CVE-2022-39393 is a high-severity (CVSS 8.6) CWE-226 vulnerability in wasmtime. A fix is available for wasmtime — see the affected versions and patch details below.
Wasmtime vulnerable to data leakage between instances in the pooling allocator
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2022-39393.
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.
How urgent is this, really
CVE-2022-39393 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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
Impact
There is a bug in Wasmtime's implementation of it's pooling instance allocator where when a linear memory is reused for another instance the initial heap snapshot of the prior instance can be visible, erroneously to the next instance. The pooling instance allocator in Wasmtime works by preallocating virtual memory for a fixed number of instances to reside in and then new instantiations pick a slot to use. Most conventional modules additionally have an initial copy-on-write "heap image" which is mapped in Wasmtime into the linear memory slot. When a heap slot is deallocated Wasmtime resets all of its contents back to the initial state but it does not unmap the image in case the next instance is an instantiation of the same module.
The bug in Wasmtime occurs when a slot in the pooling allocator previously was used for a module with a heap image, meaning that its current state of memory contains the initial heap contents of that module. If the next instantiation within that slot does not itself contain a heap image then Wasmtime would leave the old heap image in place erroneously and continue with instantiation. This means that instantiations of modules without a heap image can see the initial heap image of the prior instantiation within that slot.
Heap images in Wasmtime are created by precomputing WebAssembly data segments into one large mapping to be placed into linear memory at a particular offset. Most modules produced by toolchains today will have a heap image and an initialization snapshot. Creating a module without a heap image would require a hand-crafted *.wat file or a specially crafted source program. This consequence means that this bug is highly unlikely to be accidentally triggered and would otherwise require an intentional trigger with a hand-crafted module.
One important part of this vulnerability is Wasmtime is highly likely to segfault when the slot is reused again with a module that itself has an initialization image. For example if module A has a heap initialization image and module B does not have a heap initialization image, then the following sequence of events could happen if they all are instantiated into the same instance slot:
- Module A is instantiated, and then deallocated. This leaves A's heap image in place, reset to its initial contents.
- Module B is instantiated and erroneously can see the initial heap contents of A. Module B is then deallocated and the entire heap is unmapped and reset back to zero.
- Module A is instantiated again, but the state tracking the slot did not account for module B so it thinks the module image is still mapped and proceeds with instantiation. Any action on A's part to access linear memory will then trap and if the host accesses A's memory it will segfault because the data that's supposed to be mapped is all unmapped.
Adding this all together this means that in practice modules must be deliberately crafted to not have an initial heap image to view the contents of a prior image. If this module is instantiated though then when the slot is reused the next, likely image-using, module will believe its memory is mapped when it isn't, causing the host to segfault on unmapped memory it believed was mapped.
Patches
This bug has been patched and users should upgrade to Wasmtime 2.0.2.
Workarounds
Trigging this bug requires the pooling allocator to be configured and for copy-on-write heap images to also be enabled. Pooling allocation is not enabled by default but copy-on-write heap images are. Mitigations for this bug include:
- Disabling the pooling allocator - note that pooling allocation is not enabled by default in Wasmtime
- Disabling the
memory-init-cowfeature or withConfig::memory_init_cow
References
Config::allocation_strategy- configuration required to enable the pooling allocator.Config::memory_init_cow- configuration required to enable or disable copy-on-write (this is enabled by default).- Mailing list announcement
- Patch for
release-2.0.0branch - Patch for
main
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/wasmtime repository
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | wasmtime | ≥ 2.0.0&&< 2.0.2 | 2.0.2cargo update -p wasmtime --precise 2.0.2 |
| 🦀crates.io | wasmtime | all versions | 1.0.2cargo update -p wasmtime --precise 1.0.2 |
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 2.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2022-39393 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-2022-39393 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2022-39393. 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-2022-39393 in your dependencies?
O3 Security finds CVE-2022-39393 across crates.io dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.