Proxy: Domain Fronting

Adversaries may take advantage of routing schemes in Content Delivery Networks (CDNs) and other services which host multiple domains to obfuscate the intended destination of HTTPS traffic or traffic tunneled through HTTPS. [1] Domain fronting involves using different domain names in the SNI field of the TLS header and the Host field of the HTTP header. If both domains are served from the same CDN, then the CDN may route to the address specified in the HTTP header after unwrapping the TLS header. A variation of the the technique, "domainless" fronting, utilizes a SNI field that is left blank; this may allow the fronting to work even when the CDN attempts to validate that the SNI and HTTP Host fields match (if the blank SNI fields are ignored).

For example, if domain-x and domain-y are customers of the same CDN, it is possible to place domain-x in the TLS header and domain-y in the HTTP header. Traffic will appear to be going to domain-x, however the CDN may route it to domain-y.

ID: T1090.004
Sub-technique of:  T1090
Tactic: Command And Control
Platforms: Linux, Windows, macOS
Data Sources: Packet capture, SSL/TLS inspection
Contributors: Matt Kelly, @breakersall
Version: 1.0
Created: 14 March 2020
Last Modified: 20 June 2020

Procedure Examples

Name Description
APT29

APT29 has used the meek domain fronting plugin for Tor to hide the destination of C2 traffic.[2]

meek

meek uses Domain Fronting to disguise the destination of network traffic as another server that is hosted in the same Content Delivery Network (CDN) as the intended desitnation.

Mitigations

Mitigation Description
SSL/TLS Inspection

If it is possible to inspect HTTPS traffic, the captures can be analyzed for connections that appear to be domain fronting.

Detection

If SSL inspection is in place or the traffic is not encrypted, the Host field of the HTTP header can be checked if it matches the HTTPS SNI or against a blocklist or allowlist of domain names. [1]

References