Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access

{
  "id": "f412962270dce8b3a12f423e8064902a349f4f1bb699cdb6eb90de67862eeb3f",
  "url": "https://osv.dev/vulnerability/CVE-2026-34987",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access",
    "summary": "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, an",
    "evidence": "osv:CVE-2026-34987",
    "products": [],
    "severity": "critical",
    "structured": {
      "id": "CVE-2026-34987",
      "aliases": [
        "CVE-2026-34987",
        "GHSA-xx5w-cvp6-jv83",
        "RUSTSEC-2026-0095"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "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"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/bytecodealliance/wasmtime",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0b195ef5db76c02fb5392ec1418c58bdc5537d41"
                },
                {
                  "fixed": "09539085b9b514b65d5c51e2905dc38f6eb414b9"
                },
                {
                  "introduced": "7b3d6ae79e9153a2477668062f5622c10333925f"
                },
                {
                  "fixed": "403f9929d4e13ed87c76471d4f4bc8d381c876eb"
                },
                {
                  "introduced": "be23469ece57c0be64904f12111c8d808b0ce4ac"
                },
                {
                  "last_affected": "be23469ece57c0be64904f12111c8d808b0ce4ac"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:bytecodealliance:wasmtime:*:*:*:*:*:rust:*:*",
                    "range": [
                      {
                        "introduced": "25.0.0"
                      },
                      {
                        "fixed": "36.0.7"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "cpe": "cpe:2.3:a:bytecodealliance:wasmtime:*:*:*:*:*:rust:*:*",
                    "range": [
                      {
                        "introduced": "37.0.0"
                      },
                      {
                        "fixed": "42.0.2"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "cpe": "cpe:2.3:a:bytecodealliance:wasmtime:43.0.0:*:*:*:*:rust:*:*",
                    "range": [
                      {
                        "introduced": "43.0.0"
                      },
                      {
                        "last_affected": "43.0.0"
                      }
                    ],
                    "source": "CPE_STRING"
                  }
                ]
              }
            }
          ],
          "versions": [
            "43.0.0",
            "v43.0.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-34987.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:42.918433436Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/34xxx/CVE-2026-34987.json",
        "https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-xx5w-cvp6-jv83",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-34987"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-125",
          "CWE-787"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/34xxx/CVE-2026-34987.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-34987",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 510752,
  "published_at": "2026-04-09T18:48:33.552+00:00"
}