# CVE-2026-34946: Wasmtime's host panics when Winch compiler executes `table.fill` > A malicious WebAssembly guest can cause a host panic in Wasmtime when using the Winch compiler, leading to denial of service. - **Severity:** Medium 5.9 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`) - **Exploited in the wild:** Not known - **Login needed:** Any signed-in user - **Attackable:** Over the internet - **Victim action needed:** A user has to click or open something - **Fix available:** Yes - **Published:** 9 Apr 2026 - **Updated:** 9 Apr 2026 - **Also known as:** DEBIAN-CVE-2026-34946, GHSA-q49f-xg75-m9xw, JLSEC-2026-1359, RUSTSEC-2026-0089, UBUNTU-CVE-2026-34946 ## What is CVE-2026-34946? CVE-2026-34946 is a medium-severity denial of service vulnerability in bytecodealliance wasmtime, published on 9 Apr 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link. ### 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 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. Weakness: CWE-670 (Always-Incorrect Control Flow Implementation) ## Which versions are affected by CVE-2026-34946? 7 open source packages ship an affected version, including wasmtime. It affects 1 product. | Package | Ecosystem | 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 | | `wasmtime` | crates.io | From 43.0.0 | 43.0.1 | Products: Bytecodealliance Wasmtime. ## How do I fix CVE-2026-34946? Upgrade wasmtime to a fixed version for your release line: 36.0.7, 42.0.2 or 43.0.1. ## Is CVE-2026-34946 being exploited? There is no known exploitation of CVE-2026-34946 in the wild and no public exploit code. ## Other vulnerabilities in bytecodealliance wasmtime - [CVE-2026-34971](https://insights.sbomify.com/CVE-2026-34971/): Wasmtime miscompiled guest heap access enables sandbox escape on aarch64 Cranelift (critical 9.0) - [CVE-2026-34987](https://insights.sbomify.com/CVE-2026-34987/): Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access (critical 9.0) - [CVE-2022-39393](https://insights.sbomify.com/CVE-2022-39393/): Wasmtime vulnerable to data leakage between instances in the pooling allocator (high 8.6) - [CVE-2022-24791](https://insights.sbomify.com/CVE-2022-24791/): Use after free in Wasmtime (high 8.1) - [CVE-2026-47261](https://insights.sbomify.com/CVE-2026-47261/): Wasmtime: WASI path_open(TRUNCATE) bypasses `FilePerms::WRITE` host restriction (high 7.5) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-34946 and fix it if it is affected. What it is: CVE-2026-34946 is a medium-severity denial of service vulnerability in bytecodealliance wasmtime, published on 9 Apr 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link. Exploitation: There is no known exploitation of CVE-2026-34946 in the wild and no public exploit code. Affected packages and fixed versions: - wasmtime (crates.io): affected from 0.0.0-0; fixed in 36.0.7, 42.0.2, 43.0.1 - wasmtime (crates.io): affected from 25.0.0; fixed in 36.0.7 - wasmtime (crates.io): affected from 25.0.0; fixed in 36.0.7 - wasmtime (crates.io): affected from 37.0.0; fixed in 42.0.2 - wasmtime (crates.io): affected from 37.0.0; fixed in 42.0.2 - wasmtime (crates.io): affected from 43.0.0; fixed in 43.0.1 - wasmtime (crates.io): affected from 43.0.0; fixed in 43.0.1 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-34946/ (Markdown: https://insights.sbomify.com/CVE-2026-34946/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-34946 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-34946 - OSV: https://osv.dev/vulnerability/CVE-2026-34946 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-34946/ Summaries are generated and may contain mistakes; the official description above is the record.