Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access
Key facts
- Attack vector
- Network
- Privileges required
- Low
- User interaction
- None
- Attack complexity
- Low
- Attack requirements
- Present
- Impact type
- Code execution
- Confidentiality · integrity · availability
- High · High · High
- Fix status
- Fixed
Attack profile from CVSS 4.0 · CNA. Fix status from CNA and GitHub. Impact type from CWE-787 and the CVSS impact.
Summary
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- No
- Technical impact
- Total
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| bytecodealliance | wasmtime | 25.0.0 to before 36.0.7; 37.0.0 to before 42.0.2; 43.0.0 to before 44.0.1 | 36.0.7, 42.0.2, 44.0.1 | Patch available | CNA |
| — | wasmtime (crates.io) | 25.0.0 to before 36.0.7 | 36.0.7 | Patch available | GHSA-xx5w-cvp6-jv83 |
| — | wasmtime (crates.io) | 37.0.0 to before 42.0.2 | 42.0.2 | Patch available | GHSA-xx5w-cvp6-jv83 |
| — | wasmtime (crates.io) | 43.0.0 to before 43.0.1 | 43.0.1 | Patch available | GHSA-xx5w-cvp6-jv83 |
- wasmtime Vendorbytecodealliance Affected 25.0.0 to before 36.0.7; 37.0.0 to before 42.0.2; 43.0.0 to before 44.0.1 Fixed in 36.0.7, 42.0.2, 44.0.1 Patch available CNA
- wasmtime (crates.io) Vendor— Affected 25.0.0 to before 36.0.7 Fixed in 36.0.7 Patch available GHSA-xx5w-cvp6-jv83
- wasmtime (crates.io) Vendor— Affected 37.0.0 to before 42.0.2 Fixed in 42.0.2 Patch available GHSA-xx5w-cvp6-jv83
- wasmtime (crates.io) Vendor— Affected 43.0.0 to before 43.0.1 Fixed in 43.0.1 Patch available GHSA-xx5w-cvp6-jv83
CPE configurations NVD · 2
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:bytecodealliance:wasmtime:*:*:*:*:*:rust:*:* | 25.0.0 to before 36.0.7; 37.0.0 to before 42.0.2 | Vulnerable | bytecodealliance |
| cpe:2.3:a:bytecodealliance:wasmtime:43.0.0:*:*:*:*:rust:*:* | 43.0.0 | Vulnerable | bytecodealliance |
- cpe:2.3:a:bytecodealliance:wasmtime:*:*:*:*:*:rust:*:* Versions 25.0.0 to before 36.0.7; 37.0.0 to before 42.0.2 Vulnerable ·bytecodealliance
- cpe:2.3:a:bytecodealliance:wasmtime:43.0.0:*:*:*:*:rust:*:* Versions 43.0.0 Vulnerable ·bytecodealliance
Remediation
- Fix · CNA wasmtime (bytecodealliance) Fixed in 36.0.7, 42.0.2, 44.0.1
- Fix · GHSA-xx5w-cvp6-jv83 wasmtime (crates.io) Fixed in 36.0.7 Advisory →
- Fix · GHSA-xx5w-cvp6-jv83 wasmtime (crates.io) Fixed in 42.0.2 Advisory →
- Fix · GHSA-xx5w-cvp6-jv83 wasmtime (crates.io) Fixed in 43.0.1 Advisory →
Vendor assessments
Red Hat
Not affected: 2 products
- Red Hat Connectivity Link 1
- Red Hat Enterprise Linux 10
Timeline
- 2026-10-02 Threatist record created from CVE-2026-34987
- 2026-10-03 Alias added: GHSA-xx5w-cvp6-jv83
References
More bytecodealliance Wasmtime vulnerabilities
- TI-2026-21632 Wasmtime is vulnerable to panic when dropping a `[Typed]Func::call_async` future CVSS 4.0MEDIUM 6.9
- TI-2023-11636 Guest-controlled out-of-bounds read/write on x86_64 in wasmtime CVSS 3.1CRITICAL 10.0
- TI-2022-18765 Wasmtime vulnerable to data leakage between instances in the pooling allocator CVSS 3.1HIGH 8.6
- TI-2022-08604 Use after free in Wasmtime CVSS 3.1CRITICAL 9.8
- TI-2022-18766 wasmtime_trap_code C API function has out of bounds write vulnerability CVSS 3.1CRITICAL 9.8
- TI-2026-27193 Wasmtime miscompiled guest heap access enables sandbox escape on aarch64 Cranelift CVSS 4.0CRITICAL 9.0
Sources
- CVE-2026-34987 · CVE Program
- CVE-2026-34987 · NVD
- FIRST EPSS
- GHSA-xx5w-cvp6-jv83 · GitHub Advisory Database · CC BY 4.0
- CVE-2026-34987 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences