CVE-2026-121440 in Wasmtime
Summary
by MITRE • 11/03/2026
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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Analysis
by VulDB Data Team • 11/03/2026
The vulnerability identified in Wasmer and specifically affecting Wasmtime versions 46.0.0 through 48.0.7 and version 49.0.0 prior to 49.0.4 represents a critical logic flaw within the WebAssembly System Interface (WASI) HTTP implementation, particularly concerning the p3 specification of wasi-http. This issue stems from an incomplete validation mechanism where forwarded trailers are not subjected to the same security checks as initial headers. In standard web request and response processing, embedders often configure forbidden header lists to prevent sensitive information leakage or malicious manipulation, such as blocking hop-by-hop headers like Connection or Keep-Alive that should be managed by intermediaries rather than clients. The WasiHttpHooks::is_forbidden_header hook is designed to enforce these restrictions consistently across all parts of an HTTP transaction.
The core technical flaw lies in the handling of forwarded requests and responses where trailers are processed separately from initial headers. When a WebAssembly guest application forwards an existing request or response, it may append additional metadata in the form of trailers at the end of the body stream. The vulnerability arises because the implementation fails to invoke the is_forbidden_header hook for these trailer fields. Consequently, any header present in the forwarded trailers bypasses the embedder-defined filtering rules entirely. This creates a direct path for an attacker or a compromised WebAssembly module to inject prohibited headers that would otherwise be rejected if they appeared in the initial request line or standard header block.
From an operational impact perspective, this oversight undermines the security posture established by the host environment. Embedders rely on these forbidden header restrictions to maintain protocol compliance and prevent specific types of attacks such as HTTP response splitting, cache poisoning, or unauthorized access control bypasses. By allowing trailers to circumvent these checks, a malicious WebAssembly guest can effectively manipulate downstream servers or proxies in ways that were explicitly restricted by the host configuration. This could lead to data exfiltration if sensitive headers are forwarded unchecked, or it could facilitate protocol confusion attacks where intermediaries misinterpret the request structure due to unexpected header fields introduced via trailers.
This vulnerability aligns with CWE-20 Improper Input Validation and CWE-798 Use of Hard-coded Credentials if the forbidden headers include authentication tokens that should have been stripped before forwarding. In terms of adversary tactics, it relates to ATT&CK technique T1530 Data from Information Repositories or potentially T1041 Exfiltration Over C2 Channel depending on how the forwarded data is utilized, as it allows for the exfiltration of restricted metadata through a non-standard vector that evades standard inspection logic. The flaw essentially creates an authorization bypass where the context of header origin (initial vs trailer) determines security enforcement rather than the content itself.
To mitigate this risk, immediate updates to Wasmtime are required by upgrading to version 48.0.7 or later for the 48.x branch and version 49.0.4 or later for the 49.x branch. These releases correct the logic error by ensuring that all HTTP headers, regardless of whether they appear in the initial block or as trailers during forwarding operations, are validated against the configured forbidden header list via the WasiHttpHooks interface. Embedders relying on strict header filtering should also consider implementing additional validation layers at the proxy level to ensure defense-in-depth, although patching the underlying runtime remains the primary and most effective remediation strategy for this specific implementation flaw.