---
title: "CVE-2026-34987: Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access"
url: https://erroneo.us/security/cve/CVE-2026-34987/
description: "CVE-2026-34987 (wasmtime (crates.io (Rust))): Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime with its…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-04-09
date_modified: 2026-04-10
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-34987"
kev: false
cvss: 9
epss: 0.0049
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-34987: Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access

**Signal:** Critical, preconditions apply. Severe if exploited, but exploitation needs conditions that may not apply to you. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.5% , percentile 41: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 41% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Needs an account (low (any account)). Exposure depends on who can log in.
- Only applies if your dependency tree includes wasmtime (crates.io (Rust)) >= 25.0.0, < 36.0.7, fixed in 36.0.7 ; wasmtime (crates.io (Rust)) >= 37.0.0, < 42.0.2, fixed in 42.0.2 ; wasmtime (crates.io (Rust)) = 43.0.0, fixed in 43.0.1 . Check your lockfile, including transitive dependencies.

## What it is

Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, 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 a 32-bit memory offset stored in a 64-bit register has its upper bits cleared when it may not, and so closely related variants of the initial proof-of-concept may be able to access truly arbitrary memory in-process. This could result in a host process segmentation fault (DoS), an arbitrary data leak from the host process, or with a write, potentially an arbitrary RCE. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.

## Affected packages

- crates.io (Rust) `wasmtime` >= 25.0.0, < 36.0.7 → fixed: 36.0.7
- crates.io (Rust) `wasmtime` >= 37.0.0, < 42.0.2 → fixed: 42.0.2
- crates.io (Rust) `wasmtime` = 43.0.0 → fixed: 43.0.1

## Affected products (CVE record)

- bytecodealliance wasmtime: >= 25.0.0, < 36.0.7, >= 37.0.0, < 42.0.2, >= 43.0.0, < 44.0.1

## Severity

- GitHub_M CVSS 4.0: 9 `CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H`
- GitHub 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`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.49% probability of exploitation activity in the next 30 days; percentile 41.

## Sources

- [GitHub Advisory GHSA-xx5w-cvp6-jv83](https://github.com/advisories/GHSA-xx5w-cvp6-jv83)
- [CVE.org record CVE-2026-34987](https://www.cve.org/CVERecord?id=CVE-2026-34987)
- [NVD entry CVE-2026-34987](https://nvd.nist.gov/vuln/detail/CVE-2026-34987)
- [github.com](https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-xx5w-cvp6-jv83)
- [rustsec.org](https://rustsec.org/advisories/RUSTSEC-2026-0095.html)

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Source page: https://erroneo.us/security/cve/CVE-2026-34987/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt