GHSA-v4cp-h94r-m7xf
MEDIUMUse after free passing `externref`s to Wasm in Wasmtime
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.
Blast Radius
wasmtime🐍wasmtimeReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io, PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Impact
There was a use-after-free bug when passing externrefs from the host to guest Wasm content.
To trigger the bug, you have to explicitly pass multiple externrefs from the host to a Wasm instance at the same time, either by
- passing multiple
externrefs as arguments from host code to a Wasm function, - or returning multiple
externrefs to Wasm from a multi-value return function defined in the host.
If you do not have host code that matches one of these shapes, then you are not impacted.
If Wasmtime's VMExternRefActivationsTable became filled to capacity after passing the first externref in, then passing in the second externref could trigger a garbage collection. However the first externref is not rooted until we pass control to Wasm, and therefore could be reclaimed by the collector if nothing else was holding a reference to it or otherwise keeping it alive. Then, when control was passed to Wasm after the garbage collection, Wasm could use the first externref, which at this point has already been freed.
We have reason to believe that the effective impact of this bug is relatively small because usage of externref is currently quite rare.
Patches
The bug has been fixed, and users should upgrade to Wasmtime 0.30.0.
Additionally, we have updated our primary externref fuzz target such that it better exercises these code paths and we can have greater confidence in their correctness going forward.
Workarounds
If you cannot upgrade Wasmtime yet, you can avoid the bug by disabling reference types support in Wasmtime by passing false to wasmtime::Config::wasm_reference_types.
References
For more information
If you have any questions or comments about this advisory:
- Reach out to us on the Bytecode Alliance Zulip chat
- Open an issue in the
bytecodealliance/wasmtimerepository
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | wasmtime | all versions | 0.30.0 |
| 🐍PyPI | wasmtime | all versions | 0.30.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for wasmtime. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update wasmtime to 0.30.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-v4cp-h94r-m7xf 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 pinpoints whether GHSA-v4cp-h94r-m7xf is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-v4cp-h94r-m7xf. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-v4cp-h94r-m7xf in your dependencies?
O3 detects GHSA-v4cp-h94r-m7xf across crates.io, PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.