CVE-2026-34983: Wasmtime Use-After-Free Vulnerability

CVE-2026-34983 Overview

CVE-2026-34983 is a use-after-free vulnerability affecting Wasmtime, a runtime for WebAssembly. In version 43.0.0, cloning a wasmtime::Linker is unsound and can result in use-after-free bugs. While this vulnerability is not controllable by guest Wasm programs, it can be triggered by a specific sequence of embedder API calls made by the host application.

Critical Impact

Potential memory corruption and undefined behavior in applications embedding Wasmtime runtime version 43.0.0 through improper Linker cloning operations.

Affected Products

• Wasmtime version 43.0.0

Discovery Timeline

• 2026-04-09 - CVE CVE-2026-34983 published to NVD
• 2026-04-09 - Last updated in NVD database

Technical Details for CVE-2026-34983

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption vulnerability that occurs when memory is accessed after it has been freed. The bug manifests specifically in the wasmtime::Linker cloning functionality where the cloned linker maintains references to memory owned by the original linker instance.

The vulnerability requires a specific sequence of operations to trigger: first, the embedder must clone a wasmtime::Linker instance; second, the original linker must be dropped (deallocated); and third, the cloned linker must be used after the original has been freed. This creates a dangling pointer scenario where the cloned linker attempts to access memory that no longer belongs to the process.

Notably, this vulnerability cannot be exploited by malicious WebAssembly code running within the sandbox. The exploitation vector is limited to host applications that incorrectly manage Linker lifecycle through the embedder API, requiring physical access to the system and privileged user interaction.

Root Cause

The root cause of CVE-2026-34983 lies in the implementation of the Clone trait for wasmtime::Linker in version 43.0.0. When a Linker is cloned, the new instance does not properly copy or take ownership of the underlying data structures. Instead, it appears to maintain shallow references to memory owned by the original Linker. When the original Linker is dropped, this memory is deallocated while the cloned instance still holds references to it, resulting in a classic use-after-free condition.

Attack Vector

The attack vector for this vulnerability requires physical access (AV:P) to the system running a Wasmtime-embedded application. The attacker must have high-level privileges and the attack requires active user interaction. The exploitation path involves manipulating the host application to perform the following sequence:

1. Clone a wasmtime::Linker instance
2. Trigger deallocation of the original linker
3. Force usage of the cloned linker instance

This specific sequence causes the cloned linker to access freed memory, potentially allowing an attacker to corrupt memory, crash the application, or in some cases, achieve code execution depending on how the freed memory is subsequently used.

Detection Methods for CVE-2026-34983

Indicators of Compromise

• Application crashes or segmentation faults in Wasmtime-embedded applications during Linker operations
• Memory corruption indicators such as unexpected behavior following Linker clone operations
• Core dumps showing use-after-free access patterns in Wasmtime library code

Detection Strategies

• Review application code for patterns that clone wasmtime::Linker and subsequently drop the original instance
• Enable address sanitizer (ASan) during testing to detect use-after-free conditions
• Monitor for abnormal process terminations in applications embedding Wasmtime 43.0.0

Monitoring Recommendations

• Implement memory debugging tools in development and staging environments running Wasmtime
• Review application logs for crashes or undefined behavior coinciding with Linker lifecycle operations
• Track Wasmtime version usage across your infrastructure to identify affected deployments

How to Mitigate CVE-2026-34983

Immediate Actions Required

• Upgrade Wasmtime to version 43.0.1 or later immediately
• Audit application code for Linker cloning patterns that may trigger the vulnerability
• Avoid cloning wasmtime::Linker instances in applications that cannot be immediately upgraded

Patch Information

The vulnerability is fixed in Wasmtime version 43.0.1. The fix addresses the unsound Clone implementation for wasmtime::Linker to ensure proper memory ownership semantics when cloning Linker instances. For more details, see the GitHub Security Advisory.

Workarounds

• Refactor application code to avoid cloning wasmtime::Linker instances until the upgrade is complete
• Ensure the original Linker instance is not dropped while any clones are still in use
• If cloning is necessary, maintain a reference to the original Linker for the entire lifetime of any clones