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CVE-2026-34942: Wasmtime panics when transcoding misaligned utf-16 strings
- Severity: medium (CVSS 6.5 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.4% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-04-09 · updated 2026-10-26
- Weakness: CWE-129
Description
Wasmtime is a runtime for WebAssembly. Prior to 24.0.7, 36.0.7, 42.0.2, and 43.0.1, Wasmtime's implementation of transcoding strings into the Component Model's utf16 or latin1+utf16 encodings improperly verified the alignment of reallocated strings. This meant that unaligned pointers could be passed to the host for transcoding which would trigger a host panic. This panic is possible to trigger from malicious guests which transfer very specific strings across components with specific addresses. Host panics are considered a DoS vector in Wasmtime as the panic conditions are controlled by the guest in this situation. This vulnerability is fixed in 24.0.7, 36.0.7, 42.0.2, and 43.0.1.
Affected products
- bytecodealliance wasmtime: before 24.0.7; from 25.0.0 before 36.0.7; from 37.0.0 before 42.0.2; from 43.0.0 before 43.0.1 — fixed in 24.0.7, 36.0.7, 42.0.2, 43.0.1
How to fix
Upgrade to a fixed version: bytecodealliance wasmtime 24.0.7, 36.0.7, 42.0.2, 43.0.1. Check the vendor advisory for mitigations.
References
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