CVE-2026-34987

Full vulnerability record from the local CVENow database.

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.

Severity

CRITICAL

CVSS score

9.0 (v4.0)

Vulnerability status

Analyzed

Source identifier

[email protected]

Published

2026-04-09 19:16:25

Last modified

2026-10-26 22:10:00.163000

View original CVE JSON

{
  "id": "CVE-2026-34987",
  "sourceIdentifier": "[email protected]",
  "published": "2026-04-09T19:16:25.000",
  "lastModified": "2026-10-26T22:10:00.163",
  "vulnStatus": "Analyzed",
  "cveTags": [],
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      "lang": "en",
      "value": "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."
    },
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      "lang": "es",
      "value": "Wasmtime es un entorno de ejecución para WebAssembly. Desde la versión 25.0.0 hasta antes de las versiones 36.0.7, 42.0.2 y 43.0.1, Wasmtime con su backend de compilador Winch (línea base) no predeterminado puede permitir que un Wasm invitado correctamente construido acceda a la memoria del host fuera de su sandbox de memoria lineal. Esta vulnerabilidad requiere el uso del compilador Winch (-Ccompiler=winch). Por defecto, Wasmtime utiliza su backend Cranelift, no Winch. Con Winch, la misma suposición incorrecta está presente en teoría tanto en aarch64 como en x86-64. El caso de aarch64 tiene una prueba de concepto observada y funcional, mientras que el caso de x86-64 es teórico y puede no ser alcanzable en la práctica. Este error del compilador Winch puede permitir que el Wasm invitado acceda a la memoria antes o después de la región de memoria lineal, independientemente de si las regiones de pre-guardia o post-guardia están configuradas. El rango accesible en la prueba de concepto inicial del error es de hasta 32 KiB antes del inicio de la memoria, o ~4 GiB después del inicio de la memoria, independientemente del tamaño de las regiones de pre-guardia o post-guardia o del uso de comprobación de límites explícita o basada en regiones de guardia. Sin embargo, el error subyacente asume que un desplazamiento de memoria de 32 bits almacenado en un registro de 64 bits tiene sus bits superiores borrados cuando puede que no sea así, y por lo tanto, variantes estrechamente relacionadas de la prueba de concepto inicial pueden ser capaces de acceder a memoria verdaderamente arbitraria dentro del proceso. Esto podría resultar en una falla de segmentación del proceso host (DoS), una fuga de datos arbitraria del proceso host, o con una escritura, potencialmente una RCE arbitraria. Esta vulnerabilidad está corregida en las versiones 36.0.7, 42.0.2 y 43.0.1."
    }
  ],
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