Lease, Renew, and Revoke
With every dynamic secret and
service type authentication token, Vault
creates a lease: metadata containing information such as a time duration,
renewability, and more. Vault promises that the data will be valid for the
given duration, or Time To Live (TTL). Once the lease is expired, Vault can
automatically revoke the data, and the consumer of the secret can no longer be
certain that it is valid.
The benefit should be clear: consumers of secrets need to check in with Vault routinely to either renew the lease (if allowed) or request a replacement secret. This makes the Vault audit logs more valuable and also makes key rolling a lot easier.
All dynamic secrets in Vault are required to have a lease. Even if the data is meant to be valid for eternity, a lease is required to force the consumer to check in routinely.
In addition to renewals, a lease can be revoked. When a lease is revoked, it invalidates that secret immediately and prevents any further renewals. For example, with the AWS secrets engine, the access keys will be deleted from AWS the moment a lease is revoked. This renders the access keys invalid from that point forward.
Revocation can happen manually via the API, via the
vault revoke cli command,
or automatically by Vault. When a lease is expired, Vault will automatically
revoke that lease. When a token is revoked, Vault will revoke all leases that
were created using that token.
Note: The Key/Value Backend which stores arbitrary secrets does not issue leases although it will sometimes return a lease duration; see the documentation for more information.
When reading a dynamic secret, such as via
vault read, Vault always returns a
lease_id. This is the ID used with commands such as
vault renew and
vault revoke to manage the lease of the secret.
Lease Durations and Renewal
Along with the lease ID, a lease duration can be read. The lease duration is a Time To Live value: the time in seconds for which the lease is valid. A consumer of this secret must renew the lease within that time.
When renewing the lease, the user can request a specific amount of time they
want remaining on the lease, termed the
increment. This is not an increment
at the end of the current TTL; it is an increment from the current time. For
vault lease renew -increment=3600 my-lease-id would request that the TTL of the lease
be adjusted to 1 hour (3600 seconds). Having the increment be rooted at the
current time instead of the end of the lease makes it easy for users to reduce
the length of leases if they don't actually need credentials for the full
possible lease period, allowing those credentials to expire sooner and
resources to be cleaned up earlier.
The requested increment is completely advisory. The backend in charge of the secret can choose to completely ignore it. For most secrets, the backend does its best to respect the increment, but often limits it to ensure renewals every so often.
As a result, the return value of renewals should be carefully inspected to determine what the new lease is.
In addition to revoking a single secret, operators with proper access control can revoke multiple secrets based on their lease ID prefix.
Lease IDs are structured in a way that their prefix is always the path where
the secret was requested from. This lets you revoke trees of secrets. For
example, to revoke all AWS access keys, you can do
vault revoke -prefix aws/.
This is very useful if there is an intrusion within a specific system: all secrets of a specific backend or a certain configured backend can be revoked quickly and easily.