CVE-2025-66406 — certificates
MEDIUMCVE-2025-66406 is a medium-severity (CVSS 5) CWE-863 vulnerability in github.com/smallstep/certificates. A fix is available for github.com/smallstep/certificates — see the affected versions and patch details below.
Improper Authorization Check for SSH Certificate Revocation
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-66406.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/smallstep/certificatesReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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.
Impact
There is no way to mitigate this attack. It is recommended to update to v0.29.0 or newer.
Fix
In v0.29.0, the token validation logic was strengthened to bind each token to a specific SSH certificate serial number.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/smallstep/certificates | all versions | 0.29.0go get github.com/smallstep/certificates@v0.29.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/smallstep/certificates, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/smallstep/certificates to 0.29.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-66406 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.
Frequently Asked Questions
Is CVE-2025-66406 in your dependencies?
Find it across Go, including transitive dependencies.