CVE-2026-34941
HighCVSS 8.1Exploitation Probability (EPSS)
Low risk31th percentile - higher than 31% of all known CVEs
Summary
Wasmtime contains a byte-length validation flaw when transcoding a UTF-16 string to the latin1+utf16 component-model encoding. It checks the number of code units instead of the byte length, which can cause reads beyond the end of WebAssembly linear memory. In the default configuration this triggers a segfault (denial of service), and with guard pages disabled it may allow reading host memory.
Risk Assessment
The vulnerability can be exploited to cause remote denial of service (segfault) via a malicious WebAssembly module. In nonstandard configurations without guard pages it may additionally leak host memory data.
Recommendation
Update Wasmtime to version 24.0.7, 36.0.7, 42.0.2, or 43.0.1. Do not disable guard pages in production configurations.
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. Prior to 24.0.7, 36.0.7, 42.0.2, and 43.0.1, Wasmtime contains a vulnerability where when transcoding a UTF-16 string to the latin1+utf16 component-model encoding it would incorrectly validate the byte length of the input string when performing a bounds check. Specifically the number of code units were checked instead of the byte length, which is twice the size of the code units. This vulnerability can cause the host to read beyond the end of a WebAssembly's linear memory in an attempt to transcode nonexistent bytes. In Wasmtime's default configuration this will read unmapped memory on a guard page, terminating the process with a segfault. Wasmtime can be configured, however, without guard pages which would mean that host memory beyond the end of linear memory may be read and interpreted as UTF-16. A host segfault is a denial-of-service vulnerability in Wasmtime, and possibly being able to read beyond the end of linear memory is additionally a vulnerability. Note that reading beyond the end of linear memory requires nonstandard configuration of Wasmtime, specifically with guard pages disabled. This vulnerability is fixed in 24.0.7, 36.0.7, 42.0.2, and 43.0.1.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS