CVE-2026-34971
Published: 09 April 2026
Summary
CVE-2026-34971 is a high-severity Out-of-bounds Read (CWE-125) vulnerability in Bytecodealliance Wasmtime. Its CVSS base score is 7.8 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 2.6th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 CM-6 (Configuration Settings) and SI-16 (Memory Protection).
Threat & Defense at a Glance
Threat & Defense Details
Mitigating Controls (NIST 800-53 r5)AI
Mandates timely flaw remediation by patching the miscompilation bug in Wasmtime's Cranelift backend on aarch64 that enables sandbox escape via arbitrary host memory access.
Requires secure configuration settings for Wasmtime, such as enabling Spectre mitigations or signals-based traps, to block generation of the vulnerable load(iadd(base, ishl(index, amt))) instruction pattern.
Implements memory protection safeguards to prevent unauthorized guest WebAssembly access to host memory despite miscompiled heap accesses that bypass bounds checks.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
The vulnerability provides an arbitrary read/write primitive on host memory by bypassing WASM sandbox bounds checks, directly enabling a local attacker to escalate privileges from the guest WebAssembly module to full host access.
NVD 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…
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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.
Deeper analysisAI
CVE-2026-34971 affects Wasmtime, a runtime for WebAssembly, specifically its Cranelift compilation backend on aarch64 architectures. The vulnerability impacts versions from 32.0.0 up to but excluding 36.0.7, 42.0.2, and 43.0.1. It stems from a miscompilation bug during heap accesses of the form load(iadd(base, ishl(index, amt))), where amt is a constant. Cranelift incorrectly masks the amt value during instruction selection, erroneously pattern-matching a lowering rule. This causes diverging address computations: one for bounds checking and another for the actual load/store, allowing a guest WebAssembly module to access unintended addresses despite passing explicit bounds checks. The issue is limited to 64-bit WebAssembly linear memories or when Config::wasm_memory64 is enabled; 32-bit WebAssembly is unaffected. It is classified under CWE-125 (out-of-bounds read) and CWE-787 (out-of-bounds write), with a CVSS v3.1 score of 7.8 (AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H).
A local attacker with low privileges can exploit this by running a malicious guest WebAssembly module in an affected Wasmtime instance. Exploitation requires Spectre mitigations and signals-based traps to be disabled, as enabling either prevents generation of the vulnerable load shape. Under these conditions, the attacker crafts code where the bounds-checked address wraps around (e.g., to a smaller value), passing checks, while the load/store uses an unmasked, larger offset, such as one much further away. This yields an arbitrary read/write primitive on host memory, effecting a full sandbox escape as the guest bypasses isolation to access arbitrary host resources.
The Wasmtime security advisory (GHSA-jhxm-h53p-jm7w) confirms fixes in versions 36.0.7, 42.0.2, and 43.0.1. Security practitioners should update to these releases immediately. As interim mitigations, enable Spectre mitigations or signals-based traps, which disable the vulnerable compilation pattern and automatically enforce Spectre protections when signals are active.
Details
- CWE(s)