEST_URI} [P,L]
# Everything else -> decoy site
RewriteRule ^.*$ https://www.legitimate-decoy.com/ [R=302,L]

sudo a2ensite redirector && sudo apache2ctl configtest && sudo systemctl reload apache2

5. Generate rules from a malleable profile

Let cs2modrewrite build the Apache or nginx rules directly from your Cobalt Strike profile so the filter exactly matches beacon traffic.

cd cs2modrewrite
# Apache mod_rewrite rules
python3 cs2modrewrite.py -i havex.profile -c https://10.0.0.2 \
  -r https://www.legitimate-decoy.com -o /etc/apache2/redirect.rules
# nginx config
python3 cs2nginx.py -i havex.profile -c https://10.0.0.2 \
  -r https://www.legitimate-decoy.com -H cdn.example.com > /etc/nginx/sites-available/c2.conf

6. Terminate TLS with Let's Encrypt

Issue a valid certificate so beacon HTTPS does not throw TLS warnings and the edge looks legitimate.

sudo certbot --nginx -d cdn.example.com --agree-tos -m ops@example.com --redirect
# Verify auto-renewal
sudo certbot renew --dry-run

7. Apply OPSEC controls

Layer defenses against blue-team analysis: validate headers, geofence to the target country, divert sandboxes, and rotate edges. Consider CDN/domain fronting (T1090.004) where supported.

# Example: drop non-target geographies at the firewall with ipset/GeoIP,
# require a custom auth header in the profile, and rotate the redirector
# domain/IP on a schedule. Check the redirector only forwards matched traffic:
curl -k https://cdn.example.com/                       # expect 302 to decoy
curl -k -A "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36" \
  https://cdn.example.com/api/v2/jobs                  # expect proxied response

Tools and Resources

Validation Criteria

• [ ] Team server firewalled to accept only the redirector source IP on the C2 port.
• [ ] Redirector deployed (dumb pipe and/or filtering reverse proxy).
• [ ] Filter rules derived from the actual malleable C2 profile (URI + User-Agent + headers).
• [ ] Non-matching requests return a 302 to a benign decoy site (verified with curl).
• [ ] Matching beacon requests are proxied to the hidden team server (verified with curl).
• [ ] Valid TLS certificate issued and auto-renewal confirmed.
• [ ] OPSEC controls applied (UA/header validation, geofencing, decoy diversion).
• [ ] Domain categorization / CDN fronting considered where applicable.
• [ ] Infrastructure rotation and decommissioning plan documented.
• [ ] All activity confined to the authorized engagement scope.