Findings

CVE-2026-27195

Wasmtime is a runtime for WebAssembly.

Published Feb 24, 2026 · updated Jun 17, 2026 · bytecodealliance wasmtime

Wasmtime is a runtime for WebAssembly. Starting with Wasmtime 39.0.0, the feature became the default, which brought with it a new implementation of which made it capable of calling async-typed guest export functions. However, that implementation had a bug leading to a panic under certain circumstances: First, the host embedding calls on a function exported by a component, polling the returned once. Second, the component function yields control to the async runtime (e.g. Tokio), e.g. due to a call to host function registered using which yields, or due an epoch interruption. Third, the host embedding drops the after polling it once. This leaves the component instance in a non-reenterable state since the call never had a chance to complete. Fourth, the host embedding calls again, polling the returned . Since the component instance cannot be entered at this point, the call traps, but not before allocating a task and thread for the call. Fifth, the host embedding ignores the trap and drops the . This panics due to the runtime attempting to dispose of the task created above, which panics since the thread has not yet exited. When a host embedder using the affected versions of Wasmtime calls on a guest export and then drops the returned future without waiting for it to resolve, and then does so again with the same component instance, Wasmtime will panic. Embeddings that have the compile-time feature disabled are unaffected. Wasmtime 40.0.4 and 41.0.4 have been patched to fix this issue. Versions 42.0.0 and later are not affected. If an embedding is not actually using any component-model-async features then disabling the Cargo feature can work around this issue. This issue can also be worked around by either ensuring every future is awaited until it completes or refraining from using the again after dropping a not-yet-resolved future.