The essence of software engineering is making trade-offs, and sometimes engineers even trade away security for other priorities. When it comes to running untrusted code from unknown sources, however, exceptionally strong security is simply the bar to clear for serious participation: consider the extraordinary efforts that Web browser and hypervisor maintainers take to preserve their systems’ integrity. WebAssembly runtimes also run untrusted code from untrusted sources, and therefore such efforts are also a hard requirement for WebAssembly runtimes.

WebAssembly programs are sandboxed and isolated from one another and from the host, so they can’t read or write external regions of memory, transfer control to arbitrary code in the process, or freely access the network and filesystem. This makes it safe to run untrusted WebAssembly programs: they cannot escape the sandbox to steal private data from elsewhere on your laptop or run a botnet on your servers. But these security properties only hold true if the WebAssembly runtime’s implementation is correct. This article will highlight the ways we are ensuring correctness in the Wasmtime WebAssembly runtime and in its compiler, Cranelift.

This is our second blog post leading up to Wasmtime’s upcoming 1.0 release on September 20th, 2022. The first blog post focused on Wasmtime’s performance. We’re ready to release Wasmtime 1.0 because we believe not only that it solidly clears the bar for security and correctness, but also that we have the momentum, processes, and multi-stakeholder investment in place to keep it that way in the future.

A Safe Implementation Language

Wasmtime is implemented in the Rust programming language. Google, Microsoft, and Mozilla have each independently found that around 70% of security bugs in their browsers were memory safety bugs, such as use-after-free bugs and out-of-bounds heap accesses, including security bugs within those browsers’ WebAssembly implementations. Rust helps us avoid this whole class of bugs without sacrificing the low-level control we need to efficiently implement a language runtime. Large portions of Wasmtime even have zero unsafe blocks — such as our WebAssembly parser, which is the first component to process potentially-malicious input — and the parts that necessarily use unsafe to implement primitives are carefully vetted.

Rust does not prevent all bugs, of course. It doesn’t save us from miscompilations due to logic errors in our compiler, for example, that could ultimately lead to Wasm sandbox escapes. We’ll discuss the techniques we use to address these bugs and others that Rust doesn’t catch throughout the rest of this post.

The benefits of a safe implementation language, however, extend to applications embedding Wasmtime. Even a correct WebAssembly runtime’s utility is weakened if the interface to that runtime is unsafe or so clunky that it pushes embedders towards unsafe code out of convenience or to meet performance goals. That’s why we designed Wasmtime’s user-facing API such that misusing it is nearly impossible, using it doesn’t require any unsafe Rust, and that this safety does not also sacrifice performance. Our typed function API, for example, leverages Rust’s type system to do a single type check up front when taking a reference to a WebAssembly function, and subsequent uses — such as calls into WebAssembly through that function — don’t do repeated checks. Our strongly-typed APIs let us statically maintain critical safety invariants for users, avoiding the potential for misuse and the overhead of repeated dynamic checks.

Securing Our Supply Chain

Malicious dependencies are becoming more common. An attacker gains control over a library that your application depends on, adds code to steal your SSH keys, and your world falls apart the next time you run a build. We cannot let Wasmtime — and by extension, any application that embeds Wasmtime — be compromised by malicious third-party dependencies.

The WebAssembly component model will help protect against these attacks with capabilities-based security and lightweight isolation between software components. Unfortunately that can’t be a solution for Wasmtime itself, since Wasmtime needs to implement the component model and sits below that abstraction level.

To secure Wasmtime against malicious dependencies, we are using cargo vet. Mozilla created this tool to mechanically ensure that all third-party Rust libraries used inside Firefox have been manually reviewed by a trusted auditor.

When performing an audit, reviewers double check:

  • the use of unsafe,
  • that potentially-malicious, user-supplied data is handled with care (e.g. there is no recursion over user input that could let attackers craft inputs that cause the library to blow the stack),
  • that a markdown-parsing library, for example, doesn’t access the file system or network when it shouldn’t need those capabilities,
  • and that using the crate won’t otherwise open the door to security vulnerabilities in production.

Using cargo vet we now require that a trusted Wasmtime maintainer manually reviews all new dependencies and the delta for updates to existing dependencies. At the same time, we are burning down the list of yet-to-be-reviewed libraries that Wasmtime already depended upon before we adopted cargo vet.

cargo vet benefits from network effects: it allows us to import audits from another organization, so the more trustworthy organizations that start using cargo vet and auditing dependencies, then the fewer audits we will have to perform ourselves. And the more organizations that trust our audits, the more utility each of our audits provides. Right now, Wasmtime imports and trusts Firefox’s audits, Firefox likewise imports and trusts Wasmtime’s audits, and we hope to expand this as the cargo vet community grows.

Enabling Secure Application Designs

The security of applications using Wasmtime isn’t just determined by Wasmtime’s development process. It is also determined by how Wasmtime unlocks more-secure application designs that couldn’t have been considered before, because the performance overhead was impractical. One example is our ongoing standardization and implementation work on the previously-mentioned WebAssembly component model, and composing WebAssembly programs while maintaining isolation and performance. Another is the “disposable instance” paradigm.

We’ve worked hard to make instantiating WebAssembly instances so fast that you can create a fresh instance per task, throw it away when the task is completed, and create another new instance for the next task. This means that you can instantiate a fresh WebAssembly instance per HTTP request in a serverless application, for example. It provides isolation between tasks, so if the WebAssembly module has a bug that is triggered by the input for one task, that bug can’t automatically infect all other subsequent tasks. This wouldn’t be possible without Wasmtime’s fast instantiation.

Ubiquitous Fuzzing

Fuzzing is a software testing technique used to find security and correctness issues by feeding pseudo-random data as input into the system you’re testing:

fn fuzz() {
    loop {
        // Generate some new input.
        let input = generate_pseudo_random_data();

        // Feed that input into the system under test.
        let result = run_system_under_test(input);

        // Finally, if the system under test crashed,
        // failed an assertion, etc... then report
        // that!
        if result.is_interesting() {

We love fuzzing. We do continuous fuzzing in the background, 24/7. We do targeted fuzzing while developing new features. We fuzz in the large (e.g. all of Wasmtime) and the small (e.g. just our WebAssembly text format parser). We contribute to and help maintain some of the core fuzzing infrastructure for the whole Rust ecosystem. Our pervasive fuzzing is probably the biggest single contributing factor towards Wasmtime’s code quality.

We fuzz because writing tests by hand, while necessary, is not enough. We are fallible humans and will inevitably miss an edge case. Our minds aren’t twisted enough to come up with the kinds of inputs that a fuzzer will eventually find.

Fuzzing can be as simple as throwing random bytes at a WebAssembly binary parser and looking for any crashes. It can be as complex as generating arbitrary, guaranteed-valid WebAssembly modules, compiling them with and without optimizations, and asserting that running them yields the same results either way. We do both.

We primarily use libFuzzer, a coverage-guided fuzzing engine developed as part of the LLVM project, for our fuzzing. Our fuzzers run 24/7 as part of the OSS-Fuzz project. We contribute to and help maintain the cargo fuzz tool that makes building and running fuzzers in Rust easy, the arbitrary crate for fuzzing with structured data, and the libfuzzer-sys crate that provides Rust bindings to libFuzzer.

We use generators to create new, pseudo-random test cases, and oracles to check security and correctness properties when evaluating those test cases in Wasmtime.

We have a variety of generators, but the one we use most is wasm-smith. We wrote wasm-smith to produce pseudo-random WebAssembly modules that are guaranteed valid. It helps us test deeper within Wasmtime and Cranelift by not bouncing off the WebAssembly parser because of a malformed memory definition or failing the validator because of a type error inside a function body. It has configuration options to avoid generating code that will trap at runtime, to only generate certain kinds of instructions such as numeric instructions, and to turn various WebAssembly proposals on and off, among many other things. We like to use swarm testing to let the fuzzer dynamically configure the kinds of test cases we generate, improving the diversity of our generated test cases. Firefox has also started using wasm-smith to exercise its WebAssembly engine.

We use a variety of oracles in our fuzzing:

  • Did the program crash or fail an assertion?
  • If we capture the WebAssembly’s stack, do we see the expected stack frames?
  • Do we have the expected number of garbage collector-managed allocations and deallocations? Are we unexpectedly leaking?
  • Can we round trip a WebAssembly module through our parser, disassembler, and assembler and get the original input again?
  • Do we get the same results evaluating the input in:
    • Wasmtime with and without compiler optimizations enabled?
    • Wasmtime and V8?
    • Wasmtime and (a formally verified version of) the WebAssembly specification’s reference interpreter?
  • And many more.

We even have a symbolic checker for register allocation that we use as an oracle. The checker proves that a given allocation to a bounded number of physical registers correctly implements the original program that used an unbounded number of virtual registers, regardless of what actual values are given to the program or which control-flow paths are taken. We then generate arbitrary control-flow graphs of basic blocks containing instructions that operate on virtual registers, ask the register allocator to assign the virtual registers to physical registers, and finally use this checker as an oracle to assert that the assignment is correct.

When we implement new features in Wasmtime, we write generators and oracles specifically designed to exercise these new features. The symbolic register allocation checker is one example, as it was developed alongside a new register allocator for Cranelift. When implementing new WebAssembly proposals in Wasmtime, the baseline is adding support for the new proposal in wasm-smith. But we will also do things like create generators for testing the inline garbage collector write barriers that we emit in our compiled code when WebAssembly’s reference types proposal is enabled. And we developed a fuzzer for the component model’s interface functions in concert with their implementation in Wasmtime. We have fully embraced “fuzz-driven development”.

Formal Verification Efforts

Fuzzing gives us a statistical claim that our program is correct with respect to what the fuzzer is exercising. The longer we run the fuzzer, the closer that claim gets to 100%, but in general we’ll never reach 100% because our input space is way too large or even infinite. This is where our efforts to formally verify parts of Wasmtime and Cranelift come in.

The VeriWasm project — a collaboration between UCSD, Stanford, and Fastly — is a translation validator for WebAssembly programs compiled with Cranelift. It proves that the compiled program’s control-flow and memory accesses cannot escape its isolated sandbox. This is not a claim about a handful of inputs that we ran with a fuzzer, it proves this true for all inputs that could be given to the compiled program.

We’ve recently redesigned instruction selection in Cranelift to be defined via rewrite rules in a domain-specific language we call ISLE (Instruction Selection and Lowering Expressions). We have an ongoing collaboration with some folks from Cornell and CMU to formally verify the correctness of our ISLE-based instruction selection, proving that the machine instructions we emit do in fact implement the input Cranelift IR for all values they could be given. If it discovers an incorrect rule, the verifier will give us a counterexample. The counterexample is an input where the original Cranelift IR evaluates to one value, and the lowered machine instructions evaluate to a different value. The counterexample and its divergent results will help us diagnose and fix our buggy rule.

Looking further ahead, we are investigating refactoring Cranelift’s middle end to use ISLE and rewrite rules. This will let us formally verify the correctness of these rewrite rules, and further shrink our unverified, trusted compute base. We intend to keep applying this process to the whole compiler pipeline.

Spectre Mitigations

Spectre is a class of attacks exploiting speculative execution in modern processors. Speculative execution is when the processor guesses where control will flow — even though it has not actually computed branch conditions or indirect jump targets yet — and starts tentatively executing its guess. When the processor guesses correctly, the speculative execution’s results are used, speeding up the program; when it guesses incorrectly, they are discarded. Unfortunately, even discarded speculations can still affect the contents of caches and other internal processor state. An attacker can indirectly observe these effects by measuring the time it takes to perform operations that access that same internal state. Under the right conditions, this allows the attacker to deduce what happened in discarded speculative executions and effectively “see past” bounds checks and other security measures.

It is tempting to take a heavy-handed approach to defending against Spectre attacks. Operating system process boundaries are a common mitigation, however one of WebAssembly’s most enticing features is its lighter-weight-than-a-process isolation. Additionally, attackers must have access to a timer to pull off a Spectre attack, and while it is tempting to block access to timer APIs, it is surprisingly easy to find widgets that can be made into timers. The nature and severity of Spectre vulnerabilities depend greatly on context; the mitigations described below can form part of overall protection.

Wasmtime implements a number of Spectre mitigations to prevent speculative execution from leaking information to malicious programs:

  • Function table bounds checks are protected from speculative attack, ensuring that speculated call_indirect instructions cannot transfer control to an arbitrary location.

  • The br_table instruction is protected from speculative attack, ensuring that speculation cannot transfer control to an arbitrary location.

  • Wasmtime’s default configuration for WebAssembly linear memories elides explicit bounds checks, relying on virtual memory guard pages instead. However, when virtual memory guard pages are disabled and we must emit explicit bounds checks, we additionally emit mitigation code that prevents speculated accesses from escaping the linear memory.

  • We’re implementing support for hardware control-flow integrity features which could help mitigate Spectre attacks, such as BTI on aarch64.

Security researchers keep discovering new Spectre attacks and inventing better mitigations for them. Therefore, we expect we will keep expanding and refining Wasmtime’s Spectre mitigations in the future as well.

A Plan When Things Go Wrong

Even the most carefully crafted plans can go wrong, so we have backup plans for bugs that slip past our safeguards. It begins with our guidelines for reporting security bugs and our disclosure policy. Handling a security bug is a delicate matter, and we don’t want to make mistakes, so we have a vulnerability response runbook to walk ourselves through responding to security bugs in the moment. Once a patch is written, we backport security fixes to the two most-recent Wasmtime releases, as per our release process.

We’ve tested this safety net when faced with Cranelift miscompilations and incomplete stack maps for garbage collection.


This article detailed how Wasmtime uses language safety, fine-grained isolation, dependency auditing, fuzzing, and verification to bolster its security posture and the security postures of applications embedding Wasmtime and the WebAssembly programs Wasmtime runs. We believe that these are the minimum practices you should demand from WebAssembly runtimes when running untrusted or security-sensitive code, and we are constantly trying to raise this bar and strengthen Wasmtime’s security and correctness assurances.