GHSA-v4cp-h94r-m7xf · Severity: medium · Ecosystem: rust — Use after free passing `externref`s to Wasm in Wasmtime
Wasmtime is an open source runtime for WebAssembly & WASI. In Wasmtime from version 0.19.0 and before version 0.30.0 there was a use-after-free bug when passing `externref`s from the host to guest Wasm content. To trigger the bug, you have to explicitly pass multiple `externref`s from the host to a Wasm instance at the same time, either by passing multiple `externref`s as arguments from host code to a Wasm function, or returning multiple `externref`s 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. The bug has been fixed, and users should upgrade to Wasmtime 0.30.0. 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`.
Conclusion & alert: CVE-2021-39216 is rated Low Risk (32.7/100): CVSS Medium severity, with low exploitation likelihood (EPSS 0.30%). Mandatory action: Monitor for updates and reassess as exploit intelligence or EPSS changes.
Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.
EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).
| # | Date | Old EPSS score | New EPSS score | Delta (New - Old) |
|---|---|---|---|---|
| 1 | 2026-06-15 | 0.15% | 0.30% | +0.14% |
| 2 | 2025-11-21 | 0.07% | 0.15% | +0.08% |
| 3 | 2025-11-18 | — | 0.07% | — |
Full EPSS history (15 records total)
CVSS metrics for this CVE.
| Base score | Version | Severity | Vector | Exploitability | Impact | Score source |
|---|---|---|---|---|---|---|
| 6.3 | 3.1 | MEDIUM |
|
1.0 | 5.2 | [email protected] |
| 3.3 | 2.0 | LOW |
|
3.4 | 4.9 | [email protected] |
GHSA-v4cp-h94r-m7xf · Severity: medium · Ecosystem: rust — Use after free passing `externref`s to Wasm in Wasmtime
| vendor | priority | summary | link |
|---|---|---|---|
debian
|
unimportant | CVE-2021-39216 unimportant priority: Debian including 1 source packages (rust-wasmtime), 3 status rows across 3 suites (forky, sid, trixie): resolved 3. | https://security-tracker.debian.org/tracker/CVE-2021-39216 |
| Vendor | Product | Version | Raw CPE |
|---|---|---|---|
| bytecodealliance | wasmtime | >= 0.19.0, < 0.30.0 | cpe:2.3:a:bytecodealliance:wasmtime:*:*:*:*:*:*:*:* |
| fedoraproject | fedora | 34 | cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* |
| fedoraproject | fedora | 35 | cpe:2.3:o:fedoraproject:fedora:35:*:*:*:*:*:*:* |