CVE-2021-43790 is a high-severity (CVSS 8.1) Use After Free vulnerability in lucet-runtime. 1 public exploit reference exists, so weaponization risk is real. No vendor fix is recorded yet; mitigation options are listed below.
Use After Free in lucet
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-43790 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
lucet-runtimeReal-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 the main branch of Lucet's lucet-runtime that allows a use-after-free in an Instance object that could result in memory corruption, data race, or other related issues. This bug was introduced early in the development of Lucet and is present in all releases. As a result of this bug, and dependent on the memory backing for the Instance objects, it is possible to trigger a use-after-free when the Instance is dropped.
Patches
Users should upgrade to the main branch of the Lucet repository. Lucet does not provide versioned releases on crates.io.
Workarounds
There is no way to remediate this vulnerability without upgrading.
Description
Lucet uses a "pool" allocator for new WebAssembly instances that are created. This pool allocator manages everything from the linear memory of the wasm instance, the runtime stack for async switching, as well as the memory behind the Instance itself. Instances are referred to via an InstanceHandle type which will, on drop, release the memory backing the Instance back to the pool.
When an Instance is dropped, the fields of the Instance are destructed top-to-bottom, however when the alloc: Alloc field is destructed, the memory backing the Instance is released back to the pool before the destructors of the remaining fields are run. If another thread allocates the same memory from the pool while these destructors are still running, a race condition occurs that can lead to use-after-free errors.
The bug was corrected by changing how the InstanceHandle destructor operates to ensure that the memory backing an Instance is only returned to the pool once the Instance has been completely destroyed.
This security advisory has been assigned CVE-2021-43790.
For more information
If you have any questions or comments about this advisory:
- Open an issue in lucet repository
- Email the lucet team
- See the Bytecode Alliance security policy
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | lucet-runtime | all versions | No fix |
Affected Products
lucetbytecodeallianceResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for lucet-runtime, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Remediation status
No patched version of lucet-runtime has shipped for CVE-2021-43790 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.
Mitigate without a patch
Constrain what reaches the vulnerable code: limit the size and shape of untrusted input, isolate the affected component in a sandboxed or least-privileged process, and enable the platform's memory-safety mitigations (ASLR, stack protector, hardened allocator) so an out-of-bounds access is more likely to fail closed than to be exploitable.
Frequently Asked Questions
Is CVE-2021-43790 in your dependencies?
Find it across crates.io, including transitive dependencies.