CVE-2026-34945 Wasmtime leaks host data with 64-bit tables and Winch
A bug in Wasmtime's Winch compiler allows WebAssembly guests to read sensitive host stack data when using 64-bit tables.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-34945?
CVE-2026-34945 is a low-severity vulnerability in bytecodealliance wasmtime, published on 9 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime's Winch compiler contains a bug where a 64-bit table, part of the memory64 proposal of WebAssembly, incorrectly translated the table.size instruction. This bug could lead to disclosing data on the host's stack to WebAssembly guests. The host's stack can possibly contain sensitive data related to other host-originating operations which is not intended to be disclosed to guests. This bug specifically arose from a mistake where the return value of table.size was statically typed as a 32-bit integer, as opposed to consulting the table's index type to see how large the returned register could be. When combined with details about Wnich's ABI, such as multi-value returns, this can be combined to read stack data from the host, within a guest. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.
Weakness: CWE-681
Which versions are affected by CVE-2026-34945?
7 open source packages ship an affected version, including wasmtime. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
wasmtime crates.io | From 0.0.0-0 | 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 |
wasmtime crates.io | From 37.0.0 | 42.0.2 |
wasmtime crates.io | From 43.0.0 | 43.0.1 |
1 more packages
wasmtime crates.io | From 43.0.0 | 43.0.1 |
Linux distribution packages (1)
tree-sitter openSUSE:Tumbleweed | All versions | 0.26.8-1.1 |
Products include Bytecodealliance Wasmtime.
How do I fix CVE-2026-34945?
Upgrade wasmtime to a fixed version for your release line: 36.0.7, 42.0.2 or 43.0.1.
Is CVE-2026-34945 being exploited?
There is no known exploitation of CVE-2026-34945 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-2026-34945, GHSA-m9w2-8782-2946, JLSEC-2026-1358, RUSTSEC-2026-0086, UBUNTU-CVE-2026-34945, openSUSE-SU-2026:10598-1.