CVE-2026-34943 Wasmtime panics when lifting `flags` component value
A guest-controlled panic in Wasmtime when lifting flags values allows attackers to cause a denial of service against the host runtime.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-34943?
CVE-2026-34943 is a medium-severity denial of service vulnerability in bytecodealliance wasmtime, published on 9 Apr 2026. An attacker needs administrator access to exploit it, 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 contains a possible panic which can happen when a flags-typed component model value is lifted with the Val type. If bits are set outside of the set of flags the component model specifies that these bits should be ignored but Wasmtime will panic when this value is lifted. This panic only affects wasmtime's implementation of lifting into Val, not when using the flags! macro. This additionally only affects flags-typed values which are part of a WIT interface. This has the risk of being a guest-controlled panic within the host which Wasmtime considers a DoS vector. This vulnerability is fixed in 24.0.7, 36.0.7, 42.0.2, and 43.0.1.
Weakness: CWE-248
Which versions are affected by CVE-2026-34943?
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-2026-34943?
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-2026-34943 being exploited?
There is no known exploitation of CVE-2026-34943 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-2026-34943, GHSA-m758-wjhj-p3jq, JLSEC-2026-1356, RUSTSEC-2026-0085, UBUNTU-CVE-2026-34943.