CVE-2026-34987
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.
- CVSS 4.0
- 9.0
CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H - CVSS 3.1
- 9.9
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H - EPSS
- 0.3% chance of exploitation in the next 30 days; higher than 20% of CVEs (scores of 2026-10-26)
- CWEs
- CWE-125, CWE-787
- Published
- Modified
- Record from
- the NVD, synced
Affected
- bytecodealliance:wasmtime
Packages (OSV)
| Advisory | Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|---|
| GHSA-xx5w-cvp6-jv83 | crates.io | wasmtime | >=25.0.0 <36.0.7 | 36.0.7 |
| GHSA-xx5w-cvp6-jv83 | crates.io | wasmtime | >=37.0.0 <42.0.2 | 42.0.2 |
| GHSA-xx5w-cvp6-jv83 | crates.io | wasmtime | >=43.0.0 <43.0.1 | 43.0.1 |
| RUSTSEC-2026-0095 | crates.io | wasmtime | >=0.0.0-0 <36.0.7 >=37.0.0 <42.0.2 >=43.0.0 <43.0.1 | 36.0.7, 42.0.2, 43.0.1 |