Response wrapping

Overview

In many Vault deployments, clients can access Vault directly and consume returned secrets. In other situations, it may make sense to or be desired to separate privileges such that one trusted entity is responsible for interacting with most of the Vault API and passing secrets to the end consumer.

However, the more relays a secret travels through, the more possibilities for accidental disclosure, especially if the secret is being transmitted in plaintext. For instance, you may wish to get a TLS private key to a machine that has been cold-booted, but since you do not want to store a decryption key in persistent storage, you cannot encrypt this key in transit.

To help address this problem, Vault includes a feature called response wrapping. When requested, Vault can take the response it would have sent to an HTTP client and instead insert it into the cubbyhole of a single-use token, returning that single-use token instead.

Logically speaking, the response is wrapped by the token, and retrieving it requires an unwrap operation against this token. Functionally speaking, the token provides authorization to use an encryption key from Vault's keyring to decrypt the data.

This provides a powerful mechanism for information sharing in many environments. In the types of scenarios, described above, often the best practical option is to provide cover for the secret information, be able to detect malfeasance (interception, tampering), and limit lifetime of the secret's exposure. Response wrapping performs all three of these duties:

• It provides cover by ensuring that the value being transmitted across the wire is not the actual secret but a reference to such a secret, namely the response-wrapping token. Information stored in logs or captured along the way do not directly see the sensitive information.

• It provides malfeasance detection by ensuring that only a single party can ever unwrap the token and see what's inside. A client receiving a token that cannot be unwrapped can trigger an immediate security incident. In addition, a client can inspect a given token before unwrapping to ensure that its origin is from the expected location in Vault.

• It limits the lifetime of secret exposure because the response-wrapping token has a lifetime that is separate from the wrapped secret (and often can be much shorter), so if a client fails to come up and unwrap the token, the token can expire very quickly.

Response-Wrapping tokens

When a response is wrapped, the normal API response from Vault does not contain the original secret, but rather contains a set of information related to the response-wrapping token:

• TTL: The TTL of the response-wrapping token itself

• Token: The actual token value

• Creation Time: The time that the response-wrapping token was created

• Creation Path: The API path that was called in the original request

• Wrapped Accessor: If the wrapped response is an authentication response containing a Vault token, this is the value of the wrapped token's accessor. This is useful for orchestration systems (such as Nomad) to be able to control the lifetime of secrets based on their knowledge of the lifetime of jobs, without having to actually unwrap the response-wrapping token or gain knowledge of the token ID inside.

Vault currently does not provide signed response-wrapping tokens, as it provides little extra protection. If you are being pointed to the correct Vault server, token validation is performed by interacting with the server itself; a signed token does not remove the need to validate the token with the server, since the token is not carrying data but merely an access mechanism and the server will not release data without validating it. If you are being attacked and pointed to the wrong Vault server, the same attacker could trivially give you the wrong signing public key that corresponds to the wrong Vault server. You could cache a previously valid key, but could also cache a previously valid address (and in most cases the Vault address will not change or will be set via a service discovery mechanism). As such, we rely on the fact that the token itself is not carrying authoritative data and do not sign it.

Response-Wrapping token operations

Via the sys/wrapping path, several operations can be run against wrapping tokens:

• Lookup (sys/wrapping/lookup): This allows fetching the response-wrapping token's creation time, creation path, and TTL. This path is unauthenticated and available to response-wrapping tokens themselves. In other words, a response-wrapping token holder wishing to perform validation is always allowed to look up the properties of the token.

• Unwrap (sys/wrapping/unwrap): Unwrap the token, returning the response inside. The response that is returned will be the original wire-format response; it can be used directly with API clients.

• Rewrap (sys/wrapping/rewrap): Allows migrating the wrapped data to a new response-wrapping token. This can be useful for long-lived secrets. For example, an organization may wish (or be required in a compliance scenario) to have the pki backend's root CA key be returned in a long-lived response-wrapping token to ensure that nobody has seen the key (easily verified by performing lookups on the response-wrapping token) but available for signing CRLs in case they ever accidentally change or lose the pki mount. Often, compliance schemes require periodic rotation of secrets, so this helps achieve that compliance goal without actually exposing what's inside.

• Wrap (sys/wrapping/wrap): A helper endpoint that echoes back the data sent to it in a response-wrapping token. Note that blocking access to this endpoint does not remove the ability for arbitrary data to be wrapped, as it can be done elsewhere in Vault.

Response-Wrapping token creation

Response wrapping is per-request and is triggered by providing to Vault the desired TTL for a response-wrapping token for that request. This is set by the client using the X-Vault-Wrap-TTL header and can be either an integer number of seconds or a string duration of seconds (15s), minutes (20m), or hours (25h). When using the Vault CLI, you can set this via the -wrap-ttl parameter. When using the Go API, wrapping is triggered by setting a helper function that tells the API the conditions under which to request wrapping, by mapping an operation and path to a desired TTL.

If a client requests wrapping:

1. The original HTTP response is serialized

2. A new single-use token is generated with the TTL supplied by the client

3. Internally, the original serialized response is stored in the single-use token's cubbyhole

4. A new response is generated, with the token ID, TTL, and path stored in the new response's wrap information object

5. The new response is returned to the caller

Note that policies can control minimum/maximum wrapping TTLs; see the policies concepts page for more information.

Response-Wrapping token validation

Proper validation of response-wrapping tokens is essential to ensure that any malfeasance is detected. It's also pretty straightforward.

Validation is best performed by the following steps:

1. If a client has been expecting delivery of a response-wrapping token and none arrives, this may be due to an attacker intercepting the token and then preventing it from traveling further. This should cause an alert to trigger an immediate investigation.

2. Perform a lookup on the response-wrapping token. This immediately tells you if the token has already been unwrapped or is expired (or otherwise revoked). If the lookup indicates that a token is invalid, it does not necessarily mean that the data was intercepted (for instance, perhaps the client took a long time to start up and the TTL expired) but should trigger an alert for immediate investigation, likely with the assistance of Vault's audit logs to see if the token really was unwrapped.

3. With the token information in hand, validate that the creation path matches expectations. If you expect to find a TLS key/certificate inside, chances are the path should be something like pki/issue/.... If the path is not what you expect, it is possible that the data contained inside was read and then put into a new response-wrapping token. (This is especially likely if the path starts with cubbyhole or sys/wrapping/wrap.) Particular care should be taken with kv secrets engine: exact matches on the path are best there. For example, if you expect a secret to come from secret/foo and the interceptor provides a token with secret/bar as the path, simply checking for a prefix of secret/ is not enough.

4. After prefix validation, unwrap the token. If the unwrap fails, the response is similar to if the initial lookup fails: trigger an alert for immediate investigation.

Following those steps provides very strong assurance that the data contained within the response-wrapping token has never been seen by anyone other than the intended client and that any interception or tampering has resulted in a security alert.