What is CVE-2021-43790?

CVE-2021-43790 is a high-severity use after free vulnerability in lucet-runtime (rust), affecting versions <= 0.6.1. No fixed version is listed yet. Memory is accessed after it has been freed, leading to undefined behavior in native code.

How severe is CVE-2021-43790?

CVE-2021-43790 has a CVSS score of 8.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Which versions of lucet-runtime are affected by CVE-2021-43790?

lucet-runtime (rust) versions <= 0.6.1 is affected.

Is there a fix for CVE-2021-43790?

No fixed version is listed for CVE-2021-43790 yet. Monitor the advisory for updates and apply mitigations in the interim.

Is CVE-2021-43790 exploitable, and should I be worried?

Whether CVE-2021-43790 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.

What actually determines whether CVE-2021-43790 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.

How do I fix CVE-2021-43790?

No fixed version is listed yet. In the interim: Keep the dependency up to date. In native-code projects, use memory-safe data structures and address sanitizers during testing.

CVE-2021-43790 in lucet-runtime

Q: How severe is CVE-2021-43790?

CVE-2021-43790 has a CVSS score of 8.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Q: Which versions of lucet-runtime are affected by CVE-2021-43790?

lucet-runtime (rust) versions <= 0.6.1 is affected.

Q: Is there a fix for CVE-2021-43790?

No fixed version is listed for CVE-2021-43790 yet. Monitor the advisory for updates and apply mitigations in the interim.

Q: Is CVE-2021-43790 exploitable, and should I be worried?

Whether CVE-2021-43790 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.

Q: What actually determines whether CVE-2021-43790 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.

Q: How do I fix CVE-2021-43790?

No fixed version is listed yet. In the interim: Keep the dependency up to date. In native-code projects, use memory-safe data structures and address sanitizers during testing.

Summary

Use After Free in lucet

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

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.

Memory is accessed after it has been freed, leading to undefined behavior in native code. Typical impact: memory corruption, crash, or potential code execution.

CVE-2021-43790 has a CVSS score of 8.5 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Affected versions

lucet-runtime (<= 0.6.1)

Security releases

Not available

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Users should upgrade to the main branch of the Lucet repository. Lucet does not provide versioned releases on crates.io.

Frequently Asked Questions

What is CVE-2021-43790? CVE-2021-43790 is a high-severity use after free vulnerability in lucet-runtime (rust), affecting versions <= 0.6.1. No fixed version is listed yet. Memory is accessed after it has been freed, leading to undefined behavior in native code.
How severe is CVE-2021-43790? CVE-2021-43790 has a CVSS score of 8.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
Which versions of lucet-runtime are affected by CVE-2021-43790? lucet-runtime (rust) versions <= 0.6.1 is affected.
Is there a fix for CVE-2021-43790? No fixed version is listed for CVE-2021-43790 yet. Monitor the advisory for updates and apply mitigations in the interim.
Is CVE-2021-43790 exploitable, and should I be worried? Whether CVE-2021-43790 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
What actually determines whether CVE-2021-43790 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.
How do I fix CVE-2021-43790? No fixed version is listed yet. In the interim: Keep the dependency up to date. In native-code projects, use memory-safe data structures and address sanitizers during testing.