🛡️ CVE-2026-40097 — step-ca

🟢 CVSS 3.7 — Low ✅ No Known Exploit CWE-129 NVD
3.7
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

Step CA affected by an index out of bounds panic in TPM attestation EKU validation

# Summary

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).

Fix

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.

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.

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is high, an attacker needs no privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality none, integrity none, availability low.

Weakness class

CVE-2026-40097 is classified as CWE-129: Improper Validation of Array Index. An index derived from input is used without a range check, so the access lands outside the array.

Affected software

CVE-2026-40097 is recorded against 2 packages.

  • github.com/smallstep/certificates
  • step-ca

Timeline and source

Published on 10 April 2026 and last revised on 25 June 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from NVD.

References

github.com (Web)
nvd.nist.gov (Advisory)
github.com (Web)
github.com (Web)
github.com (Package)
github.com (Web)

Details

Severity LOW
CVSS Score 3.7
CVSS Vector CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
CWE CWE-129
Public Exploit ✅ No
Source NVD
Published 2026-04-10
Updated 2026-08-12
Modified 2026-06-25

Affected Packages

Software From version Fixed in
github.com/smallstep/certificates — —
step-ca — —

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