CVE-2026-27195 — wasmtime
Fix: bytecodealliance/wasmtime@9e51c0dCVE-2026-27195 is a CWE-755 vulnerability in wasmtime. A fix is available for wasmtime — see the affected versions and patch details below.
Wasmtime is vulnerable to panic when dropping a `[Typed]Func::call_async` future
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-27195.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
wasmtime🦀wasmtimeReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.
Description
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:
- The host embedding calls
[Typed]Func::call_asyncon a function exported by a component, polling the returnedFutureonce. - 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_asyncwhich yields, or due an epoch interruption. - The host embedding drops the
Futureafter polling it once. This leaves the component instance in a non-reenterable state since the call never had a chance to complete. - The host embedding calls
[Typed]Func::call_asyncagain, polling the returnedFuture. Since the component instance cannot be entered at this point, the call traps, but not before allocating a task and thread for the call. - 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.
Impact
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.
Patches
Wasmtime 40.0.4 and 41.0.4 have been patched to fix this issue. Versions 42.0.0 and later are not affected.
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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | wasmtime | ≥ 39.0.0&&< 40.0.4 | 40.0.4cargo update -p wasmtime --precise 40.0.4 |
| 🦀crates.io | wasmtime | ≥ 41.0.0&&< 41.0.4 | 41.0.4cargo update -p wasmtime --precise 41.0.4 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for wasmtime, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update wasmtime to 40.0.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27195 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-27195 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-27195. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-27195 in your dependencies?
O3 Security finds CVE-2026-27195 across crates.io dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.