‘Underminr’ Vulnerability Lets Attackers Hide Malicious Connections Behind Trusted Domains

Threat actors are exploiting a vulnerability in shared content delivery network (CDN) infrastructure to hide connections to malicious domains.

Dubbed Underminr, the issue is a variant of domain fronting, a now-mitigated type of attack that enabled threat actors to place an allowed domain in the SNI and TLS certificate validation fields of an HTTPS request, while embedding a different target domain in the TLS tunnel’s encrypted HTTP host header.

Because CDNs routed requests internally based on the host headers, the request reached the hidden destination, while traffic would appear to be going to a reputable front domain.

Instead of using a front domain, Underminr presents the SNI and HTTP Host of a domain while forcing a request to the IP address of another tenant on the same shared edge.

The mismatch, ADAMnetworks reports, has been exploited in attacks targeting large-scale hosting providers, including those that have implemented mitigations against domain fronting.