---
id: "CVE-2026-121441"
kind: "vulnerability"
aliases: ["CVE-2026-121441"]
title: "Wasmtime: wasi-http lets guests send conflicting content-length headers"
severity: "medium"
cvss: 6.3
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["plugin-hosts", "runtime", "wasi-http", "wasmtime", "webassembly"]
not_affected_if: ["no-wasi-http", "trusted-guests-only", "versions-patched"]
published_at: "2026-11-03T15:17:35+00:00"
first_seen_at: "2026-11-03T15:22:00+00:00"
updated_at: "2026-11-03T15:23:15+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-121441", "first_seen_at": "2026-11-03T15:22:00+00:00"}]
---

# Wasmtime: wasi-http lets guests send conflicting content-length headers

Wasmtime's wasi-http layer accepts multiple conflicting content-length headers from a guest module. An attacker running code inside a guest can make the host send requests that servers or proxies may read differently, enabling request smuggling.

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

## How to check

- Check the Wasmtime version in use with `wasmtime --version` or the wasmtime crate version in Cargo.lock
- Look for wasi-http usage (outgoing HTTP from guest modules) in your embedding code

## Fix

- Wasmtime `36.0.19` for the 36 line
- Wasmtime `48.0.7` for the 48 line
- Wasmtime `49.0.4` for the 49 line

## Timeline

- 2026-11-03T15:22:00+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-121441

## Documents

- [advisory] Vendor advisory: github.com https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-r8xh-cgf7-392j
- [advisory] Third-party advisory: www.vulncheck.com https://www.vulncheck.com/advisories/wasmtime-before-36.0.19-37.0.0-before-48.0.7-and-49.0.0-before-49.0.4-wasi-http-request-smuggling

## From the sources

Wasmtime before 36.0.19, 37.0.0 before 48.0.7, and 49.0.0 before 49.0.4 contains a request smuggling vulnerability in wasi-http that allows guests to set multiple conflicting content-length headers. Malicious guests can make the host send HTTP requests with conflicting content-length values to smuggle requests through servers or intermediaries that do not reject them.

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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_