A vulnerability exists in the Community Tier of Harden-Runner that allows bypassing the egress-policy: block network restriction using DNS queries over TCP.
Harden-Runner enforces egress policies on GitHub runners by filtering outbound connections at the network layer. When egress-policy: block is enabled with a restrictive allowed-endpoints list (e.g., only github.com:443), all non-compliant traffic should be denied. However, DNS queries over TCP, commonly used for large responses or fallback from UDP, are not adequately restricted. Tools like dig can explicitly initiate TCP-based DNS queries (+tcp flag) without being blocked.
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.
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 egress block policy by initiating DNS queries over TCP to external resolvers. This allows outbound network communication that evades the configured network restrictions.
This vulnerability affects only the Community Tier. It requires the attacker to already have code execution capabilities within the GitHub Actions workflow.
Upgrade to Harden-Runner v2.16.0 or later.
No action required. Enterprise tier customers are not affected by this vulnerability.
We would like to thank Devansh Batham for responsibly disclosing this vulnerability through our security reporting process.
2.16.0Exploitability
AV:NAC:LAT:PPR:HUI:NVulnerable System
VC:NVI:NVA:NSubsequent System
SC:HSI:NSA:N4.6/CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:N/VC:N/VI:N/VA:N/SC:H/SI:N/SA:N