GHSA-46g3-37rh-v698 Egress Policy Bypass via DNS over HTTPS (DoH) in Harden-Runner (Community Tier)

• GitHub Actions step-security/harden-runner: < 2.16.0 → fixed in 2.16.0

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 for responsibly disclosing this vulnerability through our security reporting process.