CVE-2026-40097

An attacker can trigger an index out-of-bounds panic in Step CA by sending a crafted attestation key (AK) certificate with an empty Extended Key Usage (EKU) extension during TPM device attestation.

Details

When processing a device-attest-01 ACME challenge using TPM attestation, Step CA validates that the AK certificate contains the tcg-kp-AIKCertificate Extended Key Usage OID. During this validation, the EKU extension value is decoded from its ASN.1 representation and the first element is checked. A crafted certificate could include an EKU extension that decodes to an empty sequence, causing the code to panic when accessing the first element of the empty slice.

This vulnerability is only reachable when a device-attest-01 ACME challenge with TPM attestation is configured. Deployments not using TPM device attestation are not affected.

Mitigations

If you are unable to upgrade to v0.30.0 or newer, the attack can be mitigated by disabling or removing any ACME provisioners that use TPM device attestation (device-attest-01).

Acknowledgements

This issue was identified and reported by @1seal (Oleh Konko).

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.

CVE-2026-40097 has a CVSS score of 3.7 (Low). The vector is network-reachable, no 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 (0.30.0); upgrading removes the vulnerable code path.

Kodem intelligence

In v0.30.0, a bounds check was added to validateAKCertificateExtendedKeyUsage so that an empty EKU sequence is treated as a validation failure rather than causing a panic.

1. What is CVE-2026-40097? CVE-2026-40097 is a low-severity security vulnerability in github.com/smallstep/certificates (go), affecting versions >= 0.24.0, < 0.30.0. It is fixed in 0.30.0.
2. How severe is CVE-2026-40097? CVE-2026-40097 has a CVSS score of 3.7 (Low). 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.
3. Which versions of github.com/smallstep/certificates are affected by CVE-2026-40097? github.com/smallstep/certificates (go) versions >= 0.24.0, < 0.30.0 is affected.
4. Is there a fix for CVE-2026-40097? Yes. CVE-2026-40097 is fixed in 0.30.0. Upgrade to this version or later.
5. Is CVE-2026-40097 exploitable, and should I be worried? Whether CVE-2026-40097 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
6. What actually determines whether CVE-2026-40097 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.
7. How do I fix CVE-2026-40097? Upgrade github.com/smallstep/certificates to 0.30.0 or later.