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

Critical severity GitHub Reviewed Published Oct 19, 2026 in bytecodealliance/wasmtime • Updated Oct 23, 2026

Package

cargo wasmtime (Rust)

Affected versions

< 36.0.19
>= 37.0.0, < 43.0.0

Patched versions

36.0.19
43.0.0

Description

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.

References

@pchickey pchickey published to bytecodealliance/wasmtime Oct 19, 2026
Published to the GitHub Advisory Database Oct 23, 2026
Reviewed Oct 23, 2026
Last updated Oct 23, 2026

Severity

Critical

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(39th percentile)

Weaknesses

Numeric Truncation Error

Truncation errors occur when a primitive is cast to a primitive of a smaller size and data is lost in the conversion. Learn more on MITRE.

Access of Resource Using Incompatible Type ('Type Confusion')

The product allocates or initializes a resource such as a pointer, object, or variable using one type, but it later accesses that resource using a type that is incompatible with the original type. Learn more on MITRE.

CVE ID

CVE-2026-115889

GHSA ID

GHSA-897f-g5gm-fmxm

Source code

Credits

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