Raw vector
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/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:XSummary
CVE-2026-34987 is a critical-severity Out-of-bounds Read (CWE-125) vulnerability in Bytecodealliance Wasmtime. Its CVSS base score is 9.0 (Critical).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 20th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SA-15 (Development Process, Standards, and Tools) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-34987 is a high-severity vulnerability (CVSS 3.1 score of 9.9) affecting Wasmtime, a runtime for WebAssembly, in versions from 25.0.0 up to but not including 36.0.7, 42.0.2, and 43.0.1. The issue resides in the non-default Winch (baseline) compiler backend, enabled via the -Ccompiler=winch flag; Wasmtime defaults to the Cranelift backend, which is unaffected. It stems from an incorrect assumption that a 32-bit memory offset stored in a 64-bit register has its upper bits cleared, potentially allowing properly constructed guest WebAssembly code to access host memory outside the linear-memory sandbox on both aarch64 (with an observed proof-of-concept) and x86-64 (theoretical). This flaw, linked to CWE-125 (out-of-bounds read) and CWE-787 (out-of-bounds write), enables access up to 32 KiB before the memory start or ~4 GiB after, independent of guard regions or bounds checking, with variants possibly reaching arbitrary in-process memory.
Exploitation requires an attacker to supply malicious guest Wasm executed in a Wasmtime instance configured with the Winch compiler. Per the CVSS vector (AV:N/AC:L/PR:L/UI:N/S:C), it is remotely exploitable over a network with low privileges in a scoped context, without user interaction. Successful exploitation can cause a host process segmentation fault for denial of service, leak arbitrary data from the host, or enable arbitrary read/write primitives leading to remote code execution.
The Wasmtime security advisory at https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-xx5w-cvp6-jv83 details the fix in releases 36.0.7, 42.0.2, and 43.0.1. Mitigation involves upgrading to these patched versions or avoiding the Winch compiler backend by sticking to the default Cranelift configuration.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-21031
Vulnerability Data
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…
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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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and evaluation directly finds out-of-bounds read flaws through static analysis, fuzzing, and dynamic bounds checks.
Requiring documented secure-development standards and tools can mandate bounds-checked coding practices that avoid the weakness.
Secure engineering principles require bounds checking and memory-safe constructs that stop out-of-bounds reads from being introduced.
Process isolation confines the effects of an out-of-bounds read to the compromised process.
Input validation rejects malformed indices or lengths that would otherwise cause reads outside buffer bounds.
Memory-protection mechanisms limit the exploitability and blast radius of a successful out-of-bounds write.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure-development practices such as bounds checking and memory-safe languages directly prevent out-of-bounds reads.
Vulnerability scanning and recording can discover instances of out-of-bounds reads after code is deployed.
Routine patching replaces vulnerable code containing out-of-bounds read flaws.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing in development and acceptance includes fuzzing and static analysis that detect out-of-bounds read defects before release.
Logging can record evidence of an out-of-bounds read but does not prevent the weakness itself.
Secure development life cycle mandates input validation and bounds checking that directly prevent out-of-bounds reads.
Application security requirements include explicit bounds and memory-safety specifications that mitigate buffer over-reads.
Secure system architecture and engineering principles require memory-safe design patterns and runtime protections against out-of-bounds access.
Secure coding standards explicitly forbid unsafe pointer arithmetic and mandate bounds-checked reads, eliminating CWE-125.