# CVE-2026-34943: Wasmtime panics when lifting `flags` component value > A guest-controlled panic in Wasmtime when lifting flags values allows attackers to cause a denial of service against the host runtime. - **Severity:** Medium 5.6 (`CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:A/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`) - **Exploited in the wild:** Not known - **Login needed:** An administrator - **Attackable:** Over the internet - **Victim action needed:** A user has to click or open something - **Fix available:** Yes - **Published:** 9 Apr 2026 - **Updated:** 13 Apr 2026 - **Also known as:** DEBIAN-CVE-2026-34943, GHSA-m758-wjhj-p3jq, JLSEC-2026-1356, RUSTSEC-2026-0085, UBUNTU-CVE-2026-34943 ## What is CVE-2026-34943? CVE-2026-34943 is a medium-severity denial of service vulnerability in bytecodealliance wasmtime, published on 9 Apr 2026. An attacker needs administrator access to exploit it, and only if a user opens a file or follows a link. ### Official description Wasmtime is a runtime for WebAssembly. Prior to 24.0.7, 36.0.7, 42.0.2, and 43.0.1, Wasmtime contains a possible panic which can happen when a flags-typed component model value is lifted with the Val type. If bits are set outside of the set of flags the component model specifies that these bits should be ignored but Wasmtime will panic when this value is lifted. This panic only affects wasmtime's implementation of lifting into Val, not when using the flags! macro. This additionally only affects flags-typed values which are part of a WIT interface. This has the risk of being a guest-controlled panic within the host which Wasmtime considers a DoS vector. This vulnerability is fixed in 24.0.7, 36.0.7, 42.0.2, and 43.0.1. Weakness: CWE-248 (Uncaught Exception) ## Which versions are affected by CVE-2026-34943? 9 open source packages ship an affected version, including wasmtime. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `wasmtime` | crates.io | All versions | 24.0.7 | | `wasmtime` | crates.io | All versions | 24.0.7 | | `wasmtime` | crates.io | From 0.0.0-0 | 24.0.7, 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-2026-34943? Upgrade wasmtime to a fixed version for your release line: 24.0.7, 36.0.7, 42.0.2 or 43.0.1. ## Is CVE-2026-34943 being exploited? There is no known exploitation of CVE-2026-34943 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-2026-34943 and fix it if it is affected. What it is: CVE-2026-34943 is a medium-severity denial of service vulnerability in bytecodealliance wasmtime, published on 9 Apr 2026. An attacker needs administrator access to exploit it, and only if a user opens a file or follows a link. Exploitation: There is no known exploitation of CVE-2026-34943 in the wild and no public exploit code. Affected packages and fixed versions: - wasmtime (crates.io): affected all versions; fixed in 24.0.7 - wasmtime (crates.io): affected all versions; fixed in 24.0.7 - wasmtime (crates.io): affected from 0.0.0-0; fixed in 24.0.7, 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-2026-34943/ (Markdown: https://insights.sbomify.com/CVE-2026-34943/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-34943 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-34943 - OSV: https://osv.dev/vulnerability/CVE-2026-34943 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-34943/ Summaries are generated and may contain mistakes; the official description above is the record.