Affected: Go: github.com/smallstep/certificates

## Summary An authorized attacker can bypass authorization checks and revoke any SSH certificate issued by Step CA by using a valid revocation token.

## Details Step CA users can obtain SSH certificates from a few provisioners. The SSHPOP provisioner allows revocation of the SSH certificate (preventing future certificate renewals) using a token. Due to a missing validity check, this token could be used to revoke any SSH certificate issued by the CA.

To create a token, an attacker must have access to the CA endpoint and a valid SSH certificate, meaning they were already authorized to obtain an SSH certificate. The attacker must also know the serial number of the certificate they want to revoke.

## Impact There is no way to mitigate this attack. It is recommended to update to v0.29.0 or newer.

## Fix In v0.29.0, the token validation logic was strengthened to bind each token to a specific SSH certificate serial number.

## Acknowledgements This issue was identified and reported by Gabriel Departout and Andy Russon, from [AMOSSYS](http://amossys.fr/). This audit was sponsored by [ANSSI](https://cyber.gouv.fr/) (French Cybersecurity Agency) based on [their Open-Source security audit program](https://cyber.gouv.fr/open-source-lanssi#:~:text=Financement%20d%27%C3%A9valuations%20de%20s%C3%A9curit%C3%A9%20de%20logiciels%20libres).

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