Summary
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.
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 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.
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.
devanshbatham Reporter
Summary
A vulnerability exists in the Community Tier of Harden-Runner that allows bypassing the
egress-policy: blocknetwork 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: blockis enabled with a restrictive allowed-endpoints list (e.g., onlygit.832008.xyz: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 likedigcan explicitly initiate TCP-based DNS queries (+tcpflag) 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.
Impact
When Harden-Runner is configured with
egress-policy: blockand a restrictiveallowed-endpointslist, 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.
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.