CVE-2026-115889: Wasmtime: Bit-packing in Cranelift compiler causes silent overflow and miscompilation

Should you act?

Critical, preconditions apply

Severe if exploited, but exploitation needs conditions that may not apply to you. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog. That can change quickly after a public write-up; re-check if you are affected.
• Needs an account (low (any account)). Exposure depends on who can log in.
• Only applies if your dependency tree includes wasmtime (crates.io (Rust)) < 36.0.19, fixed in 36.0.19; wasmtime (crates.io (Rust)) >= 37.0.0, < 43.0.0, fixed in 43.0.0. Check your lockfile, including transitive dependencies.

What it is

Wasmtime is a runtime for WebAssembly. Prior to 36.0.19 and 43.0.0, the Cranelift compiler backend bit-packs SSA value indices in a way that can silently overflow in release builds when compiling a sufficiently large or specially crafted Wasm function with many SSA values. The overflow can make Cranelift reference incorrect IR values, causing arbitrary miscompilation, type confusion, and other attacker-controlled effects that may escape the Wasm sandbox on any architecture using the default Cranelift backend. This issue is fixed in versions 36.0.19 and 43.0.0.

From the GitHub 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.

Weakness type: CWE-197, CWE-843.

Product: wasmtime (crates.io (Rust)) · Assigned by: GitHub_M · Published: 2026-10-23

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: low (any account)
• User interaction: none
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).

Timeline

• 2026-10-23 — CVE record last updated
• 2026-10-23 — CVE record published by GitHub_M
• 2026-10-23 — GitHub-reviewed advisory GHSA-897f-g5gm-fmxm published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-897f-g5gm-fmxm github.com
2. CVE.org record CVE-2026-115889 cve.org
3. NVD entry CVE-2026-115889 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com
6. github.com github.com
7. github.com github.com
8. rustsec.org rustsec.org

Critical crates.io (Rust) advisories · Today’s Security Digest