Sandbox escape via miscompiled shifted heap address
Published Apr 9, 2026 · Updated Apr 9, 2026
Out-of-bounds access in affected Bytecode Alliance Wasmtime releases allows guest OS users to read and write arbitrary host memory. Cranelift masks a constant shift amount incorrectly when lowering a shifted heap address, so the checked address can differ from the address ultimately loaded. Exploitation requires an aarch64 host, memory64, and disabled Spectre mitigations or signal-based traps, permitting escape from the WebAssembly sandbox.
Summary
What happened
Out-of-bounds access in affected Bytecode Alliance Wasmtime releases allows guest OS users to read and write arbitrary host memory. Cranelift masks a constant shift amount incorrectly when lowering a shifted heap address, so the checked address can differ from the address ultimately loaded. Exploitation requires an aarch64 host, memory64, and disabled Spectre mitigations or signal-based traps, permitting escape from the WebAssembly sandbox.
The record
- CVE
- CVE-2026-34971
- Published
- Apr 9, 2026
- Updated
- Apr 9, 2026
- Vendor
- Bytecode Alliance
- Product
- Wasmtime
- Classifications
- CWE-125, CWE-787
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- Apr 9, 2026CVE publishedPublication date reported by the CVE source.
- Apr 9, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=>= 32.0.0, < 36.0.7
- Affected versionversion=>= 37.0.0, < 42.0.2
- Affected versionversion=>= 43.0.0, < 44.0.1
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Public exploit references
No public exploit references are available in this record.
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Technologies
Your stack
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Your stack
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