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

What is CVE-2026-34987?

CVE-2026-34987 is a critical memory corruption vulnerability in bytecodealliance wasmtime, published on 9 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.

Official description

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.

Weakness: CWE-787, CWE-125

Which versions are affected by CVE-2026-34987?

7 open source packages ship an affected version, including wasmtime. It affects 1 product.

Products include Bytecodealliance Wasmtime.

How do I fix CVE-2026-34987?

Upgrade wasmtime to a fixed version for your release line: 36.0.7, 42.0.2 or 43.0.1.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2026-34987 being exploited?

There is no known exploitation of CVE-2026-34987 in the wild and no public exploit code.

Other vulnerabilities in bytecodealliance wasmtime

• CVE-2026-34971 Wasmtime miscompiled guest heap access enables sandbox escape on aarch64 Cranelift Critical 9.0
• CVE-2022-39393 Wasmtime vulnerable to data leakage between instances in the pooling allocator High 8.6
• CVE-2022-24791 Use after free in Wasmtime High 8.1
• CVE-2026-47261 Wasmtime: WASI path_open(TRUNCATE) bypasses `FilePerms::WRITE` host restriction High 7.5
• CVE-2026-34941 Wasmtime has a Heap OOB read in component model UTF-16 to latin1+utf16 string transcoding Medium 6.9

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2026-34987, GHSA-xx5w-cvp6-jv83, JLSEC-2026-1361, RUSTSEC-2026-0095, UBUNTU-CVE-2026-34987.