# CVE-2026-34987: Bytecode Alliance Wasmtime sandbox escape in Winch compiler backend

Source: https://junglewise.ai/threats/cve-2026-34987-bytecode-alliance-wasmtime-sandbox-escape-in-winch-compiler  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-09. Updated: 2026-05-10  
License: CC BY 4.0 with a link back

CVE-2026-34987 · Severity: critical · CVSS 9.9 · Published 2026-04-09

Wasmtime is a runtime for executing WebAssembly (Wasm) code in a secure, isolated 'sandbox.' A vulnerability in its optional Winch compiler backend allows malicious WebAssembly code to break out of this sandbox and access the host computer's memory. This could allow an attacker to crash the service, steal sensitive data from the host process, or potentially execute unauthorized commands on the underlying system.

Technologies: [Bytecodealliance Wasmtime](https://junglewise.ai/threats/technologies/bytecodealliance-wasmtime), [wasmtime (crates.io)](https://junglewise.ai/threats/technologies/wasmtime). Vendors: [Bytecodealliance](https://junglewise.ai/threats/vendors/bytecodealliance), [crates.io](https://junglewise.ai/threats/vendors/crates-io).

## Quick answers

**Is CVE-2026-34987 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-34987 being exploited in the wild.

**How severe is CVE-2026-34987?** Critical severity, with a CVSS score of 9.9, published 9 April 2026.

**Which products and versions are affected?** wasmtime (crates.io) SEMVER:introduced=0.0.0-0,fixed=36.0.7,introduced=37.0.0,fixed=42.0.2,introduced=43.0.0,fixed=43.0.1, SEMVER:introduced=25.0.0,fixed=36.0.7.

**Is there a fix for CVE-2026-34987?** Yes. wasmtime (crates.io): upgrade to 36.0.7, 43.0.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-34987 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-34987?** Check whether you run Bytecodealliance Wasmtime or wasmtime (crates.io) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- wasmtime (crates.io): upgrade to 36.0.7, 43.0.1 or later

  ```sh
  cargo update -p wasmtime
  ```


## Technical details

A vulnerability exists in the Winch (baseline) compiler backend of Wasmtime when targeting aarch64 (and theoretically x86-64). The root cause is an incorrect assumption that a 32-bit memory offset stored in a 64-bit register would have its upper bits cleared; when these bits are not cleared, the resulting address calculation can point outside the intended linear-memory sandbox. An attacker providing a specially crafted Wasm module can exploit this to perform out-of-bounds reads or writes against the host process memory, bypassing guard regions. This can lead to a process crash (DoS), data leakage, or arbitrary code execution (RCE). The vulnerability is only present when the non-default Winch compiler is enabled via '-Ccompiler=winch'.

## Affected products

- Bytecode Alliance Wasmtime >= 25.0.0, < 36.0.7; >= 37.0.0, < 42.0.2; 43.0.0

## Identifiers

- CVE-2026-34987
- GHSA-xx5w-cvp6-jv83
- RUSTSEC-2026-0095

## Timeline

- 2026-04-09: disclosed
- 2026-04-09: patched: Patched in versions 36.0.7, 42.0.2, and 43.0.1
- 2026-04-10: advisory

## References

- [https://api.github.com/users/shumbo](https://api.github.com/users/shumbo)
- [https://github.com/shumbo](https://github.com/shumbo)
- [https://api.github.com/users/shumbo/gists%7B/gist_id%7D](https://api.github.com/users/shumbo/gists%7B/gist_id%7D)
- [https://api.github.com/users/shumbo/repos](https://api.github.com/users/shumbo/repos)
- [https://avatars.githubusercontent.com/u/10496155?v=4](https://avatars.githubusercontent.com/u/10496155?v=4)
- [https://api.github.com/users/shumbo/events%7B/privacy%7D](https://api.github.com/users/shumbo/events%7B/privacy%7D)

## Related threats

- [RUSTSEC-2026-0422 - Typed functions with `i31` or `nullref` arguments may panic due to insufficient type-check](https://junglewise.ai/threats/rustsec-2026-0422-typed-functions-with-i31-or-nullref-arguments-may-0545dc2b)
- [Wasmtime type confusion with host-created struct types](https://junglewise.ai/threats/rustsec-2026-0423-cross-engine-type-confusion-with-host-created-struct-89fe3f0e)
- [Wasmtime unchecked memory usage in borrowed resource transfer](https://junglewise.ai/threats/rustsec-2026-0424-unchecked-memory-usage-in-transferring-borrowed-b297503c)
- [Wasmtime Cranelift aarch64 br_table sandbox escape](https://junglewise.ai/threats/cve-2026-115893-wasmtime-is-a-runtime-for-webassembly-prior-to-36-0-19-48-0-7)
- [Wasmtime component-model type-check bypass in stream/future lowering](https://junglewise.ai/threats/cve-2026-115892-wasmtime-is-a-runtime-for-webassembly-from-46-0-0-until-48-0-7)