CVE-2026-40097: smallstep certificates Improper Validation of Array Index

Vulnerability summary

CVE-2026-40097 is a low (CVSS 3.7) Improper Validation of Array Index vulnerability in smallstep certificates. EPSS estimates a 0.28% chance of exploitation in the next 30 days, higher than 19.6% of scored vulnerabilities.

Timeline

1. 10-04-2026
   PublishedVulnerability first disclosed.
2. 10-04-2026
   Last modifiedVulnerability information updated.

Description

Step CA is an online certificate authority for secure, automated certificate management for DevOps. From 0.24.0 to before 0.30.0-rc3, 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. 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. This vulnerability is fixed in 0.30.0-rc3.

• CWE-129Improper Validation of Array IndexThe product uses untrusted input when calculating or using an array index, but the product does not validate or incorrectly validates the index to ensure the index references a valid position within the array.