# CVE-2026-121439: Wasmtime wasi-http response splitting via unvalidated headers

Source: https://junglewise.ai/threats/cve-2026-121439-wasmtime-46-0-0-before-48-0-7-and-49-0-0-before-49-0-4-contains  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-03. Updated: 2026-11-03  
License: CC BY 4.0 with a link back

CVE-2026-121439 · Severity: medium · CVSS 5.3 · Published 2026-11-03

Wasmtime is a WebAssembly runtime that executes untrusted code sandboxes. When running malicious WebAssembly programs (guests) that use the wasi-http component, attackers can inject invalid HTTP headers to perform response splitting attacks, potentially allowing them to manipulate HTTP responses seen by clients or inject malicious content.

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

## Quick answers

**Is CVE-2026-121439 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-121439 being exploited in the wild.

**How severe is CVE-2026-121439?** Medium severity, with a CVSS score of 5.3, published 3 November 2026.

**Which products and versions are affected?** Bytecode Alliance Wasmtime 46.0.0 to before 48.0.7, and 49.0.0 to before 49.0.4.

**Is there a fix for CVE-2026-121439?** Yes: fixed in Wasmtime 48.0.7, 49.0.4, 50.0.0. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-121439 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-121439?** Check whether you run Bytecode Alliance Wasmtime 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

- Fixed in Wasmtime 48.0.7, 49.0.4, 50.0.0

## Technical details

A response splitting vulnerability exists in the wasi-http P3 component where guest code can set unvalidated content-length headers when splicing existing response bodies. The vulnerability requires a malicious WebAssembly guest and network attack vector, allowing integrity compromise through HTTP response injection (CWE-444: HTTP Request/Response Smuggling).

## Affected products

- Bytecode Alliance Wasmtime 46.0.0 to before 48.0.7, and 49.0.0 to before 49.0.4

## Timeline

- 2026-10-19: disclosed: GitHub security advisory GHSA-9prg-84vf-g5fc published
- 2026-11-03: advisory: CVE-2026-121439 published

## References

- [https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-9prg-84vf-g5fc](https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-9prg-84vf-g5fc)
- [https://www.vulncheck.com/advisories/wasmtime-46.0.0-before-48.0.7-and-49.0.0-before-49.0.4-http-response-splitting-via-wasi-http](https://www.vulncheck.com/advisories/wasmtime-46.0.0-before-48.0.7-and-49.0.0-before-49.0.4-http-response-splitting-via-wasi-http)

## Related threats

- [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)
- [Wasmtime host crash in async-lifted function invocation](https://junglewise.ai/threats/cve-2026-119139-wasmtime-46-0-0-before-48-0-6-and-49-0-0-before-49-0-3-contains)