Executive brief
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.
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
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