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

Vulnerability

Overview

CVE-2026-32947 is a medium-severity vulnerability in the Community Tier of Harden-Runner, a CI/CD security agent for GitHub Actions runners. The flaw resides in how Harden-Runner handles DNS over HTTPS (DoH) traffic. In versions prior to 2.16.0, DoH queries are not distinguished from legitimate HTTPS traffic, allowing an attacker to tunnel exfiltrated data through permitted resolvers like dns.google [1][3].

Exploitation

Method

To exploit this, an attacker must already have arbitrary code execution within the GitHub Actions workflow. By encoding sensitive data (e.g., the runner's hostname or secrets) as subdomains in a DoH query, the request appears to Harden-Runner's domain-based filtering as a legitimate HTTPS connection to an allowed endpoint. The resolver processes the query and forwards the subdomain data to an attacker-controlled domain, achieving exfiltration without directly connecting to a blocked destination [1][3].

Impact

Successful exploitation allows an attacker to bypass the egress-policy: block restriction and exfiltrate data from the runner even when a restrictive allowed-endpoints list is configured. This undermines the core network isolation Harden-Runner is designed to enforce. The vulnerability only affects the Community Tier; Enterprise Tier customers are not impacted [3][4].

Mitigation

The issue is fixed in version 2.16.0 of Harden-Runner [4]. Users of the Community Tier are strongly advised to upgrade immediately. No workarounds are documented, and the vendor recommends upgrading as the sole remediation [3].