Summary
An authorized attacker can bypass authorization checks and revoke any SSH certificate issued by Step CA by using a valid revocation token.
Details
Step CA users can obtain SSH certificates from a few provisioners. The SSHPOP provisioner allows revocation of the SSH certificate (preventing future certificate renewals) using a token. Due to a missing validity check, this token could be used to revoke any SSH certificate issued by the CA.
To create a token, an attacker must have access to the CA endpoint and a valid SSH certificate, meaning they were already authorized to obtain an SSH certificate. The attacker must also know the serial number of the certificate they want to revoke.
Acknowledgements
This issue was identified and reported by Gabriel Departout and Andy Russon, from AMOSSYS. This audit was sponsored by ANSSI (French Cybersecurity Agency) based on their Open-Source security audit program.
Embargo List
If your organization runs Step CA in production and would like advance, embargoed notification of future security updates, visit https://u.step.sm/disclosure to request inclusion on our embargo list.
Stay safe, and thank you for helping us keep the ecosystem secure.
Impact
There is no way to mitigate this attack. It is recommended to update to v0.29.0 or newer.
The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions. Typical impact: unauthorized data access or execution of privileged operations.
CVE-2025-66406 has a CVSS score of 5.0 (Medium). The vector is network-reachable, high privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.29.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
In v0.29.0, the token validation logic was strengthened to bind each token to a specific SSH certificate serial number.
Frequently Asked Questions
- What is CVE-2025-66406? CVE-2025-66406 is a medium-severity incorrect authorization vulnerability in github.com/smallstep/certificates (go), affecting versions <= 0.28.4. It is fixed in 0.29.0. The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions.
- How severe is CVE-2025-66406? CVE-2025-66406 has a CVSS score of 5.0 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/smallstep/certificates are affected by CVE-2025-66406? github.com/smallstep/certificates (go) versions <= 0.28.4 is affected.
- Is there a fix for CVE-2025-66406? Yes. CVE-2025-66406 is fixed in 0.29.0. Upgrade to this version or later.
- Is CVE-2025-66406 exploitable, and should I be worried? Whether CVE-2025-66406 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-66406 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-66406? Upgrade
github.com/smallstep/certificatesto 0.29.0 or later.