CVE-2026-121440
Deferred Deferred - Pending Action

Wasmtime HTTP Trailer Header Bypass Vulnerability

Publication date: 2026-11-03

Last updated on: 2026-11-03

Assigner: VulnCheck

Description
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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Meta Information

Published
2026-11-03
Last Modified
2026-11-03
Generated
2026-11-03
AI Q&A
2026-11-03
EPSS Evaluated
N/A
NVD
CVE-2026-121440
EUVD
EUVD-2026-115862

Affected Vendors & Products

Showing 2 associated CPEs
Vendor Product Version / Range
bytecodealliance wasmtime 46.0.0
bytecodealliance wasmtime 49.0.0

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Exploitability

CWE
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KEV
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CWE ID Description
CWE-424 The product does not sufficiently protect all possible paths that a user can take to access restricted functionality or resources.

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Executive Summary

This vulnerability affects Wasmtime versions 46.0.0 to 48.0.6 and 49.0.0 to 49.0.3. It allows WASIp3 guests to forward HTTP request or response body trailers in a way that bypasses the WasiHttpHooks::is_forbidden_header hook's filtering. This means unauthorized header manipulation is possible, undermining the embedder's intended header restrictions.

Detection Guidance

To detect this vulnerability, check the version of Wasmtime installed on your system. Run: wasmtime --version. If the output shows versions between 46.0.0 to 48.0.6 or 49.0.0 to 49.0.3, your system is vulnerable.

Impact Analysis

The impact is limited but could allow unauthorized header manipulation in HTTP requests or responses. Attackers might bypass security restrictions set by the embedder, potentially leading to minor data integrity issues. Exploitation requires network access but no privileges or user interaction.

Mitigation Strategies

Upgrade Wasmtime to version 48.0.7, 49.0.4, or later. For the wasmtime-wasi-http package, ensure you update to version 50.0.0 or apply patch releases. Review custom implementations of is_forbidden_header and migrate to forbidden_header_filter.

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