Bug 2457003 (CVE-2026-34987)

Summary: CVE-2026-34987 wasmtime: bytecodealliance/wasmtime: Wasmtime: Arbitrary code execution via incorrect memory offset handling in Winch compiler
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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A flaw was found in Wasmtime, a runtime for WebAssembly. When using its non-default Winch compiler backend, a properly constructed guest WebAssembly (Wasm) module can exploit an incorrect assumption in how memory offsets are handled. This allows the guest Wasm to access host memory outside its designated sandbox. Such access could lead to a denial of service, arbitrary data leakage from the host process, or potentially arbitrary code execution.
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Description OSIDB Bzimport 2026-04-09 19:02:26 UTC 
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.