CVE-2026-34987
CriticalCVSS 9.9Exploitation Probability (EPSS)
Low risk20th percentile - higher than 20% of all known CVEs
Summary
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.
Risk Assessment
This can result in a host process segmentation fault (DoS), an arbitrary data leak from the host process, or, with a write, potentially arbitrary RCE. The risk applies only to environments explicitly using the Winch backend.
Recommendation
Update Wasmtime to version 36.0.7, 42.0.2, or 43.0.1. Until patched, avoid using the Winch compiler and stay on the default Cranelift backend if possible.
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-2026-119143Medium
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.
- CVE-2026-119137Medium
Wasmtime 42.0.0 before 48.0.6 and 49.0.0 before 49.0.3 contains a stack overflow vulnerability that allows guests to crash the host through recursive type drops. Guests can define a long chain of recursive types so that removing the top-level type triggers unbounded recursive drops, exhausting the host stack.
Original NVD description (English source)
Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime with its Winch (baseline) non-default compiler backend may allow properly constructed guest Wasm to access host memory outside of its linear-memory sandbox. This vulnerability requires use of the Winch compiler (-Ccompiler=winch). By default, Wasmtime uses its Cranelift backend, not Winch. With Winch, the same incorrect assumption is present in theory on both aarch64 and x86-64. The aarch64 case has an observed-working proof of concept, while the x86-64 case is theoretical and may not be reachable in practice. This Winch compiler bug can allow the Wasm guest to access memory before or after the linear-memory region, independently of whether pre- or post-guard regions are configured. The accessible range in the initial bug proof-of-concept is up to 32KiB before the start of memory, or ~4GiB after the start of memory, independently of the size of pre- or post-guard regions or the use of explicit or guard-region-based bounds checking. However, the underlying bug assumes a 32-bit memory offset stored in a 64-bit register has its upper bits cleared when it may not, and so closely related variants of the initial proof-of-concept may be able to access truly arbitrary memory in-process. This could result in a host process segmentation fault (DoS), an arbitrary data leak from the host process, or with a write, potentially an arbitrary RCE. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS