CVE-2021-39216

MEDIUMNVD 6.36.3—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 6.3 under NVD CVSS v3. EPSS exploit probability: 0.3% (21st percentile of EPSS-scored CVEs). GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
6.3EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0.3%CVSS: 6.3Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

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 externrefs from the host to guest Wasm content. To trigger the bug, you have to explicitly pass multiple externrefs from the host to a Wasm instance at the same time, either by passing multiple externrefs as arguments from host code to a Wasm function, or returning multiple externrefs 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.

CVSS v3
6.3
EG Score
6.3MEDIUMmedium confidence
EG Risk
33
EG Risk 33/100CISA SSVC

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity63% × 45%
Exploitation0% × 40%
Automatability30% × 15%
CISA SSVC: Track at low or medium mission impact; Track or Attend at high (mission-essential systems).
Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
EPSS PROB
0.3%
EPSS %ILE
21st
KEV
Not listed

CISA SSVCTrack at low or medium mission impact; Track or Attend at high (mission-essential systems).

A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.

Exploitation none (no KEV listing, exploit record or EPSS ≥ 50%) · Automatable unknown (not published for this CVE) · Technical impact partial (CVSS below 9.0). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table

Published

September 17, 2021

Last Modified

November 21, 2024

References (10)

Affected Packages

(2 across 2 ecosystems)
crates.io(1)
PackageVulnerable rangeFix by version rangeDependents
wasmtime—
  • 0.26.0 up to 0.30.0: fixed in 0.30.0
  • every version up to 0.30.0: fixed in 0.30.0
  • 0.0.0-0 up to 0.30.0: fixed in 0.30.0
—
PyPI(1)
PackageVulnerable rangeFix by version rangeDependents
wasmtime0.0.1 ... 0.9.0 (25 versions)
  • 0.26.0 up to 0.30.0: fixed in 0.30.0
  • every version up to 0.30.0: fixed in 0.30.0
—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 7× in last 7d / 32× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 185 total refreshes for this CVE.

  1. 2026-10-10 19:56 UTCEPSS rescore
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Frequently asked(5)

What is CVE-2021-39216?
CVE-2021-39216 is a medium vulnerability published on September 17, 2021. 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 externrefs from the host to guest Wasm content. To trigger the bug, you have to explicitly pass multiple externrefs from the host to a…
When was CVE-2021-39216 disclosed?
CVE-2021-39216 was first published on September 17, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-39216 actively exploited?
CVE-2021-39216 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.3% probability of exploitation in the next 30 days (21st percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2021-39216?
CVE-2021-39216 has a CVSS v3 base score of 6.3 (NVD).
How do I remediate CVE-2021-39216?
A fix for CVE-2021-39216 is available: update to the fixed version the vendor names in its advisory.

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