CVE-2026-27195

The affected versions of Wasmtime can panic if the host embedder drops the future returned by wasmtime::component::[Typed]Func::call_async before it resolves.

Details

Starting with Wasmtime 39.0.0, the component-model-async feature became the default, which brought with it a new implementation of [Typed]Func::call_async which made it capable of calling async-typed guest export functions. However, that implementation had a bug leading to a panic under certain circumstances:

1. The host embedding calls [Typed]Func::call_async on a function exported by a component, polling the returned Future once.
2. The component function yields control to the async runtime (e.g. Tokio), e.g. due to a call to host function registered using LinkerInstance::func_wrap_async which yields, or due an epoch interruption.
3. The host embedding drops the Future after polling it once. This leaves the component instance in a non-reenterable state since the call never had a chance to complete.
4. The host embedding calls [Typed]Func::call_async again, polling the returned Future. Since the component instance cannot be entered at this point, the call traps, but not before allocating a task and thread for the call.
5. The host embedding ignores the trap and drops the Future. This panics due to the runtime attempting to dispose of the task created above, which panics since the thread has not yet exited.

Workarounds

If an embedding is not actually using any component-model-async features then disabling the component-model-async Cargo feature can work around this issue. This issue can also be worked around by either ensuring every call_async future is awaited until it completes or refraining from using the Store again after dropping a not-yet-resolved call_async future.

Resources

This was first reported in https://bytecodealliance.zulipchat.com/#narrow/channel/206238-general/topic/Panic.20in.20Wasmtime.2041.2E0.2E3.20.28runtime.2Fconcurrent.2Fcomponent.29

When a host embedder using the affected versions of Wasmtime calls wasmtime::component::[Typed]Func::call_async 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 component-model-async compile-time feature disabled are unaffected.

Kodem intelligence

Wasmtime 40.0.4 and 41.0.4 have been patched to fix this issue. Versions 42.0.0 and later are not affected.

1. What is CVE-2026-27195? CVE-2026-27195 is a medium-severity security vulnerability in wasmtime (rust), affecting versions >= 39.0.0, < 40.0.4. It is fixed in 40.0.4, 41.0.4.
2. Which versions of wasmtime are affected by CVE-2026-27195? wasmtime (rust) versions >= 39.0.0, < 40.0.4 is affected.
3. Is there a fix for CVE-2026-27195? Yes. CVE-2026-27195 is fixed in 40.0.4, 41.0.4. Upgrade to this version or later.
4. Is CVE-2026-27195 exploitable, and should I be worried? Whether CVE-2026-27195 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-27195 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-27195?
   • Upgrade wasmtime to 40.0.4 or later
   • Upgrade wasmtime to 41.0.4 or later