critical · 9.9 Patched

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

CVE-2026-115889 · GHSA-897f-g5gm-fmxm · RUSTSEC-2026-0380 · published 19 Oct 2026

How serious is it

  • Severity: critical, CVSS score 9.9 out of 10.
  • No exploit prediction (EPSS) score yet.
  • Not on the CISA list of exploited vulnerabilities.
  • No public exploit in Exploit-DB.

Is your app affected by CVE-2026-115889?

Paste this prompt into Claude Code, Cursor or any AI coding tool: it runs a free Shippable scan of your project and fixes what it finds. Built with Lovable, Bolt or v0? Scan your live app. Or upload your package file.

Who is affected

  • wasmtime (crates.io) < 36.0.19
  • wasmtime (crates.io) >= 37.0.0, < 43.0.0

How to check

  1. Run cargo tree -i wasmtime in your project to see the installed version, including copies pulled in by other packages.
  2. Or paste your lockfile into the checker: it lists every package with a known vulnerability, this one included.

How to fix

Copies a prompt for Claude Code, Cursor or Codex: it checks your lockfiles for this and upgrades it.
  1. On < 36.0.19: Upgrade wasmtime to 36.0.19 or later, commit the lockfile and redeploy.
  2. On >= 37.0.0, < 43.0.0: Upgrade wasmtime to 43.0.0 or later, commit the lockfile and redeploy.
  3. If a vulnerable version was live, check your logs and rotate the secrets that server could read.

The advisory

## Summary 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. ## Impact 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. ## Patches Wasmtime v36.0.19 has been released which fixes this issue.

Check your whole app

Dependencies are only half of it. Free and read-only: before you push, run npx shippable-dev scan localhost:3000 in your project folder, no account needed. For your live app, you prove it's yours with a one-line tag first.

References

Merged from GitHub Advisory Database, OSV. Last updated 26 Oct 2026. All vulnerabilities · RSS · JSON

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