Bug 2457006 (CVE-2026-35186)

Summary: CVE-2026-35186 Wasmtime: github.com/bytecodealliance/wasmtime: Wasmtime: Denial of Service and potential information disclosure via Winch compiler type confusion
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in Wasmtime, a runtime for WebAssembly. The Winch compiler backend incorrectly handles the `table.grow` operator, leading to an internal type mismatch. This can result in a Denial of Service (DoS), where the host process crashes. Additionally, under specific configurations with disabled memory guard pages, this flaw could lead to the disclosure of up to 16 bytes of sensitive host data.
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Bug Depends On: 2457175    
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Description OSIDB Bzimport 2026-04-09 19:02:40 UTC 
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.