CVE-2026-121441
Deferred Deferred - Pending Action

WASM-HTTP Request Smuggling in Wasmtime

Publication date: 2026-11-03

Last updated on: 2026-11-03

Assigner: VulnCheck

Description
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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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-2026-121441
EUVD
EUVD-2026-115863

Affected Vendors & Products

Showing 3 associated CPEs
Vendor Product Version / Range
bytecodealliance wasmtime 0
bytecodealliance wasmtime 37.0.0
bytecodealliance wasmtime 49.0.0

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-444 The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.

Attack-Flow Graph

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Executive Summary

This vulnerability is a request smuggling issue in Wasmtime's wasi-http component. Malicious guests can set multiple conflicting Content-Length headers in HTTP requests, causing the host to send requests with inconsistent content-length values. This allows requests to be smuggled through servers or intermediaries that do not reject such malformed requests.

Detection Guidance

Detecting this vulnerability requires checking the version of Wasmtime in use. Compare your installed version against the patched versions (36.0.19, 48.0.7, 49.0.4, or later). Use commands like 'wasmtime --version' or inspect package managers (e.g., 'apt list --installed | grep wasmtime' or 'cargo install --list | grep wasmtime'). Monitor HTTP traffic for malformed requests with conflicting Content-Length headers.

Impact Analysis

An attacker could exploit this to bypass security controls, access unauthorized data, or disrupt services by smuggling requests through vulnerable systems. The impact depends on the affected system's configuration and the presence of intermediaries that fail to reject malformed requests.

Mitigation Strategies

Upgrade Wasmtime to the latest patched versions (36.0.19, 48.0.7, 49.0.4, or newer). If using a package manager, run updates (e.g., 'apt upgrade wasmtime' or 'cargo update -p wasmtime'). Disable or restrict wasi-http functionality if immediate upgrading isn't possible. Review and update intermediary servers to reject malformed requests with conflicting headers.

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