GHSA-46g3-37rh-v698

Egress Policy Bypass via DNS over HTTPS (DoH) in Harden-Runner (Community Tier)

## 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 the allowed domains check via DNS over HTTPS by proxying DNS queries through a permitted resolver (e.g., Google's DoH service). This allows data exfiltration even when `allowed-endpoints` is set to only whitelisted domains. This vulnerability affects only the Community Tier. It requires the attacker to already have code execution capabilities within the GitHub Actions workflow. ## Remediation ### For Community Tier Users Upgrade to Harden-Runner v2.16.0 or later. ### For Enterprise Tier Users No action required. Enterprise tier customers are not affected by this vulnerability. ## Credit We would like to thank [Devansh Batham](https://github.com/devanshbatham) for responsibly disclosing this vulnerability through our security reporting process.