CVE-2026-115889 in Wasmtime
Summary
by MITRE • 10/24/2026
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.
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Analysis
by VulDB Data Team • 10/24/2026
The vulnerability identified within the WebAssembly runtime environment, specifically affecting the Cranelift compiler backend prior to versions 36.0.19 and 43.0.0, represents a critical failure in intermediate representation handling during code generation. Wasmtime serves as a foundational execution engine for WebAssembly modules across various platforms, relying heavily on its internal compilers to translate high-level bytecode into efficient machine instructions. The Cranelift backend is the default compiler used by many of these implementations due to its performance characteristics and JIT compilation capabilities. However, within this architecture, SSA (Static Single Assignment) value indices are bit-packed for storage efficiency. This optimization introduces a latent arithmetic flaw where the index values can silently overflow in release builds when processing sufficiently large or specially crafted WebAssembly functions that contain an excessive number of SSA values. The silent nature of this overflow is particularly dangerous because it does not trigger standard runtime exceptions or assertions, allowing corrupted state to propagate through subsequent compilation stages without detection by developers or automated testing suites during normal operation.
From a technical perspective, the core flaw lies in how Cranelift manages memory references for intermediate representation nodes. When an SSA value index exceeds its allocated bit width due to overflow, it wraps around to a lower, incorrect numerical value. Consequently, when the compiler attempts to reference specific IR values based on these corrupted indices, it retrieves data associated with entirely different variables or instructions. This misalignment leads directly to arbitrary miscompilation, where the generated machine code performs operations unrelated to the original WebAssembly intent. More critically, this mechanism facilitates type confusion, a severe security vulnerability class defined by CWE-843, wherein objects are treated as having types they do not possess. In the context of WebAssembly, which relies on strict memory safety and sandboxing boundaries, such miscompilation can result in out-of-bounds memory accesses or unauthorized control flow transfers. The attacker-controlled nature of this effect means that a maliciously crafted Wasm module could exploit these compiler errors to bypass security checks embedded within the runtime environment.
The operational impact of this vulnerability is profound, as it potentially allows an attacker to escape the WebAssembly sandbox on any architecture utilizing the default Cranelift backend. Sandbox escapes are among the most severe outcomes in web-based execution environments because they grant code privileges equivalent to those of the host process or browser engine. By leveraging type confusion and miscompilation, a malicious actor could execute arbitrary native code, read sensitive memory regions belonging to other tabs or processes, or modify critical application state that was previously protected by WebAssembly's isolation guarantees. This risk is amplified in release builds where debug assertions are disabled, meaning the vulnerability remains dormant until triggered by specific input patterns designed to maximize SSA value counts. The lack of immediate error reporting ensures that compromised binaries may be deployed and executed before the underlying compiler defect is recognized, posing a significant threat to applications relying on Wasmtime for secure execution of untrusted code.
Mitigation strategies primarily involve upgrading the WebAssembly runtime environment to patched versions where this arithmetic overflow has been resolved. Specifically, users must ensure they are running Cranelift backend versions 36.0.19 or later, as well as any related releases at version 43.0.0 and above that incorporate these fixes. For organizations unable to immediately update their runtime dependencies, implementing strict input validation on WebAssembly modules before compilation can help mitigate the risk by preventing excessively large functions with high SSA value counts from reaching the vulnerable compiler path. Additionally, enabling debug builds or assertion checks in development environments can expose such overflow conditions early in the software lifecycle, although this is not a viable production solution due to performance overheads. Security teams should also monitor for new advisories related to Cranelift and Wasmtime, as further hardening measures may be introduced to prevent similar index-based vulnerabilities in future releases.
This incident aligns with several established security frameworks that categorize the nature of these flaws. The underlying issue is best described by CWE-190 Integer Overflow or Wraparound, which occurs when an arithmetic operation exceeds the maximum value a variable can hold, leading to unexpected results. Furthermore, the exploitation technique involving type confusion maps directly to CWE-843, highlighting how incorrect typing leads to security vulnerabilities. In terms of offensive tactics, this vulnerability facilitates techniques associated with MITRE ATT&CK ID T1059 Command and Scripting Interpreter if used in conjunction with other exploits, but more specifically it enables privilege escalation within the sandboxed environment akin to T1620 Reflective Code Loading or escape mechanisms that allow lateral movement. Understanding these classifications helps security professionals prioritize remediation efforts and communicate the severity of the flaw to stakeholders effectively. By addressing this vulnerability through timely updates and rigorous input validation, organizations can maintain the integrity of their WebAssembly execution environments and prevent potential sandbox escapes that could compromise broader system security.