CVE-2021-39218

Workarounds

If you cannot upgrade Wasmtime at this time, you can avoid this bug by disabling the reference types proposal by passing false to wasmtime::Config::wasm_reference_types

References

• The Wasm reference types proposal, which introduces externref

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

There was an invalid free and out-of-bounds read and write bug when running Wasm that uses externrefs in Wasmtime.

To trigger this bug, Wasmtime needs to be running Wasm that uses externrefs, the host creates non-null externrefs, Wasmtime performs a garbage collection (GC), and there has to be a Wasm frame on the stack that is at a GC safepoint where

• there are no live references at this safepoint, and
• there is a safepoint with live references earlier in this frame's function.

Under this scenario, Wasmtime would incorrectly use the GC stack map for the safepoint from earlier in the function instead of the empty safepoint. This would result in Wasmtime treating arbitrary stack slots as externrefs that needed to be rooted for GC. At the next GC, it would be determined that nothing was referencing these bogus externrefs (because nothing could ever reference them, because they are not really externrefs) and then Wasmtime would deallocate them and run <ExternRef as Drop>::drop on them. This results in a free of memory that is not necessarily on the heap (and shouldn't be freed at this moment even if it was), as well as potential out-of-bounds reads and writes.

Even though support for externrefs (via the reference types proposal) is enabled by default, unless you are creating non-null externrefs in your host code or explicitly triggering GCs, you cannot be affected by this bug.

We have reason to believe that the effective impact of this bug is relatively small because usage of externref is currently quite rare.

A read operation accesses a memory location beyond the intended buffer boundary. Typical impact: sensitive data disclosure or crash.

CVE-2021-39218 has a CVSS score of 6.3 (Medium). The vector is requires local access, 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. A fixed version is available (0.30.0); upgrading removes the vulnerable code path.

Kodem intelligence

This bug has been patched and users should upgrade to Wasmtime version 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.

1. What is CVE-2021-39218? CVE-2021-39218 is a medium-severity out-of-bounds read vulnerability in wasmtime (rust), affecting versions >= 0.26.0, < 0.30.0. It is fixed in 0.30.0. A read operation accesses a memory location beyond the intended buffer boundary.
2. How severe is CVE-2021-39218? CVE-2021-39218 has a CVSS score of 6.3 (Medium). 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.
3. Which versions of wasmtime are affected by CVE-2021-39218? wasmtime (rust) versions >= 0.26.0, < 0.30.0 is affected.
4. Is there a fix for CVE-2021-39218? Yes. CVE-2021-39218 is fixed in 0.30.0. Upgrade to this version or later.
5. Is CVE-2021-39218 exploitable, and should I be worried? Whether CVE-2021-39218 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
6. What actually determines whether CVE-2021-39218 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.
7. How do I fix CVE-2021-39218?
   • Upgrade wasmtime to 0.30.0 or later
   • Upgrade wasmtime to 0.30.0 or later