CVE-2021-39218

Memory corruption via incorrect GC stack-map selection

Published Sep 17, 2021 · Updated Aug 4, 2024

Memory corruption in Bytecode Alliance Wasmtime 0.26.0 through 0.29.0 allows local users to corrupt memory via crafted WebAssembly. The registered-module stack-map lookup selects an earlier GC safepoint when the current safepoint has no live references, causing arbitrary stack slots to be treated as externrefs and later freed. Triggering the flaw requires host-created non-null externrefs, garbage collection, and a specific pair of safepoints; the resulting invalid frees and out-of-bounds accesses can corrupt or crash the host process.

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

What happened

Memory corruption in Bytecode Alliance Wasmtime 0.26.0 through 0.29.0 allows local users to corrupt memory via crafted WebAssembly. The registered-module stack-map lookup selects an earlier GC safepoint when the current safepoint has no live references, causing arbitrary stack slots to be treated as externrefs and later freed. Triggering the flaw requires host-created non-null externrefs, garbage collection, and a specific pair of safepoints; the resulting invalid frees and out-of-bounds accesses can corrupt or crash the host process.

The record

CVE
CVE-2021-39218
Published
Sep 17, 2021
Updated
Aug 4, 2024
Vendor
Bytecode Alliance
Product
Wasmtime
Classifications
CWE-125, CWE-787, CWE-590
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=>= 0.26.0, <= 0.29.0

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

Bytecode Alliance · Wasmtimeversion=>= 0.26.0, <= 0.29.0

Published CVSS scores

6.3GitHub, Inc.CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H
3.3NIST NVDAV:L/AC:M/Au:N/C:N/I:P/A:P

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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-125Out-of-bounds Read
CWE-787Out-of-bounds Write
CWE-590Free of Memory not on the Heap

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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