Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access_CVE-2026-34987

{
    "lastseen": "",
    "description": "Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime with its Winch (baseline) non-default compiler backend may allow properly constructed guest Wasm to access host memory outside of its linear-memory sandbox. This vulnerability requires use of the Winch compiler (-Ccompiler=winch). By default, Wasmtime uses its Cranelift backend, not Winch. With Winch, the same incorrect assumption is present in theory on both aarch64 and x86-64. The aarch64 case has an observed-working proof of concept, while the x86-64 case is theoretical and may not be reachable in practice. This Winch compiler bug can allow the Wasm guest to access memory before or after the linear-memory region, independently of whether pre- or post-guard regions are configured. The accessible range in the initial bug proof-of-concept is up to 32KiB before the start of memory, or ~4GiB after the start of memory, independently of the size of pre- or post-guard regions or the use of explicit or guard-region-based bounds checking. However, the underlying bug assumes a 32-bit memory offset stored in a 64-bit register has its upper bits cleared when it may not, and so closely related variants of the initial proof-of-concept may be able to access truly arbitrary memory in-process. This could result in a host process segmentation fault (DoS), an arbitrary data leak from the host process, or with a write, potentially an arbitrary RCE. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.",
    "published": "2026-04-09T18:48:33.552Z",
    "modified": "2026-04-10T14:12:55.374Z",
    "type": "cve",
    "title": "Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access",
    "source": "GitHub_M",
    "references": "https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-xx5w-cvp6-jv83",
    "id": "CVE-2026-34987",
    "bulletinFamily": "",
    "cwe": [
        "CWE-125",
        "CWE-787"
    ],
    "cvelist": null,
    "sourceData": "bytecodealliance wasmtime >= 25.0.0, < 36.0.7\nbytecodealliance wasmtime >= 37.0.0, < 42.0.2\nbytecodealliance wasmtime >= 43.0.0, < 44.0.1",
    "sourceHref": "",
    "cvss": {
        "score": 9,
        "severity": "CRITICAL",
        "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",
        "version": "4.0"
    },
    "cvss2": [],
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        "baseScore": 0,
        "baseSeverity": "",
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        "privilegesRequired": "",
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        "scope": "",
        "confidentialityImpact": "",
        "integrityImpact": "",
        "availabilityImpact": "",
        "cvssV3": {
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    },
    "href": "",
    "category_name": "CVE",
    "post_link": "",
    "product": "wasmtime",
    "version": ">= 25.0.0, < 36.0.7",
    "vendor": "bytecodealliance",
    "ai_description": "",
    "ai_severity": "",
    "ai_vendor": "",
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    "ai_score": 0
}