Sandbox escape via uncleared memory-offset bits
Published Apr 9, 2026 · Updated Apr 9, 2026
Out-of-bounds access in affected Wasmtime releases allows guest OS users to read or write host memory via crafted WebAssembly. Winch assumes that a 32-bit memory offset held in a 64-bit register has cleared upper bits, allowing a bounds-checked access to resolve outside linear memory. Exploitation requires the non-default Winch backend; the demonstrated aarch64 case can leak or modify host data or crash the process, while x86-64 reachability remains theoretical.
Summary
What happened
Out-of-bounds access in affected Wasmtime releases allows guest OS users to read or write host memory via crafted WebAssembly. Winch assumes that a 32-bit memory offset held in a 64-bit register has cleared upper bits, allowing a bounds-checked access to resolve outside linear memory. Exploitation requires the non-default Winch backend; the demonstrated aarch64 case can leak or modify host data or crash the process, while x86-64 reachability remains theoretical.
The record
- CVE
- CVE-2026-34987
- 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=>= 25.0.0, < 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.
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
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Your stack
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