Harden-Runner is a CI/CD security agent that works like an EDR for GitHub Actions runners. In versions 2.15.1 and below, a DNS over HTTPS (DoH) vulnerability allows attackers to bypass egress-policy: block network restrictions by tunneling exfiltrated data through permitted HTTPS endpoints like dns.google. The attack works by encoding sensitive data (e.g., the runner's hostname) as subdomains in DoH queries, which appear as legitimate HTTPS traffic to Harden-Runner's domain-based filtering but are ultimately forwarded to an attacker-controlled domain. This effectively enables data exfiltration without directly connecting to any blocked destination. Exploitation requires the attacker to already have code execution within the GitHub Actions workflow. The issue was fixed in version 2.16.0.

Affected: GitHub Actions: step-security/harden-runner

## Summary

A vulnerability exists in the Community Tier of Harden-Runner that allows bypassing the `egress-policy: block` network restriction using DNS over HTTPS (DoH).

Harden-Runner secures GitHub Actions workflows on runners by applying network policies, including an `allowed-endpoints` configuration that limits outbound traffic to specified domains and ports (e.g., `github.com:443`). In `egress-policy: block` mode, non-compliant connections are intercepted and denied.

This vulnerability exploits DoH, a protocol that encapsulates DNS queries within HTTPS requests. By crafting a DNS query that embeds exfiltrated data as a subdomain (e.g., encoding the runner's hostname into a label), an attacker can route the request through a permitted HTTPS endpoint like `dns.google` (`8.8.8.8`'s DoH service). The resolver processes the query and forwards it to the attacker's controlled domain, achieving exfiltration without directly accessing the blocked destination. This evades Harden-Runner's domain-based filtering, as the initial HTTPS connection appears legitimate.

This vulnerability requires the attacker to already have code execution capabilities within the GitHub Actions workflow.

The Enterprise Tier of Harden-Runner is **not affected** by this vulnerability.

## Impact

When Harden-Runner is configured with `egress-policy: block` and a restrictive `allowed-endpoints` list, an attacker with existing code execution capabilities within a GitHub Actions workflow can bypass th…