CVE-2026-40097: CVE-2026-40097: Index Out-of-Bounds Panic in Step CA TPM Attestation

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CVE-2026-40097: Index Out-of-Bounds Panic in Step CA TPM Attestation

Vulnerability ID: CVE-2026-40097
CVSS Score: 3.7
Published: 2026-04-10

Step CA versions prior to 0.30.0-rc3 contain a vulnerability (CWE-129) where processing a malformed TPM Attestation Key certificate results in a Go runtime panic. This flaw causes a Denial of Service condition when the device-attest-01 ACME challenge is enabled and triggered by an unauthenticated attacker.

TL;DR

A missing bounds check in Step CA's TPM attestation logic allows an attacker to crash the application via an empty Extended Key Usage (EKU) ASN.1 sequence.

Technical Details

CWE ID: CWE-129
Attack Vector: Network
CVSS Score: 3.7
EPSS Score: 0.0003
Impact: Denial of Service (DoS)
Exploit Status: None
KEV Status: Not Listed

Affected Systems

smallstep/certificates (Step CA)
smallstep/certificates: >= 0.24.0, < 0.30.0-rc3 (Fixed in: 0.30.0-rc3)

Code Analysis

Commit: ffd31ac

Fix index out of bounds panic in validateAKCertificateExtendedKeyUsage

@@ -1250,7 +1250,7 @@ func validateAKCertificateExtendedKeyUsage(c *x509.Certificate) error {
    )
    for _, ext := range c.Extensions {
        if ext.Id.Equal(oidExtensionExtendedKeyUsage) {
-           if _, err := asn1.Unmarshal(ext.Value, &ekus); err != nil || !ekus[0].Equal(oidTCGKpAIKCertificate) {
+           if _, err := asn1.Unmarshal(ext.Value, &ekus); err != nil || len(ekus) == 0 || !ekus[0].Equal(oidTCGKpAIKCertificate) {
                return errors.New("AK certificate is missing Extended Key Usage value tcg-kp-AIKCertificate (2.23.133.8.3)")
            }
            valid = true

Mitigation Strategies

Upgrade the smallstep/certificates package to version 0.30.0 or greater.
Disable the device-attest-01 ACME challenge in Step CA configuration if TPM attestation is not required.

Remediation Steps:

Review Step CA configurations to determine if the device-attest-01 challenge is active.
Plan a maintenance window to update the Step CA binary.
Download version 0.30.0 or the latest stable release from the official smallstep GitHub repository.
Deploy the updated binary and restart the CA service.
Verify service stability by monitoring application logs for unexpected Go runtime panics.

References

Read the full report for CVE-2026-40097 on our website for more details including interactive diagrams and full exploit analysis.

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