151 lines
8.5 KiB
Markdown
151 lines
8.5 KiB
Markdown
---
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layout: "docs"
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page_title: "Security Model"
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sidebar_current: "docs-internals-security"
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description: |-
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Learn about the security model of Vault.
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---
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# Security Model
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Due to the nature of Vault and the confidentiality of data it is managing,
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the Vault security model is very critical. The overall goal of Vault's security
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model is to provide [confidentiality, integrity, availability, accountability,
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authentication](http://en.wikipedia.org/wiki/Information_security).
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This means that data at rest and in transit must be secure from eavesdropping
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or tampering. Clients must be appropriately authenticated and authorized
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to access data or modify policy. All interactions must be auditable and traced
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uniquely back to the origin entity. The system must be robust against intentional
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attempts to bypass any of its access controls.
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# Threat Model
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The following are the various parts of the Vault threat model:
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* Eavesdropping on any Vault communication. Client communication with Vault
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should be secure from eavesdropping as well as communication from Vault to
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its storage backend.
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* Tampering with data at rest or in transit. Any tampering should be detectable
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and cause Vault to abort processing of the transaction.
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* Access to data or controls without authentication or authorization. All requests
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must be proceeded by the applicable security policies.
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* Access to data or controls without accountability. If audit logging
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is enabled, requests and responses must be logged before the client receives
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any secret material.
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* Confidentiality of stored secrets. Any data that leaves Vault to rest in the
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storage backend must be safe from eavesdropping. In practice, this means all
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data at rest must be encrypted.
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* Availability of secret material in the face of failure. Vault supports
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running in a highly available configuration to avoid loss of availability.
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The following are not parts of the Vault threat model:
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* Protecting against arbitrary control of the storage backend. An attacker
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that can perform arbitrary operations against the storage backend can
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undermine in any number of ways that are difficult or impossible to protect
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against. As an example, an attacker could delete or corrupt all the contents
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of the storage backend causing total data loss for Vault. The ability to controls
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reads would allow an attacker to snapshot in a well-known state and rollback state
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changes if that would be beneficial to them.
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* Protecting against the leakage of the existance of secret material. An attacker
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that can read from the storage backend may observe that secret material exists
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and is stored, even if it is kept confidential.
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* Protecting against memory analysis of a running Vault. If an attacker is able
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to inspect the memory state of a running Vault instance then the confidentiality
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of data may be compromised.
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# External Threat Overview
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Given the architecture of Vault, there are 3 distinct systems we are concerned with
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for Vault. There is the client, which is speaking to Vault over an API. There is Vault
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or the server more accurately, which is providing an API and serving requests. Lastly,
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there is the storage backend, which the server is utilizing to read and write data.
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There is no mutual trust between the Vault client and server. Clients use
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[TLS](http://en.wikipedia.org/wiki/Transport_Layer_Security) to verify the identity
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of the server and to establish a secure communication channel. Servers require that
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a client provides a client token for every request which is used to identify the client.
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A client that does not provide their token is only permitted to make login requests.
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The storage backends used by Vault are also untrusted by design. Vault uses a security
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barrier for all requests made to the backend. The security barrier automatically encrypts
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all data leaving Vault using the [Advanced Encryption Standard (AES)](http://en.wikipedia.org/wiki/Advanced_Encryption_Standard)
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cipher in the [Galois Counter Mode (GCM)](http://en.wikipedia.org/wiki/Galois/Counter_Mode).
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The nonce is randomly generated for every encrypted object. When data is read from the
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security barrier the GCM authentication tag is verified prior to decryption to detect
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any tampering.
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Depending on the backend used, Vault may communicate with the backend over TLS
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to provide an added layer of security. In some cases, such as a file backend this
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is not applicable. Because storage backends are untrusted, an eavesdropper would
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only gain access to encrypted data even if communication with the backend was intercepted.
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# Internal Threat Overview
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Within the Vault system, a critical security concern is an attacker attempting
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to gain access to secret material they are not authorized to. This is an internal
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threat if the attacker is already permitted some level of access to Vault and is
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able to authenticate.
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When a client first authenticates with Vault, a credential backend is used to
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verify the identity of the client and to return a list of associated ACL policies.
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This association is configured by operators of Vault ahead of time. For example,
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GitHub users in the "engineering" team may be mapped to the "engineering" and "ops"
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Vault policies. Vault then generates a client token which is a randomly generated
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UUID and maps it to the policy list. This client token is then returned to the client.
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On each request a client provides this token. Vault then uses it to check that the token
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is valid and has not been revoked or expired, and generates an ACL based on the associated
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policies. Vault uses a strict default deny or whitelist enforcement. This means unless
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an associated policy allows for a given action, it will be denied. Each policy specifies
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a level of access granted to a path in Vault. When the policies are merged (if multiple
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policies are associated with a client), the highest access level permitted is used.
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For example, if the "engineering" policy permits read/write access to the "eng/" path,
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and the "ops" policy permits read access to the "ops/" path, then the user gets the
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union of those. Policy is matched using a longest-prefix match, which is the most
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specific definied policy.
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Certain operations are only permitted by "root" users, which is a distinguished
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policy built into Vault. This is similar to the concept of a root user on a Unix system
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or an Administrator on Windows. Although clients could be provided with root tokens
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or associated with the root policy, instead Vault supports the notion of "sudo" privilege.
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As part of a policy, users may be granted "sudo" privileges to certain paths, so that
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they can still perform security sensitive operations without being granted global
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root access to Vault.
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Lastly, Vault supports using a [Two-man rule](http://en.wikipedia.org/wiki/Two-man_rule) for
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unsealing using [Shamir's Secret Sharing technique](http://en.wikipedia.org/wiki/Shamir's_Secret_Sharing).
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When Vault is started, it starts in an _sealed_ state. This means that the encryption key
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needed to read and write from the storage backend is not yet known. The process of unsealing
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requires providing the master key so that the encryption key can be retrieved. The risk of distributing
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the master key is that a single malicious actor with access to it can decrypt the entire
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Vault. Instead, Shamir's technique allows us to split the master key into multiple shares or parts.
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The number of shares and the threshold needed is configurable, but by default Vault generates
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5 shares, any 3 of which must be provided to reconstruct the master key.
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By using a secret sharing technique, we avoid the need to place absolute trust in the holder
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of the master key, and avoid storing the master key at all. The master key is only
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retrievable by reconstructing the shares. The shares are not useful for making any requests
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to Vault, and can only be used for unsealing. Once unsealed the standard ACL mechanisms
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are used for all requests.
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To make an analogy, a bank puts security deposit boxes inside of a vault.
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Each security deposit box has a key, while the vault door has both a combination and a key.
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The vault is encased in steel and concrete so that the door is the only practical entrance.
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The analogy to Vault, is that the cryptosystem is the steel and concrete protecting the data.
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While you could tunnel through the concrete or brute force the encryption keys, it would be
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prohibitively time consuming. Opening the bank vault requires two-factors: the key and the combination.
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Similarly, Vault requires multiple shares be provided to reconstruct the master key.
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Once unsealed, each security deposit boxes still requires the owner provide a key, and similarly
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the Vault ACL system protects all the secrets stored.
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