CVE-2021-39216

Host memory corruption via unrooted externref collection

Published Sep 17, 2021 · Updated Aug 4, 2024

Use-after-free in Wasmtime 0.19.0 through 0.29.0 allows context-dependent attackers to corrupt host memory via multiple externref transfers. The VMExternRefActivationsTable can fill after the first externref, trigger garbage collection on the second, and reclaim the first before Wasm receives control. Reachability requires host code to pass or return multiple externrefs simultaneously; the stale reference can damage host-process integrity or availability.

CVSS severity6.3
Medium
EPSS probability0.31%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Wasmtime 0.19.0 through 0.29.0 allows context-dependent attackers to corrupt host memory via multiple externref transfers. The VMExternRefActivationsTable can fill after the first externref, trigger garbage collection on the second, and reclaim the first before Wasm receives control. Reachability requires host code to pass or return multiple externrefs simultaneously; the stale reference can damage host-process integrity or availability.

The record

CVE
CVE-2021-39216
Published
Sep 17, 2021
Updated
Aug 4, 2024
Vendor
Bytecode Alliance
Product
Wasmtime
Classifications
CWE-416
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Sep 17, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>=0.19.0, <=0.29.0

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

Bytecode Alliance · Wasmtimeversion=>=0.19.0, <=0.29.0

Published CVSS scores

6.3GitHubCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-416Use After Free

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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