---
id: "CVE-2026-121440"
kind: "vulnerability"
aliases: ["CVE-2026-121440"]
title: "Wasmtime: forwarded trailers bypass embedder header filtering in WASIp3"
severity: "medium"
cvss: 6.3
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["runtime", "wasi-http", "wasip3", "wasmtime", "webassembly"]
not_affected_if: ["default-header-hooks", "no-wasip3", "trusted-guests-only"]
published_at: "2026-11-03T15:17:35+00:00"
first_seen_at: "2026-11-03T15:22:00+00:00"
updated_at: "2026-11-03T15:23:18+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-121440", "first_seen_at": "2026-11-03T15:22:00+00:00"}]
---

# Wasmtime: forwarded trailers bypass embedder header filtering in WASIp3

Wasmtime's WASI HTTP p3 implementation does not run forwarded trailers through the embedder's forbidden-header hook. A WASIp3 guest can therefore forward trailers that the host meant to block, undermining header restrictions.

_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 in the project's Cargo.lock for `wasmtime`.
- Look for use of the wasi-http p3 feature and a custom `WasiHttpHooks` implementation that defines `is_forbidden_header`.

## Fix

- Wasmtime `48.0.7` for the 48.x line
- Wasmtime `49.0.4` for the 49.x line

## Timeline

- 2026-11-03T15:22:00+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-121440

## Documents

- [advisory] Vendor advisory: github.com https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-jxcf-8qw5-5c33
- [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-forwarded-trailers-header-filter-bypass

## From the sources

Wasmtime 46.0.0 before 48.0.7 and 49.0.0 before 49.0.4 fails to check forwarded trailers against the WasiHttpHooks::is_forbidden_header hook in wasi-http p3. WASIp3 guests can forward a request or response body so its trailers skip embedder-customized forbidden header filtering, undermining header restrictions the embedder intended.

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