---
id: "CVE-2026-121439"
kind: "vulnerability"
aliases: ["CVE-2026-121439"]
title: "Wasmtime: guests can forge content-length headers, enabling HTTP response splitting"
severity: "medium"
cvss: 6.9
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["runtime", "wasi-http", "wasip3", "wasmtime", "webassembly"]
not_affected_if: ["no-wasi-http-p3", "trusted-guests-only"]
published_at: "2026-11-03T15:17:35+00:00"
first_seen_at: "2026-11-03T15:22:00+00:00"
updated_at: "2026-11-03T15:23:10+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-121439", "first_seen_at": "2026-11-03T15:22:00+00:00"}]
---

# Wasmtime: guests can forge content-length headers, enabling HTTP response splitting

Wasmtime, the WebAssembly runtime, accepts unvalidated content-length headers from wasip3 guest programs in its wasi-http implementation. A malicious guest can splice an existing body into a new response, enabling response splitting or injection.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the Wasmtime version with `wasmtime --version` or the wasmtime crate version in Cargo.lock
- Look for wasi-http p3 use in your guest components

## Fix

- Wasmtime `46.x` before `48.0.7`: move to `48.0.7` or later
- Wasmtime `49.x` before `49.0.4`: move to `49.0.4` or later

## Timeline

- 2026-11-03T15:22:00+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-121439

## Documents

- [advisory] Vendor advisory: github.com https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-9prg-84vf-g5fc
- [advisory] Third-party advisory: www.vulncheck.com 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

## From the sources

Wasmtime 46.0.0 before 48.0.7 and 49.0.0 before 49.0.4 contains a response splitting vulnerability in wasi-http p3 that allows guests to set unvalidated content-length headers. Malicious wasip3 guests can splice an existing body into a new response with an incorrect content-length to perform response splitting or injection attacks.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_