CVE-2026-34945
mediumpriority · track · 26EPSS 0.003 · 25 %ileEUUSEPSS · scored 2026-10-27
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.
cvss
Technical severity, 0–10. One row per scoring source.
base score 0–10 · higher = more severe · hover a vector to decode⚠ sources disagree · low–medium · 0.6–6.5
ssvc
A stakeholder decision — act, attend, or track — from exploitation and impact.
weaknesses
The CWE weakness classes this flaw belongs to.
affected products (10)
Products and version ranges known to be affected.
advisories (2)
Vendor and ecosystem advisories describing this.
- GHSA-m9w2-8782-2946Wasmtime has host data leakage with 64-bit tables and Winch2026-04-09ghsa · osvUS
### Impact 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 information disclosure should not be possible in WebAssembly, violates spec semantics, and is a vulnerability in Wasmtime. ### Patches Wasmtime 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime. ### Workarounds Users of Cranelift are not affected by this issue, but users of Winch have no workarounds other than disabling the `Config::wasm_memory64` proposal.
- RUSTSEC-2026-0086Host data leakage with 64-bit tables and Winch2026-04-09vendor · osvUS
This is an entry in the RustSec database for the Wasmtime security advisory located at https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-m9w2-8782-2946 For more information see the GitHub-hosted security advisory.
also known as (5)
Other names and identifiers for this entity.
sources
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references (4)
External citations — patches, advisories, analysis.
- https://github.com/bytecodealliance/wasmtime/releasesPatchcnnvd
- https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-m9w2-8782-2946cnnvd
- https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-m9w2-8782-2946enisa_euvd
- https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-m9w2-8782-2946Vendor Advisorynvd
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