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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

3.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-40097: 0.18% estimated exploitation activity in the next 30 days; percentile 7.73 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

10 Apr 2026, 20:18 UTC

Last modified by the source

10 Apr 2026, 20:18 UTC

Source last checked

26 Oct 2026, 15:19 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-9qq8-cgcv-qmc9

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# Patch Next advisory brief: CVE-2026-40097

Advisory: Step CA affected by an index out of bounds panic in TPM attestation EKU validation
IDs: CVE-2026-40097, GHSA-9qq8-cgcv-qmc9
Record: https://patchnext.com/advisories/cve-2026-40097 (revision 1, source checked 26 Oct 2026, 15:19 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/smallstep/certificates 0.30.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/smallstep/certificates: affected >= 0.24.0, < 0.30.0; fixed in 0.30.0

## Suggested inspection (read-only)
- Go github.com/smallstep/certificates: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/smallstep/certificates.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-9qq8-cgcv-qmc9 (GitHub-reviewed advisory GHSA-9qq8-cgcv-qmc9 lists affected ranges: Go github.com/smallstep/certificates >=0.24.0 <0.30.0)

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