Wasmtime provides unsound API access to a WebAssembly shared linear memory

### Impact Wasmtime's Rust embedder API contains an unsound interaction where a WebAssembly shared linear memory could be viewed as a type which provides safe access to the host (Rust) to the contents of the linear memory. This is not sound for shared linear memories, which could be modified in parallel, and this could lead to a data race in the host. Wasmtime has a `wasmtime::Memory` type which represents linear memories in a WebAssembly module. Wasmtime also has `wasmtime::SharedMemory`, however, which represents shared linear memories introduced in the WebAssembly `threads` proposal. The API of `SharedMemory` does not provide accessors which return `&[u8]` in Rust, for example, as that's not a sound type signature when other threads could be modifying memory. The `wasmtime::Memory` type, however, does provide this API as it's intended to be used with non-shared memories where static knowledge is available that no concurrent or parallel reads or writes are happening. This means that it's not sound to represent a shared linear memory with `wasmtime::Memory` and it must instead be represented with `wasmtime::SharedMemory`. There were two different, erroneous, methods of creating a `wasmtime::Memory` which represents a shared memory however: 1. The [`wasmtime::Memory::new` constructor](https://docs.rs/wasmtime/latest/wasmtime/struct.Memory.html#method.new) takes a `MemoryType` which could be `shared`. This function did not properly reject shared memory types and require usage of [`SharedMemory::new`](https://docs.rs/wasmtime/latest/wasmtime/struct.SharedMemory.html#method.new) instead. 2. Capturing a core dump with WebAssembly would expose all linear memories a `wasmtime::Memory`. This means that a core dump would perform an unsynchronized read of shared linear memory, possibly leading to data races. This is a bug in Wasmtime's safe Rust API. It should not be possible to cause unsoundness with Wasmtime's embedding API if `unsafe` is not used. Embeddings which do not use the wasm threads proposal nor created shared memories nor actually share shared memories across threads are unaffected. Only if shared memories are created across threads might an embedding be affected. ### Patches Patch releases have been issued for [all supported versions of Wasmtime](https://docs.wasmtime.dev/stability-release.html), notably: 24.0.5, 36.0.3, 37.0.3, and 38.0.4. These releases reject creation of shared memories via `Memory::new` and shared memories are now excluded from core dumps. ### Workarounds Embeddings affected by this issue should use `SharedMemory::new` instead of `Memory::new` to create shared memories. Affected embeddings should also disable core dumps if they are unable to upgrade. Note that core dumps are disabled by default but the wasm threads proposal (and shared memory) is enabled by default. It's recommended to upgrade to a patched version of Wasmtime, however.

Source published

12 Nov 2025 21:36:29 UTC

Source modified

10 Sep 2026 03:50:31 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:L/AC:H/PR:H/UI:R/S:U/C:N/I:L/A:N

Publisher rating · LOW