CVE-2026-34942 Wasmtime panics when transcoding misaligned utf-16 strings
A malicious WebAssembly guest can trigger a host panic in Wasmtime by sending misaligned UTF-16 strings, causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-34942?
CVE-2026-34942 is a medium-severity denial of service vulnerability in bytecodealliance wasmtime, published on 9 Apr 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Official description
Wasmtime is a runtime for WebAssembly. Prior to 24.0.7, 36.0.7, 42.0.2, and 43.0.1, Wasmtime's implementation of transcoding strings into the Component Model's utf16 or latin1+utf16 encodings improperly verified the alignment of reallocated strings. This meant that unaligned pointers could be passed to the host for transcoding which would trigger a host panic. This panic is possible to trigger from malicious guests which transfer very specific strings across components with specific addresses. Host panics are considered a DoS vector in Wasmtime as the panic conditions are controlled by the guest in this situation. This vulnerability is fixed in 24.0.7, 36.0.7, 42.0.2, and 43.0.1.
Weakness: CWE-129
Which versions are affected by CVE-2026-34942?
9 open source packages ship an affected version, including wasmtime. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
wasmtime crates.io | All versions | 24.0.7 |
wasmtime crates.io | All versions | 24.0.7 |
wasmtime crates.io | From 0.0.0-0 | 24.0.7, 36.0.7, 42.0.2, 43.0.1 |
wasmtime crates.io | From 25.0.0 | 36.0.7 |
wasmtime crates.io | From 25.0.0 | 36.0.7 |
wasmtime crates.io | From 37.0.0 | 42.0.2 |
3 more packages
wasmtime crates.io | From 37.0.0 | 42.0.2 |
wasmtime crates.io | From 43.0.0 | 43.0.1 |
wasmtime crates.io | From 43.0.0 | 43.0.1 |
Products include Bytecodealliance Wasmtime.
How do I fix CVE-2026-34942?
Upgrade wasmtime to a fixed version for your release line: 24.0.7, 36.0.7, 42.0.2 or 43.0.1.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-34942 being exploited?
There is no known exploitation of CVE-2026-34942 in the wild and no public exploit code.
Other vulnerabilities in bytecodealliance wasmtime
- CVE-2026-34971 Wasmtime miscompiled guest heap access enables sandbox escape on aarch64 Cranelift
- CVE-2026-34987 Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access
- CVE-2022-39393 Wasmtime vulnerable to data leakage between instances in the pooling allocator
- CVE-2022-24791 Use after free in Wasmtime
- CVE-2026-47261 Wasmtime: WASI path_open(TRUNCATE) bypasses `FilePerms::WRITE` host restriction
Sources
Also known as DEBIAN-CVE-2026-34942, GHSA-jxhv-7h78-9775, JLSEC-2026-1355, RUSTSEC-2026-0092, UBUNTU-CVE-2026-34942.