# Wasmtime panic lowering host-created component model stream

Source: https://junglewise.ai/threats/rustsec-2026-0378-panic-lowering-a-host-created-component-model-stream-b1c248ac  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-19. Updated: 2026-10-21  
License: CC BY 4.0 with a link back

Severity: medium · CVSS 4 · Published 2026-10-19

Wasmtime is a WebAssembly runtime that allows applications to execute untrusted code safely. A flaw in how it converts data streams between host and guest code can cause the runtime to crash when a type mismatch occurs, leading to denial of service. An attacker with high privileges who can craft a malicious component model stream could trigger this crash.

Technologies: [Bytecode Alliance Wasmtime](https://junglewise.ai/threats/technologies/bytecode-alliance-wasmtime), [wasmtime (crates.io)](https://junglewise.ai/threats/technologies/wasmtime). Vendors: [Bytecode Alliance](https://junglewise.ai/threats/vendors/bytecode-alliance), [crates.io](https://junglewise.ai/threats/vendors/crates-io).

## Quick answers

**Is This vulnerability exploited in the wild?** No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.

**How severe is This vulnerability?** Medium severity, with a CVSS score of 4, published 19 October 2026.

**Which products and versions are affected?** Bytecode Alliance Wasmtime 46.0.0 to 48.0.6, 49.0.0 to 49.0.3; wasmtime (crates.io) SEMVER:introduced=46.0.0,fixed=48.0.7,introduced=49.0.0,fixed=49.0.4.

**Is there a fix for This vulnerability?** Yes. wasmtime (crates.io): upgrade to 49.0.4 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is This vulnerability attacked?** It is attacked over the network.

**Am I affected?** Check whether you run Bytecode Alliance Wasmtime or wasmtime (crates.io) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- wasmtime (crates.io): upgrade to 49.0.4 or later

  ```sh
  cargo update -p wasmtime
  ```


## Technical details

The vulnerability exists in Wasmtime's component model stream lowering logic, where a host-created stream is passed to a guest expecting a different payload type. This type mismatch triggers a panic rather than graceful error handling. The attack requires network access, high privileges, and active user interaction to trigger; the impact is availability loss on the vulnerable system only.

## Affected products

- Bytecode Alliance wasmtime 46.0.0 to 48.0.6, 49.0.0 to 49.0.3

## Identifiers

- GHSA-9pvw-hwfg-92h5
- RUSTSEC-2026-0378

## Timeline

- 2026-10-19: disclosed
- 2026-10-20: advisory

## References

- [https://crates.io/crates/wasmtime](https://crates.io/crates/wasmtime)
- [https://rustsec.org/advisories/RUSTSEC-2026-0378.html](https://rustsec.org/advisories/RUSTSEC-2026-0378.html)

## Related threats

- [Wasmtime wasi-http response splitting via unvalidated headers](https://junglewise.ai/threats/cve-2026-121439-wasmtime-46-0-0-before-48-0-7-and-49-0-0-before-49-0-4-contains)
- [Wasmtime denial of service in component model stream operations](https://junglewise.ai/threats/cve-2026-119143-wasmtime-46-0-0-before-48-0-6-and-49-0-0-before-49-0-3-contains)
- [Wasmtime host panic in call_async Future drop](https://junglewise.ai/threats/cve-2026-119142-wasmtime-46-0-0-before-48-0-6-and-49-0-0-before-49-0-3-contains)
- [Wasmtime denial of service in realloc handling](https://junglewise.ai/threats/cve-2026-119141-wasmtime-46-0-0-before-48-0-6-and-49-0-0-before-49-0-3-contains)
- [Wasmtime denial of service via thread.yield in component start](https://junglewise.ai/threats/cve-2026-119140-wasmtime-46-0-0-before-48-0-6-and-49-0-0-before-49-0-3-contains)