Wasmtime has data leakage between pooling allocator instances
Date: 2026-04
Affected: rust/wasmtime, rust/wasmtime, rust/wasmtime
Attack vector: info-disclosure
Category: vulnerability-disclosure
Severity: Low
CVE: CVE-2026-34988
CWE: CWE-119

### Impact Wasmtime's implementation of its pooling allocator contains a bug where in certain configurations the contents of linear memory can be leaked from one instance to the next. The implementation of resetting the virtual memory permissions for linear memory used the wrong predicate to determine if resetting was necessary, where the compilation process used a different predicate. This divergence meant that the pooling allocator incorrectly deduced at runtime that resetting virtual memory permissions was not necessary while compile-time determine that virtual memory could be relied upon. Exposing this bug requires specific configuration values to be used. If any of these configurations are not applicable then this bug does not happen: * The pooling allocator must be in use. * The `Config::memory_guard_size` configuration option must be 0. * The `Config::memory_reservation` configuration must be less than 4GiB. * The pooling allocator must be configured with `max_memory_size` the same as the `memory_reservation` value. If all of these conditions are applicable then when a linear memory is reused the VM permissions of the previous iteration are not reset. This means that the compiled code, which is assuming out-of-bounds loads will segfault, will not actually segfault and can read the previous contents of linear memory if it was previously mapped. This represents a data leakage vulnerability between guest WebAssembly instances which breaks WebAssembly's semantics and


Mitigations:

Architectural layers (MAESTRO):