CVE-2026-115889

Wasmtime: Bit-packing in Cranelift compiler causes silent overflow and miscompilation

In Wasmtime prior to v43, the Cranelift compiler backend bit-packs its SSA value indices in a way that can result in silent overflow in release builds, and result in incorrect value references in the IR resulting in arbitrary miscompilation. A very large Wasm function body, or one that is specially crafted to use many SSA values, can use this to force type-confusion or other arbitrary side-effects and escape the sandbox.

Cranelift was changed in Wasmtime v43 to use a full u32 index, with checks if that limit is exceeded, and so is no longer susceptible to this bug.

A Wasmtime engine with default settings (using the Cranelift backend) on any architecture/platform could experience a sandbox escape from a specially-crafted Wasm module.

An object is accessed using a type that is incompatible with its actual type, causing the runtime to interpret memory incorrectly. Typical impact: memory safety violations, unexpected behavior, or code execution.

CVE-2026-115889 has a CVSS score of 9.9 (Critical). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (36.0.19, 43.0.0); upgrading removes the vulnerable code path.

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Wasmtime v36.0.19 has been released which fixes this issue.

1. What is CVE-2026-115889? CVE-2026-115889 is a critical-severity type confusion vulnerability in wasmtime (rust), affecting versions < 36.0.19. It is fixed in 36.0.19, 43.0.0. An object is accessed using a type that is incompatible with its actual type, causing the runtime to interpret memory incorrectly.
2. How severe is CVE-2026-115889? CVE-2026-115889 has a CVSS score of 9.9 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of wasmtime are affected by CVE-2026-115889? wasmtime (rust) versions < 36.0.19 is affected.
4. Is there a fix for CVE-2026-115889? Yes. CVE-2026-115889 is fixed in 36.0.19, 43.0.0. Upgrade to this version or later.
5. Is CVE-2026-115889 exploitable, and should I be worried? Whether CVE-2026-115889 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-115889 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-115889?
   • Upgrade wasmtime to 36.0.19 or later
   • Upgrade wasmtime to 43.0.0 or later

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