Executive brief
Wasmtime is a tool used to run WebAssembly applications in a secure, isolated 'sandbox' environment. A flaw in its compilation process on ARM64 systems allows a malicious application to break out of this sandbox and gain full access to the host computer's memory. This could lead to the theft of sensitive data or the ability to modify system files and operations.
Technical details
A miscompilation bug exists in Wasmtime's Cranelift backend for aarch64 during instruction selection for specific heap access patterns. The vulnerability occurs when lowering a load operation of the form 'load(iadd(base, ishl(index, amt)))' where a constant 'amt' is incorrectly masked. This leads to a divergence between the address used for explicit bounds checks and the address used for the actual load, enabling a guest module to bypass memory safety boundaries. The issue specifically affects 64-bit WebAssembly linear memories when Spectre mitigations or signals-based-traps are disabled. Successful exploitation provides an arbitrary read/write primitive against host memory, resulting in a complete sandbox escape. Patches are available in versions 36.0.7, 42.0.2, and 43.0.1.
Affected products
- Bytecode Alliance Wasmtime >= 32.0.0, < 36.0.7; >= 37.0.0, < 42.0.2; 43.0.0
Timeline
- 2026-04-09: disclosed
- 2026-04-09: advisory
- 2026-04-09: patched