CVE-2026-115892: Wasmtime: Panic lowering a host-created component model stream into a guest that expects a different payload type

Summary

Wasmtime may panic when the host lowers a component model stream or future into a guest, if the host created that stream or future with one payload type and the guest expects a different one. This can happen when an embedder creates a stream or future, passes it through a guest, receives it back under a type-erased type such as StreamAny or FutureAny, and then passes it to a guest function with a different payload type.

GHSA-63h4-6cx9-pjfj is a related advisory also fixing issues within this area, and this advisory is being issued for portions of Wasmtime that were not fixed as part of that advisory.

Impact

This affects embedders that do all of the following:

1. Create streams or futures on the host with StreamReader::new or FutureReader::new 2. Pass them to guest code and receive them back as StreamAny, FutureAny, Val::Stream/Val::Future, or a typed reader whose payload is one of these. 3. Pass them on to a guest function whose stream or future payload type differs from the type the host created them with.

A guest can cause the mismatch in step 3 when it controls the types of the functions it exports or of the values it returns. In that case a malicious guest can make the host panic, a denial of service. Embedders that only use statically typed StreamReader<T>/FutureReader<T> with concrete payload types throughout, or that never pass streams or futures they created through a guest and back, are not affected.