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Critical, CVSS 9.2

Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access

CVE-2026-34987GHSA-xx5w-cvp6-jv83

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (wasmtime 43.0.1, 42.0.2, 36.0.7), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • crates.io wasmtime

    Affected
    >= 25.0.0, < 36.0.7>= 37.0.0, < 42.0.2>= 43.0.0, <= 43.0.0
    Fixed in
    36.0.7, 43.0.1, 42.0.2

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
9.2 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-34987: 0.28% estimated exploitation activity in the next 30 days; percentile 20.35 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Impact Wasmtime with its Winch (baseline) non-default compiler backend may allow properly constructed guest Wasm to access host memory outside of its linear-memory sandbox. This vulnerability requires use of the Winch compiler (-Ccompiler=winch). By default, Wasmtime uses its Cranelift backend, not Winch. With Winch, the same incorrect assumption is present in theory on both aarch64 and x86-64. The aarch64 case has an observed-working proof of concept, while the x86-64 case is theoretical and may not be reachable in practice. This Winch compiler bug can allow the Wasm guest to access memory before or after the linear-memory region, independently of whether pre- or post-guard regions are configured. The accessible range in the initial bug proof-of-concept is up to 32KiB before the start of memory, or ~4GiB after the start of memory, independently of the size of pre- or post-guard regions or the use of explicit or guard-region-based bounds checking. However, the underlying bug assumes…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from ghsa record GHSA-xx5w-cvp6-jv83

Agent brief

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# Patch Next advisory brief: CVE-2026-34987

Advisory: Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access
IDs: CVE-2026-34987, GHSA-xx5w-cvp6-jv83
Record: https://patchnext.com/advisories/cve-2026-34987 (revision 1, source checked 26 Oct 2026, 15:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: wasmtime 43.0.1, 42.0.2, 36.0.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io wasmtime: affected >= 25.0.0, < 36.0.7; >= 37.0.0, < 42.0.2; >= 43.0.0, <= 43.0.0; fixed in 36.0.7, 43.0.1, 42.0.2

## Suggested inspection (read-only)
- crates.io wasmtime: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on wasmtime.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-xx5w-cvp6-jv83 (GitHub-reviewed advisory GHSA-xx5w-cvp6-jv83 lists affected ranges: crates.io wasmtime >=25.0.0 =37.0.0 =43.0.0 <=43.0.0)

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