Wasmtime can panic when adding excessive fields to a `wasi:http/types.fields` instance

Summary

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.

Timeline

1. Feb 24, 2026, 9:31 PM UTCCVE published
2. Feb 27, 2026, 8:55 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: partial

3. Feb 27, 2026, 8:55 PM UTCRecord last updated

Affected versions

• bytecodealliance wasmtime

  fixed in 24.0.6; From 25.0.0 · fixed in 36.0.6; From 37.0.0 · fixed in 40.0.4; From 41.0.0 · fixed in 41.0.4

• wasmtimecrates.io

  fixed in 24.0.6

• wasmtimecrates.io

  From 25.0.0 · fixed in 36.0.6

• wasmtimecrates.io

  From 37.0.0 · fixed in 40.0.4

• wasmtimecrates.io

  From 0.0.0-0 · fixed in 24.0.6; From 25.0.0 · fixed in 36.0.6; From 37.0.0 · fixed in 40.0.4; From 41.0.0 · fixed in 41.0.4

References 7