Hunting For Domain Fronting C2 Traffic

Hunting for Domain Fronting C2 Traffic

Overview

Domain fronting (MITRE ATT&CK T1090.004) is a technique where attackers use different domain names in the TLS SNI field and the HTTP Host header to disguise C2 traffic behind legitimate CDN-hosted domains. This skill detects domain fronting by parsing proxy/web gateway logs for SNI-Host header mismatches, analyzing TLS certificates for CDN provider identification, flagging connections where the SNI points to a high-reputation domain but the Host header targets an attacker-controlled domain, and correlating with known CDN provider IP ranges.

When to Use

• When investigating security incidents that require hunting for domain fronting c2 traffic
• When building detection rules or threat hunting queries for this domain
• When SOC analysts need structured procedures for this analysis type
• When validating security monitoring coverage for related attack techniques

Detection Gaps & Validation

• ECH makes classic SNI≠Host detection blind. Encrypted Client Hello (and the earlier ESNI) encrypts the SNI entirely, so the SNI-vs-Host mismatch signal disappears. Pivot to JA3/JARM TLS fingerprinting plus destination CDN IP/ASN reputation when SNI is unreadable.
• No TLS inspection = no Host header. Without MITM/inspection the HTTP Host is inside the TLS session and the mismatch is invisible; the hunt silently produces zero findings.
• Fronting is shifting to "domainless" fronting and CDN-internal pivots as Fastly/Cloudflare/CloudFront disabled classic fronting — Azure and other providers still permit variants (T1090.004).
• Validate the hunt fires: with inspection enabled, issue curl --resolve frontable.cdn:443:<CDN_IP> -H "Host: hidden-c2.example" and confirm the proxy log captures the SNI↔Host delta and your rule alerts; baseline the default Cobalt Strike / Meterpreter JA3/JARM values.
• FP tuning: legitimate apps and CDNs that legitimately split SNI and Host, plus health-check/monitoring probes — score on reputation differential, not mismatch alone.

Prerequisites

• Web proxy or secure web gateway logs with SNI and Host header fields
• Python 3.8+ with pyOpenSSL and cryptography libraries
• TLS inspection enabled on proxy for Host header visibility
• CDN provider IP range lists (CloudFront, Azure CDN, Cloudflare)

Steps

1. Parse proxy logs for connections with both SNI and Host header fields
2. Compare SNI domain against HTTP Host header for mismatches
3. Extract TLS certificate Subject and SAN fields using pyOpenSSL
4. Identify CDN-hosted connections via certificate issuer and IP ranges
5. Flag high-confidence domain fronting where SNI and Host differ on CDN IPs
6. Score alerts based on domain reputation differential
7. Generate detection report with network flow context

Expected Output

JSON report containing detected domain fronting indicators with SNI-Host pairs, certificate details, CDN provider identification, confidence scores, and MITRE ATT&CK technique mapping.