CVE-2026-27572 — Wasmtime can panic when adding excessive fields to a `wasi:http/types.fields` instance

Affected: bytecodealliance wasmtime

Wasmtime is a runtime for WebAssembly. Prior to versions 24.0.6, 36.0.6, 4.0.04, 41.0.4, and 42.0.0, Wasmtime's implementation of the `wasi:http/types.fields` resource is susceptible to panics when too many fields are added to the set of headers. Wasmtime's implementation in the `wasmtime-wasi-http` crate is backed by a data structure which panics when it reaches excessive capacity and this condition was not handled gracefully in Wasmtime. Panicking in a WASI implementation is a Denial of Service vector for embedders and is treated as a security vulnerability in Wasmtime. Wasmtime 24.0.6, 36.0.6, 40.0.4, 41.0.4, and 42.0.0 patch this vulnerability and return a trap to the guest instead of panicking. There are no known workarounds at this time. Embedders are encouraged to update to a patched version of Wasmtime.

GHSA-243v-98vx-264h — Wasmtime can panic when adding excessive fields to a `wasi:http/types.fields` instance

Affected: crates.io: wasmtime

### Impact

Wasmtime's implementation of the `wasi:http/types.fields` resource is susceptible to panics when too many fields are added to the set of headers. Wasmtime's implementation in the `wasmtime-wasi-http` crate is backed by a data structure which panics when it reaches excessive capacity and this condition was not handled gracefully in Wasmtime. Panicking in a WASI implementation is a Denial of Service vector for embedders and is treated as a security vulnerability in Wasmtime.

### Patches

Wasmtime 24.0.6, 36.0.6, 40.0.4, 41.0.4, and 42.0.0 patch this vulnerability and return a trap to the guest instead of panicking.

### Workarounds

There are no known workarounds at this time, embedders are encouraged to update to a patched version of Wasmtime.

### Resources

* [Limitations of `http::HeaderMap`](https://docs.rs/http/1.4.0/http/header/#limitations)

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CVE-2026-27572 — Wasmtime can panic when adding excessive fields to a `wasi:http/types.fields` instance

GHSA-243v-98vx-264h — Wasmtime can panic when adding excessive fields to a `wasi:http/types.fields` instance