Sandstorm is a security product.

We like to say that Sandstorm's priorities are Usability, Security, and Freedom. In public, we tend to talk more about usability and freedom, because those are the priorities users are most excited about. However, within the team, we are just as passionate -- if not more so -- about security.

Ultimately, our goal is that, to the maximum extent possible, users need not worry about security, because using the system intuitively will result in the desired security properties "by default". Moreover, we aim to allow network administrators to be able to say: "As long as it's on Sandstorm, you can run whatever apps you want, because we trust Sandstorm to keep things secure."

Threat models

Sandstorm defends against a wide variety of threats; too many to list here. However, as a platform for apps, most of our energy goes into the following:

Mitigation of app bugs

Sandstorm's most important security goal is to ensure that security bugs in applications are contained and mitigated to the maximum extent possible. For example:

• A buggy app should not allow an attacker to compromise the rest of the system or network.
• A buggy app should not be able to grant attackers access even to itself.
• A buggy app should not be able to expose private data to the internet.

Obviously, Sandstorm cannot defend against every possible app bug, especially when apps need access to sensitive resources in order to function. However, security is about risk management, and there is much that Sandstorm can and does do to greatly reduce the user's or the network admin's overall risk.

Defense against malicious apps

Sandstorm does not just aim to defend against buggy apps, but also actively malicious apps. It is our goal that a user should be able to install and run arbitrary applications from arbitrary authors without serious consequence. This is important because Sandstorm is aimed at allowing non-technical users to administer their own server. Inevitably, such users will install malware.

Again, obviously, Sandstorm cannot prevent an app from misusing permissions that a user has explicitly granted to it. But, making an app explicitly request such permissions makes it much easier for users to understand what is happening and defend themselves.

Defense against surveillance and profiling

In the world of Software-as-a-Service, it is common practice for web apps to collect information about individual users, commonly for the purpose of building advertising profiles. We at Sandstorm feel that any such collection is only ethical with the user's full knowledge and consent. Unfortunately, in practice, profiling usually happens behind the user's back.

Sandstorm aims to prevent apps from engaging in covert surveillance while allowing statistics gathering when the user consents to it.

Case Studies

By our analysis, Sandstorm automatically protected users from over 95% of the publicly disclosed security vulnerabilities discovered in apps on the Sandstorm app market, before the vulnerabilities were even disclosed. We also mitigated most Linux kernel security issues. See Security non-events for examples of security problems which were mitigated by Sandstorm.

Strategies

Sandstorm's primary overarching security strategies are as follows.

Platform-level authentication

Sandstorm implements authentication at the platform level, so that you log into the platform once rather than to each application separately. When you open an app, the platform informs the app of your already-authenticated identity. This means that applications themselves never handle sensitive authentication credentials like passwords, which greatly reduces the damage possible if an application's database is compromised.

More generally, Sandstorm aims to ensure that applications never store sensitive secrets at all. For example, we plan to implement an outgoing OAuth proxy such that applications do not directly engage in OAuth requests but rather request the platform do so on their behalf. Thus, the platform is able to store OAuth tokens securely.

Fine-grained isolation

Sandstorm implements fine-grained containers. This means that Sandstorm does not just isolate apps from each other, but isolates individual resources within an app. For example, with Etherpad (a document editor), Sandstorm creates a new Etherpad instance in its own isolated container for every Etherpad document you create.

Fine-grained isolation allows Sandstorm to implement access control at the container level. When you share an Etherpad document, you are telling Sandstorm who should have access, not Etherpad. Thus, no bug in Etherpad can allow someone to get access to a document to which they should not have.

It's important to note that Sandstorm can only truly enforce a binary has access / does not have access. Permission levels like read vs. write are app-dependent and thus can only be implemented by the app. To that end, when a user connects, Sandstorm computes (via the sharing model) which permissions the user has, and then asks the app to enforce those permissions on the user's session. That is, when the app receives requests, those requests are annotated with information like "this user has read permission but not write". The app can then enforce said permissions without ever needing to track any information about specific users.

True Confinement

A Sandstorm app, by default, is totally isolated from the network. It cannot connect to anyone; it can only receive proxied HTTP requests from the user. Thus, by default, an app cannot "phone home" to its developers' servers, and cannot build an advertising profile on you, unless you give it permission to do so.

It's worth noting that it may be possible for a malicious app to leak small amounts of information through "covert channels". For example, if the developer is able to run another app on the same server where the user's instance of the app is running (perhaps because both are running in a shared hosting environment), then the two app instances may be able to communicate by varying their CPU usage and observing the timing changes caused by those variations. Sandstorm ultimately cannot prevent these kinds of attacks. However, since covert channels are very obviously malicious, any developer caught using one would risk serious PR and possibly legal consequences, which should hopefully deter any large company from doing such a thing. Moreover, covert channels are usually very limited in bandwidth. Sandstorm ensures that it is not possible to bootstrap a normal communications channel by leaking plain bits -- in technical terms, capabilities in Sandstorm are never just bits, and therefore you cannot leak capabilities via covert channels.

Notice: As of this writing (April 2021), Sandstorm retains limited incoming and outgoing SMTP to allow email clients to work. This will be removed once email support via the Powerbox is implemented. Additionally, there are limitations to our client-side sandboxing at this time. See the related section on this document for details.

Capability-based Usable Security

Sandstorm employs capability-based security in order to make security usable.

Security without usability is, after all, trivial: just disconnect your server from the network. Now it's secure, but useless. The real challenge in security is making sure it does not get in way of getting work done.

Since Sandstorm isolates and confines apps by default, we need a way to allow the user to connect apps to each other easily and securely. Capability-based security helps enable this by representing permissions as "capabilities", objects which the user may pass around between apps. A capability both identifies a resource (like an address) and grants its bearer permission to use that resource.

The advantage of capability-based security is that it effectively infers security from a separate action that the user had to do anyway. Consider a traditional system based on access control lists (ACLs). Normally, there are two steps required for a user to connect app A to app B.

1. The user tells app A how to find app B, for example by specifying app B's hostname.
2. The user edits app B's access control list to indicate that app A has access.

Capability-based security combines these two into one step:

1. The user gives app A a capability to app B.

Notice that this one step is something the user would need to do even if there were no security. App A always needs to be told which app B to talk to.

The Powerbox

Sandstorm uses capability-based security at every level of the platform. All intra-system communications are performed using Cap'n Proto, a capability-based transport network protocol. Sandstorm capabilities are literally Cap'n Proto capabilities in implementation.

At the user interface level, the user interacts with capabilities through "the powerbox". The basic functioning of the powerbox is as follows:

1. As the user installs apps, each app tells the platform about what kinds of APIs (Cap'n Proto interfaces) it implements.
2. At some point, an app that the user is using makes a request to the platform saying "I need a capability implementing interface Foo".
3. The platform renders a picker UI to the user, where the user can choose from among all their apps that implement API Foo.
4. The user chooses the app they want to satisfy the request.
5. The platform grants the requesting app a capability to the chosen API.

Thus, we've implemented a "service discovery" mechanism that is user-friendly and automatically handles security.

The Powerbox is an excellent example of the intersection between Sandstorm's promises of Usability, Security, and Freedom:

• Usability: The user does not need to understand concepts like IP addresses, hostnames, access credentials, etc. in order to connect two apps. The choice is presented in a way that non-technical users can grasp.

• Security: The platform can automatically infer the proper security settings from the user's choice: obviously, the user wants the requesting app to have permission to access the target they chose, and no others.

• Freedom: An app can never request permission to a specific other app, but can only ask for something implementing the desired API. The user can always substitute any compatible app. This helps to prevent vendor lock-in, where all apps from a vendor integrate only with each other and fail to give the user any ability to swap out one app for a third party app.

Tactics

At a lower level, here are some of the techniques Sandstorm uses to provide security.

Server Sandboxing

Every grain (fine-grained application instance) runs, on the server side, inside a secure sandbox. The sandbox is based on the same Linux kernel namespacing features commonly used to implement containers. However, unlike most container implementations, Sandstorm implements various measures to reduce the kernel's "attack surface". That is, Sandstorm disables many kernel APIs that apps don't need, in order to mitigate any security vulnerabilities found in those APIs. For example:

• We use seccomp-bpf to disable many exotic system calls, especially ones which have seen a lot of vulnerabilities in the past. For example, we do not allow apps to create new UID namespaces -- the source of a large number of recent kernel vulnerabilities.

• We do not mount /proc or /sys filesystems.

• The only devices exposed are /dev/zero, /dev/null, and /dev/urandom (with /dev/random symlinked to urandom, as it should be).

Over time, Sandstorm plans to disable more and more system calls by moving implementations to userspace, but even with the filter we already have, Sandstorm has avoided dozens of kernel vulnerabilities over the last few months.

Sandstorm's server container maps the app's package (libraries, assets, etc.) read-only, and maps the per-grain writable storage at /var. Thus, apps are stateful, yet assets can be shared between many instances of the app.

An app's only communication to the outside world is done through a single Cap'n Proto socket, inherited by the app's root process. For example, HTTP requests are delivered to the app as Cap'n Proto RPCs. The app may employ shims (such as sandstorm-http-bridge) to bridge between Cap'n Proto and traditional protocols, e.g. to be able to use a traditional HTTP server without modification. By using a capability-based schema-driven protocol, it is easy to review exactly what an app can do for security purposes. See for example Sandstorm's web-session.capnp which defines HTTP over Cap'n Proto: notice that this is far more readable than the HTTP specification.

Client Sandboxing

On the client side, Sandstorm isolates apps by requiring every app to run on a separate, randomly-generated hostname. Because of this, Sandstorm requires a wildcard host.

Sandstorm not only hosts each grain at a separate origin, but actually creates a new origin for every session. That is, every time a user opens a document, it is hosted at a new one-off cryptographically-random hostname which expires shortly after the document is closed.

Randomized, unguessable hostnames can help mitigate certain common security bugs in apps, such as XSRF, reflected XSS, and clickjacking attacks. All of these attacks involve an attacker tricking the user's browser into performing actions on another site using the user's credentials. But for any of these attacks to work, the attacker must know the address to attack. If every user gets a different hostname, and indeed the hostnames change frequently, then it is much harder to launch these kinds of attacks.

Note that because DNS requests are made in cleartext, random hostnames will not defend against an attacker who has the ability to snoop network traffic coming from the user's machine. Therefore, apps should still implement their own defenses against these attacks as they always have. But, when a bug slips through (as they commonly do), randomized hostnames make an attack much, much more difficult to pull off, which is still a big win.

Sandstorm will soon employ the Content-Security-Policy header to prevent an app from communicating with other origins without permission, in order to implement full confinement. As of this writing (April 2021), this has been partially implemented behind a configuration option, but is pending wider rollout on ensuring existing apps are updated to be compatible with this change. At present, remote media and images are permitted, but will be controlled via a permission prompt in the future. Unfortunately, it isn't presently possible to block WebRTC via Content Security Policy, but we are working with the standards process.