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

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-24 · updated 2026-06-17
Weakness: CWE-770

Description

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.

Affected products

bytecodealliance wasmtime: before 24.0.6; from 25.0.0 before 36.0.6; from 37.0.0 before 40.0.4; from 41.0.0 before 41.0.4 — fixed in 24.0.6, 36.0.6, 40.0.4, 41.0.4

How to fix

Upgrade to a fixed version: bytecodealliance wasmtime 24.0.6, 36.0.6, 40.0.4, 41.0.4. Check the vendor advisory for mitigations.

References

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