Wasmtime has an improperly masked return value from `table.grow` with Winch compiler backend
Key facts
- Attack vector
- Network
- Privileges required
- Low
- User interaction
- None
- Attack complexity
- High
- Attack requirements
- Present
- Impact type
- Denial of service
- Confidentiality · integrity · availability
- Low · Low · High
- Fix status
- Partial Red Hat: 2 without a fix
Attack profile from CVSS 4.0 · CNA. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-789.
Summary
Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime's Winch compiler backend contains a bug where translating the table.grow operator causes the result to be incorrectly typed. For 32-bit tables this means that the result of the operator, internally in Winch, is tagged as a 64-bit value instead of a 32-bit value. This invalid internal representation of Winch's compiler state compounds into further issues depending on how the value is consumed. The primary consequence of this bug is that bytes in the host's address space can be stored/read from. This is only applicable to the 16 bytes before linear memory, however, as the only significant return value of table.grow that can be misinterpreted is -1. The bytes before linear memory are, by default, unmapped memory. Wasmtime will detect this fault and abort the process, however, because wasm should not be able to access these bytes. Overall this this bug in Winch represents a DoS vector by crashing the host process, a correctness issue within Winch, and a possible leak of up to 16-bytes before linear memory. Wasmtime's default compiler is Cranelift, not Winch, and Wasmtime's default settings are to place guard pages before linear memory. This means that Wasmtime's default configuration is not affected by this issue, and when explicitly choosing Winch Wasmtime's otherwise default configuration leads to a DoS. Disabling guard pages before linear memory is required to possibly leak up to 16-bytes of host data. 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
- Yes
- Technical impact
- Partial
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-f984-pcp8-v2p7 |
| — | wasmtime (crates.io) | 37.0.0 to before 42.0.2 | 42.0.2 | Patch available | GHSA-f984-pcp8-v2p7 |
| — | wasmtime (crates.io) | 43.0.0 to before 43.0.1 | 43.0.1 | Patch available | GHSA-f984-pcp8-v2p7 |
- 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-f984-pcp8-v2p7
- wasmtime (crates.io) Vendor— Affected 37.0.0 to before 42.0.2 Fixed in 42.0.2 Patch available GHSA-f984-pcp8-v2p7
- wasmtime (crates.io) Vendor— Affected 43.0.0 to before 43.0.1 Fixed in 43.0.1 Patch available GHSA-f984-pcp8-v2p7
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-f984-pcp8-v2p7 wasmtime (crates.io) Fixed in 36.0.7 Advisory →
- Fix · GHSA-f984-pcp8-v2p7 wasmtime (crates.io) Fixed in 42.0.2 Advisory →
- Fix · GHSA-f984-pcp8-v2p7 wasmtime (crates.io) Fixed in 43.0.1 Advisory →
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Red Hat Connectivity Link 1 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux 10 | Affected | No fix yet: Fix deferred |
Timeline
- 2026-10-02 Threatist record created from CVE-2026-35186
- 2026-10-03 Alias added: GHSA-f984-pcp8-v2p7
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-2026-27209 Wasmtime with Winch compiler backend on aarch64 may allow a sandbox-escaping memory access CVSS 4.0CRITICAL 9.0
Sources
- CVE-2026-35186 · CVE Program
- CVE-2026-35186 · NVD
- FIRST EPSS
- GHSA-f984-pcp8-v2p7 · GitHub Advisory Database · CC BY 4.0
- CVE-2026-35186 · 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