What is the CVSS score of CVE-2026-34971?

CVE-2026-34971 has a CVSS 4.0 base score of 9.0 (Critical). CVSS vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H. EPSS exploitation probability: 0.0%.

How to fix or mitigate CVE-2026-34971?

Within 24 hours: Inventory all Wasmtime deployments and identify systems running affected versions (32.0.0-36.0.6, 42.0.0-42.0.1, 43.0.0) on aarch64 architecture; suspend execution of untrusted WebAssembly modules. Within 7 days: Disable Spectre mitigations and signal-based traps where operationally feasible as temporary mitigation (verify memory safety impact with architecture team); contact Bytecode Alliance for patch timeline and interim guidance. Within 30 days: Upgrade to a patched Wasmtime release once available; conduct security review of any WebAssembly code executed during vulnerable window.

Wasmtime CVE-2026-34971

Q: Which versions of Wasmtime are affected by CVE-2026-34971?

Bytecode Alliance Wasmtime versions 32.0.0-36.0.6, 42.0.0-42.0.1, and 43.0.0 contain a critical sandbox escape vulnerability that allows authenticated attackers to read and write arbitrary host…. A patch is available.

EUVD-2026-21027 CRITICAL

Out-of-bounds Read (CWE-125)

2026-04-09 GitHub_M

GHSA-jhxm-h53p-jm7w

Buffer Overflow Information Disclosure Wasmtime Bytecodealliance

9.0

CVSS 4.0 · Vendor: GitHub_M

View

Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

Access

Craft malicious WebAssembly module
Delivery

Exploit Cranelift aarch64 heap access bug
Exploit

Bypass bounds check validation
Execution

Access out-of-bounds memory
Impact

Read sensitive data or execute code

Vulnerability Assessment

Weakness	CWE CWE-125 ASVS 5.0 V15 - Secure Coding and Architecture Bounds checking, read limits MITRE ATT&CK 1 technique across 1 tactic: Collection The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up. Sign in to read
Exploitation Conditions	Wasmtime versions 32.0.0 to before 36.0.7, 42.0.2, and 43.0.1 on aarch64 architecture. Requires a malicious WebAssembly module with specific heap access patterns and explicit bounds checks. Attacker needs ability to load and execute untrusted WASM code on the target system.
Impact	Critical sandbox escape affecting Wasmtime 32.0.0-36.0.6, 42.0.0-42.0.1, 43.0.0 on aarch64 with 64-bit WebAssembly and Spectre mitigations disabled. Authenticated access required. Miscompiled heap load operations allow guests to bypass bounds checks and read/write arbitrary host memory, fully compromising isolation. High severity justified by sandbox bypass and arbitrary memory access despite authentication gating.
Exploit ScenarioDerived	Attacker deploys malicious WebAssembly module on vulnerable Wasmtime instance with 64-bit memory and Spectre mitigations off. Module crafts load(iadd(base, ishl(index, shift))) pattern, passing bounds check via one address computation but loading from attacker-controlled alternate address. Reads/writes host memory bypassing sandbox.

Act Now Prioritize within 24-48h. Why: CRITICAL severity.

Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Apr 09, 2026

Arbitrary memory read/write vulnerability in Bytecode Alliance Wasmtime versions 32.0.0 through 36.0.6, 42.0.0-42.0.1, and 43.0.0 allows authenticated remote attackers to escape WebAssembly sandbox restrictions. The Cranelift compilation backend on aarch64 architecture miscompiles specific heap access patterns, creating divergent address computations where bounds checks validate one address while loads access another, enabling sandbox escape through unrestricted host memory access. Exploitation requires 64-bit WebAssembly linear memories with Spectre mitigations and signals-based-traps disabled. No public exploit identified at time of analysis.

RemediationAI

Vendor-released patches: upgrade immediately to Wasmtime 36.0.7, 42.0.2, or 43.0.1 depending on deployment version branch. These releases correct the Cranelift instruction selection mask logic eliminating divergent address computation. For environments unable to patch immediately, enable Spectre mitigations via runtime configuration, which prevents generation of the vulnerable load pattern and serves as effective temporary mitigation. Alternatively, enable signals-based-traps configuration option. Organizations using 32-bit WebAssembly or non-aarch64 architectures are not affected. Full technical details and patch information: https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-jhxm-h53p-jm7w

Affected VersionsENISA

wasmtime 37.0.0, < 42.0.2

wasmtime 43.0.0, < 44.0.1

wasmtime 32.0.0, < 36.0.7

Affected Packages

cargo wasmtime fix: 36.0.7, 42.0.2, 43.0.1, 44.0.1

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 1 package in cargo

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

Technical ContextAI · show details

CWE-125 out-of-bounds read stems from incorrect instruction selection in Cranelift's aarch64 backend. Pattern-matching rule for load(iadd(base, ishl(index, amt))) applies faulty bit masking to constant shift amount, causing erroneous lowering that diverges from WebAssembly semantics. Miscompilation generates load instructions accessing unintended memory offsets, bypassing bounds validation while address wrapping occurs in bounds-check computation but not load execution.

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

Vendor (GitHub_M) PRIMARY 9.0 CRITICAL

CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

SUSE 7.8 HIGH

AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H

Red Hat 8.5 HIGH

Primary rating from Vendor (GitHub_M).

CVSS VectorVendor: GitHub_M

Attack Vector

Network

Attack Complexity

Low

Attack Requirements

Present

Privileges Required

Low

User Interaction

None

Vulnerable system: Confidentiality

High

Vulnerable system: Integrity

High

Vulnerable system: Availability

High

Subsequent system: Confidentiality

High

Subsequent system: Integrity

High

Subsequent system: Availability

High

Lifecycle Timeline

Patch released

Apr 10, 2026 - 02:30 nvd

Patch available

CVE Published

Apr 09, 2026 - 18:45 nvd

CRITICAL 9.0

DescriptionCVE.org

Wasmtime is a runtime for WebAssembly. From 32.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime's Cranelift compilation backend contains a bug on aarch64 when performing a certain shape of heap accesses which means that the wrong address is accessed. When combined with explicit bounds checks a guest WebAssembly module this can create a situation where there are two diverging computations for the same address: one for the address to bounds-check and one for the address to load. This difference in address being operated on means that a guest module can pass a bounds check but then load a different address. Combined together this enables an arbitrary read/write primitive for guest WebAssembly when accesssing host memory. This is a sandbox escape as guests are able to read/write arbitrary host memory. This vulnerability has a few ingredients, all of which must be met, for this situation to occur and bypass the sandbox restrictions. This miscompiled shape of load only occurs on 64-bit WebAssembly linear memories, or when Config::wasm_memory64 is enabled. 32-bit WebAssembly is not affected. Spectre mitigations or signals-based-traps must be disabled. When spectre mitigations are enabled then the offending shape of load is not generated. When signals-based-traps are disabled then spectre mitigations are also automatically disabled. The specific bug in Cranelift is a miscompile of a load of the shape load(iadd(base, ishl(index, amt))) where amt is a constant. The amt value is masked incorrectly to test if it's a certain value, and this incorrect mask means that Cranelift can pattern-match this lowering rule during instruction selection erroneously, diverging from WebAssembly's and Cranelift's semantics. This incorrect lowering would, for example, load an address much further away than intended as the correct address's computation would have wrapped around to a smaller value insetad. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.

Executive ViewAI

Why It Matters

Bytecode Alliance Wasmtime versions 32.0.0-36.0.6, 42.0.0-42.0.1, and 43.0.0 contain a critical sandbox escape vulnerability that allows authenticated attackers to read and write arbitrary host memory, bypassing WebAssembly isolation controls. Organizations running Wasmtime on aarch64 architecture face immediate risk of privilege escalation and data exfiltration if untrusted WebAssembly modules are executed, requiring urgent action to isolate or restrict affected deployments.

Business Impact: Severe

Compensating Controls

No vendor-released patch identified at time of analysis. Interim mitigations: disable Spectre mitigations (AArch64-specific hardening) and disable signal-based traps on affected deployments to prevent exploitation path-both require architectural review for compatibility. Restrict execution to trusted WebAssembly modules only and isolate Wasmtime processes with strict network segmentation and capability-based access controls. Monitor for exploit indicators including unusual memory access patterns and out-of-bounds operations in Wasmtime logs.

Threat Intelligence Overview

EPSS Score

0.0%

Top 97% exploitable

Threat Intel

1 source

Data Quality

8 sources

Confidence: High

EUVD

NVD

CVE.org

EPSS

Vulnrichment GHSA Vendor

GitHub

VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue

Severity CRITICAL

CISA KEV No

Public POC No

EPSS 0.0%

Patch available Yes

Active threat No

Deciding rule

CRITICAL severity

What every action level means

Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.

Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.

This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.

This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.

Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision

Scheduled

Exploitation

None

System Exposure· derived

Open

Automatable

Human Impact

Very High

Technical Impact

Total

Priority Score

Low Medium High Critical

KEV: 0

EPSS: 0

CVSS: +45

PoC: 0

Weaponized: 0

Threat intel: 0

Gov advisory: 0

Attack Technique

Buffer Overflow HIGH

A buffer overflow occurs when a program writes more data to a memory buffer than it was allocated to hold, causing the excess data to spill into adjacent memory regions.

Information Disclosure MEDIUM

Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.