CVE-2026-34946: Wasmtime's host panics when Winch compiler executes `table.fill`
Impact
Wasmtime's Winch compiler contains a vulnerability where the compilation of the table.fill instruction can result in a host panic. This means that a valid guest can be compiled with Winch, on any architecture, and cause the host to panic. This represents a denial-of-service vulnerability in Wasmtime due to guests being able to trigger a panic.
The specific issue is that a historical refactoring, #11254, changed how compiled code referenced tables within the table. instructions. This refactoring forgot to update the Winch code paths associated as well, meaning that Winch was using the wrong indexing scheme. Due to the feature support of Winch the only problem that can result is tables being mixed up or nonexistent tables being used, meaning that the guest is limited to panicking the host (using a nonexistent table), or executing spec-incorrect behavior and modifying the wrong table.
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 for users of Winch there is no workaround for this bug. Hosts are recommended to updated to a patched version of Wasmtime.
Other sources
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 vulnerability where the compilation of the table.fill instruction can result in a host panic. This means that a valid guest can be compiled with Winch, on any architecture, and cause the host to panic. This represents a denial-of-service vulnerability in Wasmtime due to guests being able to trigger a panic. The specific issue is that a historical refactoring changed how compiled code referenced tables within the table. instructions. This refactoring forgot to update the Winch code paths associated as well, meaning that Winch was using the wrong indexing scheme. Due to the feature support of Winch the only problem that can result is tables being mixed up or nonexistent tables being used, meaning that the guest is limited to panicking the host (using a nonexistent table), or executing spec-incorrect behavior and modifying the wrong table. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.
— MITRE
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-34946?
The severity of CVE-2026-34946 is considered high due to its potential to cause host panics.
How do I fix CVE-2026-34946?
To fix CVE-2026-34946, upgrade Wasmtime to version 43.0.1 or later.
What versions of Wasmtime are affected by CVE-2026-34946?
CVE-2026-34946 affects Wasmtime versions 25.0.0 to 42.0.2 and 43.0.0.
What impact does CVE-2026-34946 have on applications?
CVE-2026-34946 can lead to unexpected application crashes due to host panics triggered by the `table.fill` instruction.
Is CVE-2026-34946 specific to any architecture?
CVE-2026-34946 is not architecture-specific and can impact any architecture running Wasmtime.