cbcvebase.
CVE-2026-34987
published 2026-04-09

CVE-2026-34987: 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…

PriorityP264critical9.9CVSS 3.1
AVNACLPRLUINSCCHIHAH
EPSS
0.28%
19.5th percentile
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.

Affected

10 ranges
VendorProductVersion rangeFixed in
bytecodealliancewasmtime
bytecodealliancewasmtime
bytecodealliancewasmtime
bytecodealliancewasmtime
bytecodealliancewasmtime>= 0.0.0-0 < 36.0.736.0.7
bytecodealliancewasmtime>= 25.0.0 < 36.0.736.0.7
bytecodealliancewasmtime>= 25.0.0 < 36.0.736.0.7
bytecodealliancewasmtime>= 37.0.0 < 42.0.242.0.2
bytecodealliancewasmtime>= 37.0.0 < 42.0.242.0.2
bytecodealliancewasmtime>= 43.0.0 < 43.0.143.0.1

Detection & IOCsextracted from sources · hover to see the quote

  • Detect use of the Winch compiler backend via the command-line flag, which is required to trigger this vulnerability
  • Flag Wasmtime processes launched with the Winch compiler flag on aarch64 systems, where a working proof-of-concept has been confirmed
  • Alert on Wasmtime versions 25.0.0 through 36.0.6, 42.0.1, and 43.0.0 (inclusive) running with the Winch backend as vulnerable version range
  • Monitor host processes running Wasmtime with Winch for unexpected segmentation faults (SIGSEGV), which may indicate sandbox escape attempts
  • ·Vulnerability is only exploitable when the non-default Winch compiler backend is explicitly enabled; the default Cranelift backend is NOT affected
  • ·The sandbox escape is independent of guard region configuration — pre- or post-guard regions do NOT mitigate this bug
  • ·The root cause is a 32-bit memory offset stored in a 64-bit register with upper bits not cleared; closely related PoC variants may reach arbitrary in-process memory beyond the initial 32KiB/4GiB range
  • ·Red Hat products do not use the Winch compiler and are therefore not at risk

CVSS provenance

nvdv3.19.9CRITICALCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
nvdv4.09.0CRITICALCVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
vendor_redhat9.0CRITICAL
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