CVE-2026-34971

Full vulnerability record from the local CVENow database.

Description

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.

Severity

CRITICAL

CVSS score

9.0 (v4.0)

Vulnerability status

Modified

Source identifier

[email protected]

Published

2026-04-09 19:16:24.663000

Last modified

2026-10-26 22:10:00.163000

View original CVE JSON

{
  "id": "CVE-2026-34971",
  "sourceIdentifier": "[email protected]",
  "published": "2026-04-09T19:16:24.663",
  "lastModified": "2026-10-26T22:10:00.163",
  "vulnStatus": "Modified",
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "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."
    },
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      "lang": "es",
      "value": "Wasmtime es un entorno de ejecución para WebAssembly. Desde la versión 32.0.0 hasta antes de las versiones 36.0.7, 42.0.2 y 43.0.1, el backend de compilación Cranelift de Wasmtime contiene un error en aarch64 al realizar un cierto tipo de accesos a la pila (heap), lo que significa que se accede a la dirección incorrecta. Cuando se combina con comprobaciones de límites explícitas en un módulo WebAssembly invitado, esto puede crear una situación en la que hay dos cálculos divergentes para la misma dirección: uno para la dirección a comprobar límites y otro para la dirección a cargar. Esta diferencia en la dirección sobre la que se opera significa que un módulo invitado puede pasar una comprobación de límites pero luego cargar una dirección diferente. Combinado, esto habilita una primitiva de lectura/escritura arbitraria para WebAssembly invitado al acceder a la memoria del host. Esto es un escape de sandbox ya que los invitados pueden leer/escribir memoria arbitraria del host. Esta vulnerabilidad tiene varios ingredientes, todos los cuales deben cumplirse, para que esta situación ocurra y se eviten las restricciones del sandbox. Esta forma de carga mal compilada solo ocurre en memorias lineales de WebAssembly de 64 bits, o cuando Config::wasm_memory64 está habilitado. WebAssembly de 32 bits no se ve afectado. Las mitigaciones de Spectre o las trampas basadas en señales deben estar deshabilitadas. Cuando las mitigaciones de Spectre están habilitadas, la forma de carga ofensiva no se genera. Cuando las trampas basadas en señales están deshabilitadas, las mitigaciones de Spectre también se deshabilitan automáticamente. El error específico en Cranelift es una compilación incorrecta de una carga con la forma load(iadd(base, ishl(index, amt))) donde amt es una constante. El valor de amt se enmascara incorrectamente para probar si es un cierto valor, y esta máscara incorrecta significa que Cranelift puede hacer coincidir erróneamente esta regla de reducción (lowering rule) durante la selección de instrucciones, divergiendo de la semántica de WebAssembly y Cranelift. Esta reducción incorrecta, por ejemplo, cargaría una dirección mucho más lejana de lo previsto, ya que el cálculo de la dirección correcta habría envuelto a un valor menor en su lugar. Esta vulnerabilidad se corrige en las versiones 36.0.7, 42.0.2 y 43.0.1."
    }
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