How Capabilities Work in Austral

Code is permissionless by default.

Rather, all code within the same address space runs with uniform permissions, despite different modules having different degrees of trustworthiness. If you download a left-pad library, it might implement leftpad, but it might contain malware that hoovers up your home directory and sends it to a server somewhere. This is called a supply chain attack.

And there is nothing in the semantics of most programming languages that lets you prevent this. Anything can access the foreign function interface (FFI), the filesystem, the network, etc. Even clock access is dangerous, since timing information is useful in carrying out timing attacks.

The transitive closure of dependencies in modern applications is huge. The node_modules directory on a humble React app I have on my laptop is 460 MiB, and that’s just React and ProseMirror. Most applications are a thin layer of business logic sitting atop a huge pile of library code, out of sight and out of mind. Nobody can audit all of it (though LLMs might make a dent here), and malware can be hidden in very subtle edge cases of the semantics of a language.

What’s the solution? The graybeard will blame the programmer, say we need fewer dependencies and that any competent dev can re-write Figma in vanilla.js over a weekend. The HN commenter will blame the programmer for failing to audit the quarter-million lines of code in their dependencies.

But we shouldn’t want to give up dependencies. Leveraging dependencies increases economic productivity, reinventing the wheel (unless you have a good reason) decreases it. Modern programming is O-ring, not Cobb-Douglas. And as I wrote in the Austral intro post, blaming the programmer will change nothing:

If planes were flown like we write code, we’d have daily crashes, of course, but beyond that, the response to every plane crash would be: “only a bad pilot blames their plane! If they’d read subparagraph 71 of section of the C++, er, 737 spec, they’d know that at 13:51 PM on the vernal equinox the wings fall off the plane.”

This doesn’t happen in aviation, because in aviation we have decided, correctly, that human error is an intrinsic and inseparable part of human activity. And so we have built concentric layers of mechanical checks and balances around pilots, to take on part of the load of flying. Because humans are tired, they are burned out, they have limited focus, limited working memory, they are traumatized by writing executable YAML, etc.

Discipline doesn’t fix type errors: type systems do. Discipline doesn’t fix memory leaks and buffer overflows: ownership types do. Similarly, security vulnerabilities will not be fixed by demanding superhuman discipline but by building languages with safer semantics.


  1. The Solution
  2. Capabilities in Austral
    1. Example
    2. Capabilities vs. Values
    3. The Capability Hierarchy
    4. The Root Capability
  3. Limitations
    1. Irrevocability
    2. Global Uniqueness
    3. Unsafe FFI
    4. Summary
  4. Future Work
    1. Auditing
    2. A Stricter Model

The Solution

The solution is capability-based security. A capability is an unforgeable token that grants access to some permissioned resource, like the filesystem or the network or an accurate clock. Anything that should be locked down should require a capability to access.

And, ideally, capabilities should be arbitrarily granular: requiring a capability to access the filesystem as a whole, read and write, removes a good chunk of security vulnerabilities. But we can go further: we can constraint access to a directory and its contents, or to a specific file, or to a specific file in read-only mode, and so on.

Capabilities at the process and operating system level are widely implemented: Capscisum, Fuchsia, pledge, seccomp.

These are typically more coarse-grained than what you can do with language-level support, but they’re easier to implement, because you can implement capability security around a completely untrusted, unaudited codebase, written in any language, runtime, or era.

Language-level capabilities are harder. The language’s semantics have to be designed with capabilities in mind, trying to slap capability-security on a language after the fact is like trying to slap a type system on a dynamically-typed language. It might work, but you will have soundness issues, and you will cope and say it “doesn’t matter in practice”.

The reason it’s hard is, you’d typically represent capabilities as types, and most programming languages don’t give hard guarantees about the provenance of types. In C and C++ you can in principle cast anything into anything. In Python or Common Lisp or other dynamic languages, you can dynamically search for a class by name and instantiate it anywhere. Unsafe operations—precisely what capability-based security is meant to constrain—let you get around that very security.

The E programming language has capability security built in, but you don’t hear much about it.

Capabilities in Austral

Austral “supports” capability-based security. Supports in quotes because it is not a first-class feature: capability security is simply a consequence of linear types (except for the RootCapability—more on this later). Which makes me proud of the language design, since it’s a good sign when useful things naturally fall out of a design by logical necessity.

Capabilities are represented as linear types. For an introduction to linear types in Austral, see the introduction to Austral post, or the post where I explain how the linear type checker works.

Because capabilities are linear, they are not copyable. A piece of code in possession of a capability can destroy it, or give it to someone else, but not send a copy to someone else and keep theirs. And because there is no global mutable state, capabilities cannot surreptitiously be stored in a global variable for another piece of code to acquire them.

Because of Austral’s strict module system and encapsulation, capabilities cannot be created ex nihilo. To create a capability, you must prove that you have access to a higher, more powerful capability. This satisfies all the security properties we want.

And now if a leftpad dependency wants to read your data and send it to a server, it needs a filesystem capability and a network capability. It should be an obvious red flag if a string-manipulation library were to ask for those dependencies. And this makes it hard to hide malware.


The following is a sketch of an API for a capability-secure network socket library:

module Network is
    type NetworkCapability: Linear;

    generic [R: Region]
    function acquire(root: &![RootCapability, R]): NetworkCapability;

    function surrender(netcap: NetworkCapability): Unit;

    type Socket: Linear;

    generic [R: Region]
    function open(
        netcap: &![NetworkCapability, R],
        host: String,
        port: Nat16
    ): Socket;

    function close(socket: Socket): Unit);

    -- ... the rest of the socket API ...
end module.

The Network module exports a NetworkCapability type, and two lifecycle functions: acquire and surrender. The acquire function takes a reference to the RootCapability, which is the equivalent of global God-mode permissions, and returns a NetworkCapability.

The NetworkCapability type is opaque: it is declared, but not defined, in the module API file. Which means it can be imported and mentioned by other modules, but it cannot be constructed by code outside the Network module. Austral is absolutely strict about this. The only way to create a NetworkCapability instance outside this module is via the acquire function.

Similarly, Socket is a linear value that wraps some internal, unsafe socket handle. Since Socket is a linear type, you can also think of it as a capability: having a value of type Socket gives you the capability to read or write from that socket or close it.

Analogously, Socket has two lifecycle functions: open takes a reference to the network capability, a host and a port and returns a Socket (error handling is elided for clarity), and close takes a Socket, closes it, and consumes it.

The way you’d use this is:

-- Assuming we have a variable `root` holding the RootCapability.
let netcap: NetworkCapability := acquire(&!root);
let socket: Socket := open(&!netcap, "", 80);
-- We can surrender the capability immediately after opening the socket, since
-- we don't need it for anything else.
-- Do something with the socket.

Capabilities vs. Values

There isn’t a sharp distinction between capabilities and linear types.

Capabilities can be empty records, holding no values. In that case they are pure type-level permission slips. Typically such capabilities are “broad”: a network capability, or a filesystem capability likely wouldn’t have a pointer to anything.

But linear types that have values—like a linear File type that wraps an unsafe file handle, or a network socket, or a database handle, etc.—can also be thought of as capabilities, especially if the API is designed such that those types can only be constructed by proving you have a capability to use that API.

The Capability Hierarchy

Capabilities form a hierarchy. Again, this hierarchy isn’t an inheritance hierarchy, and it’s not built into the language. It’s a hierarchy that is implicit in the functions that let you create a capability from another, more powerful capability.

For example, in the above code example, the hierarchy looks like this:

Analogously, a capability-security filesystem API with granular permissions might look like this:

With each capability type providing functions to constraint it further, until we get to the leaf nodes like FileAttrsRead (e.g. read a file’s modification time).

The Root Capability

The root capability is the only part of Austral’s capability-security model that is built into the language.

As mentioned above, capabilities cannot be created out of thin air, you have to pass a proof (a reference) that you own a broader, more powerful capability. The root capability is the base case of the recursion: it represents the highest level of permissions.

Values of type RootCapability cannot be created in userspace. The root capability is only available as the first argument of the entrypoint function. Where in C an entrypoint might look like this:

int main(int argc, char** argv) {
    printf("Hello, world!\n");
    return 0;

In Austral the entrypoint function looks like this:

function main(root: RootCapability): ExitCode is
    -- Some code here.
    return ExitSuccess();

The entrypoint is all-powerful. The design pattern here is that the entrypoint should acquire the capabilities that it needs (e.g. filesystem access, network access), then surrender the root, and call some other function with those capabilities:

function main(root: RootCapability): ExitCode is
    -- Acquire some capabilities.
    let netcap: NetworkCapability := acquireNetwork(&!root);
    let fscap: FileSystemCapability := acquireFileSystem(&!root);
    let termcap: TerminalCapability := acquireTerminal(&!root);
    -- Surrender the root.
    -- Pass our capabilities to some other function.
    mainInner(netcap, fscap, termcap);
    -- Finally, exit.
    return ExitSuccess();

But also, a program can immediately surrender the root and acquire no capabilities. With such an entrypoint, you are guaranteed that the program is useless: that it does nothing but warm the CPU and exit.


This section describes the limitations in Austral’s current capability security model.


Capabilites are, by default, irrevocable. Code that owns a capability can surrender it (by consuming the linear value), but there’s no built-in mechanism for the owner of a higher-level capability to revoke it.

This can be implemented, however, using pointers and access control lists. E.g. a File capability might hold an internal, opaque ID and a reference to the FileSystem capability that created it. Then all operations that use File follow that reference and check the File ID against an access control table in the FileSystem capability. This can be implemented and would work, but it has to be implemented explicitly, every time you want to do this, for each capability type. There is no “automatic revoke”.

It also has the drawback that, by holding a reference, the File type is now tied to the lifetime of the FileSystem capability. This may, or may not, be a feature.

Global Uniqueness

In the above example, you can do this:

function main(root: RootCapability): ExitCode is
    -- Acquire the network capability twice.
    let netcap1: NetworkCapability := acquireNetwork(&!root);
    let netcap2: NetworkCapability := acquireNetwork(&!root);
    -- ...

And this is fine: different subsystems might each require a separate network capability, e.g. an HTTP server and a database client.

But what about the terminal? Terminal access should probably be globally unique: otherwise, you could hand a terminal capability to two separate threads, each of which logs its work to the terminal, and the output will appear interleaved in a non-deterministic way. Once you acquire a terminal capability, you shouldn’t be able to do so again until the original capability has been surrendered.

In the current model, the only way to implement the global uniqueness of capabilities something like this:

module Unique is
    type UniqueCapability: Linear;

    function acquire(root: RootCapability): UniqueCapability;

    function surrender(cap: UniqueCapability): RootCapability;
end module.

That is: the UniqueCapability contains the RootCapability, and returns it to the client at the end of its existence. And this is inconvenient, because we can only have one globally unique resource live at any one time.

So this is an area where, at present, the only solution is programmer discipline: the programmer should be informed that some capabilities should only be acquired once for the duration of the program, and they take the responsibility when they fail to do this.

Unsafe FFI

Every programming language needs a escape hatch. When interacting with the outside world, you have to do unsafe things. The question is: can you draw a boundary around unsafe code, so that it is clearly visible and demarcated and splash damage is minimized, and can you help programmers identify and audit the unsafe parts of their code?

Austral’s FFI is unsafe. The only limit to using the FFI, calling foreign functions, and doing unsafe pointer arithmetic is that any module that does this must be marked unsafe. The idea is that we can then separate safe and unsafe modules and thus reduce the auditing burden on end-users.


The model is good—but not good enough. You can still have supply-chain attacks in Austral, the only thing that is reduced is the scope of auditing: rather than having to audit every line of code, you only have to audit unsafe modules, and verify that 1) they don’t do anything nasty and 2) they wrap their unsafe internals in a safe, capability-secure API.

So the improvement in safety is only partial.

Future Work

This section describes different ways in which Austral’s capability-based security might evolve to support greater safety.


One solution is to force the user to audit new packages. When you build your project for the first time, the dependency solver will create a lockfile with the exact version of every package to build against. Initially, all of those as marked as unaudited.

Then the build system makes you go through the unsafe modules of each dependency, you read the code and accept or reject each module. If you build against unaudited packages you get a warning, if everything is audited, you get no warnings.

The pro is that this is tractable: it can be implemented using the existing capability model, using ordinary technology, and only unsafe modules have to be audited for safety, unlike most languages where you’d have to audit every single line.

The drawback is that is it still requires a lot of elbow grease from the programmer. Maybe a web-of-trust, collective (paid?) auditing solution can replace manual auditing by the end user.

A Stricter Model

The main safety limitation in Austral’s current capability security model is the FFI.

FFI code is completely unsafe, code that imports Austral.Memory is completely unsafe. The only safety check is that modules that do this must be marked unsafe with pragma Unsafe_Module;. But there is no obligation that the programmer audit these modules. So there’s a faultline here: capabilities are a userspace construct, and unsafe modules are a language-level construct.

Maybe we can unify them, and achieve greater safety?

The approach described here is much more tedious to use, but it is safer and more precise.

The basic idea is to introduce a new capability called Unsafe, that is acquired from a root capability:

module Austral.Unsafe is
    type Unsafe: Linear;

    generic [R: Region]
    function acquire(root: &![RootCapability, R]): Unsafe;

    function surrender(unsafe: Unsafe): Unit;
end module.

All unsafe operations (pointer arithmetic, pointer casting, calling an FFI function) require passing an Unsafe capability. So, while the Austral.Memory module currently looks like this:

module Austral.Memory is
    type Address[T: Type]: Free;
    type Pointer[T: Type]: Free;

    generic [T: Type]
    function allocate(): Address[T];

    generic [T: Type]
    function load(pointer: Pointer[T]): T;

    generic [T: Type]
    function positiveOffset(pointer: Pointer[T], offset: Index): Pointer[T];

    -- ...

Under this model it would look something like:

import Austral.Unsafe ( Unsafe );

module Austral.Memory is
    type Address[T: Type]: Free;
    type Pointer[T: Type]: Free;

    generic [T: Type, R: Region]
    function allocate(unsafe: &![Unsafe, R]): Address[T];

    generic [T: Type, R: Region]
    function load(unsafe: &![Unsafe, R, pointer: Pointer[T]): T;

    generic [T: Type, R: Region]
    function positiveOffset(unsafe: &![Unsafe, R, pointer: Pointer[T], offset: Index): Pointer[T];

    -- ...

Similarly, every FFI function that is defined has an implicit parameter added to at the beginning of its parameter list: it needs to take a reference to the Unsafe capability. So if you define something like:

function putChar(character: Int32): Int32 is
    pragma Foreign_Import(External_Name => "putchar");

To call putChar you need to pass a reference to the unsafe capability:

putChar('a');           -- Error: wrong number of arguments.
putChar(&!unsafe, 'a'); -- Good

This has a number of advantages:

  1. Symmetry: the concept of unsafe modules is no longer needed. So we can get rid of the Unsafe_Module pragma. The Austral.Memory module can be imported by any other module, but its functions cannot be used unless the client passes an Unsafe capability, which in turn they must acquire.

  2. Userspace: unsafe modules are a language-level concept. The Unsafe capability is mostly implementable in userspace, with the minor caveat that foreign functions having to take a reference to Unsafe is a rule that must be built into the language. It’s generally a good thing when we can implement something as a library rather than as a first-class language feature.

  3. Granularity: the scope of “unsafe” is made a lot more granular: now it is no longer that some modules are unsafe, and others are safe, and we have to audit the entirety of an unsafe module. Rather: anything that takes an Unsafe capability is potentially unsafe and becomes an auditing target.

And some disadvantages:

  1. False Positives: some foreign functions are side-effect free. Do we have to pass an Unsafe capability to calculate the sin of a number? That’s not very convenient. Maybe some foreign functions can be whitelisted, with the build system alerting the user about all the whitelisted functions so they can be audited.

  2. Allocation: any data structure that does memory allocation needs to take and hold on to an Unsafe capability, or maybe something more specific.

    Alternatively, we could have allocator objects that take and hold on to the Unsafe capability, and can dispense blocks of memory using it. Then anything that needs memory allocation takes a value that implements the Allocator type class or something along those lines.

Since Unsafe is very broad it might be sub-capabilities: a ForeignCapability type to call a foreign function, an AllocationCapability type to call the allocate function in Austral.Memory, etc.

So if you’re building an API that wraps an unsafe, foreign library, the API might look something like this:

import Austral.Unsafe ( Unsafe );

module LibFoo is
    type FooCapability: Linear;

    function acquire(unsafe: Unsafe): FooCapability;

    function surrender(cap: FooCapability): Unsafe;

    -- Other functions that use `FooCapability`.
    -- ...
end module.

Internally, FooCapability would be a record that holds on to the Unsafe capability:

import Austral.Unsafe ( Unsafe );

module body LibFoo is
    record FooCapability: Linear is
        unsafe: Unsafe;

    function acquire(unsafe: Unsafe): FooCapability is
        return FooCapability(unsafe => unsafe);

    function surrender(cap: FooCapability): Unsafe is
        let { unsafe: Unsafe } := cap;
        return unsafe;

    -- etc.
end module body.

And the rest of the API would take references to the FooCapability capability. Internally, each of those functions would then transform that into a reference to the inner Unsafe value, and pass that to any FFI functions that have to be called.


The current model works, but might benefit from being made stricter. I’ll have to experiment with this to see what works in practice.

See Also: