GHSA-9qq8-cgcv-qmc9 — Step CA affected by an index out of bounds panic in TPM attestation EKU validation

Affected: Go: github.com/smallstep/certificates

# 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

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GHSA-9qq8-cgcv-qmc9 — Step CA affected by an index out of bounds panic in TPM attestation EKU validation