Harden-Runner, the CI/CD security agent for GitHub Actions, contains a DoH vulnerability in versions 2.15.1 and earlier that lets an attacker encode sensitive data as subdomains in DNS over HTTPS queries. The queries go to a permitted HTTPS endpoint such as dns.google, pass through Harden-Runner’s domain‑based egress filtering, and are forwarded to an attacker‑controlled domain, enabling exfiltration of data without connecting to a blocked destination. This weakness is associated with CWE‑693, CWE‑807, and CWE‑863 and allows a breach of confidentiality by leaking information that should not leave the isolated network.

Step‑Security Harden‑Runner Community Tier, versions up to and including 2.15.1, is affected. The vulnerability was resolved in version 2.16.0. Any environment running a vulnerable Harden‑Runner instance on GitHub Actions is at risk.

The CVSS score of 4.6 indicates moderate severity, while the EPSS score of less than 1% suggests a low probability of wide‑scale exploitation. The vulnerability is not listed in CISA’s KEV catalog. Exploitation requires existing code execution within the GitHub Actions workflow; the attacker then crafts DoH queries that appear legitimate. Because the attack uses standard HTTPS channels that are normally permitted, defenses based solely on egress rules are ineffective. The overall risk is moderate but depends heavily on whether the runner is exposed to malicious workflow code.