CVE-2026-40097 Step CA affected by an index out of bounds panic in TPM attestation EKU validation
An attacker can cause a denial of service in Step CA by sending a crafted TPM attestation certificate with an empty EKU extension, triggering a panic.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-40097?
CVE-2026-40097 is a low-severity denial of service vulnerability in smallstep certificates, published on 10 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-129
Which versions are affected by CVE-2026-40097?
3 open source packages ship an affected version, including github.com/smallstep/certificates. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/smallstep/certificates Go | From 0.24.0 | 0.30.0 |
github.com/smallstep/certificates Go | From 0.24.0 | 0.30.0 |
github.com/smallstep/certificates Go | From 0.24.0 | 0.30.0 |
Products include Smallstep Step-Ca.
How do I fix CVE-2026-40097?
Upgrade github.com/smallstep/certificates to 0.30.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-40097 being exploited?
There is no known exploitation of CVE-2026-40097 in the wild and no public exploit code.
Other vulnerabilities in smallstep certificates
Sources
Also known as CLEANSTART-2026-LF51380, CLEANSTART-2026-XI87503, GHSA-9qq8-cgcv-qmc9, GO-2026-5297, MINI-c5wr-7mfx-jwmr, MINI-j347-365p-qh3g.