Wasmtime EUVD-2026-109187

Exploitation is possible only against embedders that do all of the following: (1) create component-model streams or futures on the host via StreamReader::new or FutureReader::new; (2) pass them into guest code and receive them back as a type-erased StreamAny, FutureAny, Val::Stream/Val::Future, or a typed reader wrapping one of these; and (3) pass that value on to a guest function whose stream/future payload type differs from the type the host originally created it with. A malicious guest triggers the mismatch by controlling the types of the functions it exports or the values it returns. Embedders that use only statically typed StreamReader<T>/FutureReader<T> with concrete payload types throughout, or that never pass host-created streams/futures through a guest and back, are NOT affected. The limiting factors are this specific component-model embedding pattern (not a default or common configuration), the requirement to execute attacker-controlled guest code, and the operator action of forwarding the type-erased value; impact is confined to a host crash with no code execution or data exposure.

This is a medium-severity denial-of-service issue, not a high-impact priority. The CVSS 4.0 score of 5.7 is driven entirely by availability loss (VA:H) with no confidentiality or integrity impact (VC:N/VI:N) - the only consequence stated in the GHSA advisory is a host panic. Although the vector is AV:N, exploitation is heavily gated: AC:H, Attack Requirements present (AT:P), high privileges/operator dependency (PR:H), and required user/embedder interaction (UI:P). No EPSS score was supplied, the CVE is not in CISA KEV, and no public exploit or PoC reference is present in the input - only vendor fix PRs and commits. The vuln.today decision label is 'this_month', consistent with a patch-this-cycle item rather than an emergency. The biggest risk-limiting factor is the narrow precondition: only embedders that create streams/futures on the host and round-trip them through a guest as type-erased values are affected. No signals conflict; all point to a real but low-urgency, DoS-only bug. Note the input Tags ('Information Disclosure', 'Memory Corruption') contradict the advisory and CVSS, which describe neither - see data_quality_flags.

An application embeds Wasmtime and creates a component-model stream or future on the host, passes it into guest code, and gets it back as a type-erased StreamAny/FutureAny or Val::Stream/Val::Future. A malicious guest that controls its exported function types returns the value typed for a different payload, and when the host lowers it back into a guest expecting that different type, a required payload-type check is skipped and the Wasmtime host panics, crashing the embedder (denial of service). No public exploit code is identified at the time of analysis.