Wasmtime is a runtime for WebAssembly. In 43.0.0, cloning a wasmtime::Linker is unsound and can result in use-after-free bugs. This bug is not controllable by guest Wasm programs. It can only be triggered by a specific sequence of embedder API calls made by the host. Specifically, the following steps must occur to trigger the bug clone a wasmtime::Linker, drop the original linker instance, use the new, cloned linker instance, resulting in a use-after-free. This vulnerability is fixed in 43.0.1.

Affected: crates.io: wasmtime

### Impact

In version 43.0.0 of the `wasmtime` crate, cloning a `wasmtime::Linker` is unsound and can result in use-after-free bugs.

This bug is not controllable by guest Wasm programs. It can only be triggered by a specific sequence of embedder API calls made by the host.

The typical symptom of this use-after-free bug is a segfault. It does not enable heap corruption or data leakage.

If you are using the `wasmtime` CLI, rather than the embedding API, you are not affected. If you are using the embedding API but are not calling `wasmtime::Linker`'s `Clone` implementation, you are not affected.

Specifically, the following steps must occur to trigger the bug:

* Clone a `wasmtime::Linker` * Drop the original linker instance * Use the new, cloned linker instance, resulting in a use-after-free

### Patches

This bug has been patched in Wasmtime version 43.0.1

### Workarounds

Wasmtime embedders are highly encouraged to upgrade their `wasmtime` crate dependency.

If upgrading is not an option, or as a temporary workaround before upgrading, you can avoid this bug by not cloning `wasmtime::Linker` and instead creating a new, empty `wasmtime::Linker` and manually reregistering the host APIs from the original linker:

```rust use wasmtime::{Linker, Result, Store};

fn clone_linker<T>(linker: &Linker<T>, store: &mut Store<T>) -> Result<Linker<T>> { let mut cloned = Linker::new(); for (module, name, item) in linker.iter(store) { cloned.define(module, name, item)?;…

Affected: PyPI: wasmtime

Wasmtime is a runtime for WebAssembly. In 43.0.0, cloning a wasmtime::Linker is unsound and can result in use-after-free bugs. This bug is not controllable by guest Wasm programs. It can only be triggered by a specific sequence of embedder API calls made by the host. Specifically, the following steps must occur to trigger the bug clone a wasmtime::Linker, drop the original linker instance, use the new, cloned linker instance, resulting in a use-after-free. This vulnerability is fixed in 43.0.1.

Affected: crates.io: wasmtime

This is an entry in the RustSec database for the Wasmtime security advisory located at https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-hfr4-7c6c-48w2 For more information see the GitHub-hosted security advisory.

Affected: Ubuntu:24.04:LTS: rust-wasmtime, Ubuntu:25.10: rust-wasmtime, Ubuntu:26.04:LTS: rust-wasmtime

Wasmtime is a runtime for WebAssembly. In 43.0.0, cloning a wasmtime::Linker is unsound and can result in use-after-free bugs. This bug is not controllable by guest Wasm programs. It can only be triggered by a specific sequence of embedder API calls made by the host. Specifically, the following steps must occur to trigger the bug clone a wasmtime::Linker, drop the original linker instance, use the new, cloned linker instance, resulting in a use-after-free. This vulnerability is fixed in 43.0.1.