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.

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

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. 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

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