CVE-2026-34988
MediumCVSS 6.3Exploitation Probability (EPSS)
Low risk21th percentile - higher than 21% of all known CVEs
Summary
A vulnerability in Wasmtime (versions 28.0.0 to before 36.0.7, 42.0.2, and 43.0.1) in the pooling allocator implementation incorrectly determines whether to reset virtual memory permissions for linear memory. Due to a divergence between compile-time and runtime predicates, under specific configurations (memory_guard_size=0, memory_reservation < 4GiB, max_memory_size equal to memory_reservation) linear memory contents can leak between instances. This breaks WebAssembly semantics and the Wasmtime sandbox.
Risk Assessment
The vulnerability allows data leakage between WebAssembly instances, potentially exposing sensitive information processed by other instances. However, it requires a non-default pooling allocator configuration, so default settings are not affected.
Recommendation
Update Wasmtime to version 36.0.7, 42.0.2, or 43.0.1. If immediate update is not possible, avoid pooling allocator configurations with memory_guard_size=0 and memory_reservation < 4GiB, and ensure max_memory_size is not equal to memory_reservation.
Other vulnerabilities in Wasmtime
See all- CVE-2026-115893Critical
Wasmtime prior to 36.0.19, 48.0.7, and 49.0.4 has a flaw in the Cranelift compiler on aarch64. A br_table instruction with more than ~256,000 targets can be miscompiled, allowing a jump to arbitrary code and sandbox escape.
- CVE-2026-115889Critical
Wasmtime prior to 36.0.19 and 43.0.0 has a flaw in the Cranelift compiler where SSA value indices can silently overflow when compiling large Wasm functions. This can cause miscompilation, type confusion, and sandbox escape.
- CVE-2026-115384Critical
Wasmtime 39.0.0 before 48.0.4 and 49.0.0 before 49.0.2 contains a stack buffer overflow that allows malicious guests to write ~16KB to the native stack via unvalidated async callback result types. Attackers can supply components with mistyped component-model-async callback functions to overwrite return addresses, crashing the host or escaping the sandbox.
- CVE-2026-34987Critical
A vulnerability in the Wasmtime WebAssembly runtime affects its Winch (baseline) non-default compiler backend. It may allow properly constructed guest Wasm to access host memory outside of its linear-memory sandbox. Exploitation requires the Winch compiler (-Ccompiler=winch); the default Cranelift backend is not affected.
- CVE-2026-119143Medium
Wasmtime 46.0.0 before 48.0.6 and 49.0.0 before 49.0.3 contains a denial of service vulnerability that allows guest code to trigger a host panic through out-of-bounds zero-length stream operations. Malicious guests can perform zero-length component model stream reads or writes with unvalidated addresses, causing Direct{Source,Destination}::remaining to panic and crash WASIp3 embedders.
- CVE-2026-119142Medium
Wasmtime 46.0.0 before 48.0.6 and 49.0.0 before 49.0.3 contains a denial of service vulnerability where dropping the Future returned by TypedFunc::call_async can panic the host. When the guest task has not started, cleanup code searches only one location for work items and unwraps the resulting error, crashing embedders that cancel pending calls.
- CVE-2026-119141Medium
Wasmtime 46.0.0 before 48.0.6 and 49.0.0 before 49.0.3 contains a denial of service vulnerability that allows malicious Wasm modules to panic the host because the may_leave flag is not cleared during realloc. Attackers can call into the host from realloc while lowering stream or future results, such as cancelling the active read or re-waiting a waitable-set, crashing the host.
- CVE-2026-119140Medium
Wasmtime 46.0.0 before 48.0.6 and 49.0.0 before 49.0.3 contains a denial of service vulnerability that allows malicious components to panic the host via thread.yield in start functions. Attackers can supply a component whose start function calls an async-lowered import then thread.yield, causing an undisposed fiber drop that crashes embedders.
- CVE-2026-119139Medium
Wasmtime 46.0.0 before 48.0.6 and 49.0.0 before 49.0.3 contains a leftover assertion in component::Func::call that rejects guest exports using the async ABI option. An attacker who supplies a component with an async-lifted export can crash the host process when an embedder invokes that export through Func::call.
- CVE-2026-119138Medium
Wasmtime 46.0.0 before 48.0.6 and 49.0.0 before 49.0.3 contains a denial of service vulnerability that allows pending calls to trigger an assertion failure after a trap poisons the Store. Attackers supplying misbehaving guest code can cause a trap and let earlier calls proceed, crashing embedders that continue using the inconsistent Store.
Original NVD description (English source)
Wasmtime is a runtime for WebAssembly. From 28.0.0 to before 36.0.7, 42.0.2, and 43.0.1, 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. The pooling allocator must be in use, Config::memory_guard_size configuration option must be 0, Config::memory_reservation configuration must be less than 4GiB, and pooling allocator must be configured with max_memory_size the same as the memory_reservation value in order to exploit this vulnerability. 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 additionally breaks the sandbox that Wasmtime provides. Wasmtime is not vulnerable to this issue with its default settings, nor with the default settings of the pooling allocator, but embeddings are still allowed to configure these values to cause this vulnerability. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS