CVE-2026-115388 in Wasmtime
Summary
by MITRE • 10/22/2026
wasmtime-wasi-http in Wasmtime before 36.0.16, 37.0.0 before 48.0.3, and 49.0.0 before 49.0.1 does not enforce the wasi:io/streams check-write allowance for WASIp2 outgoing HTTP bodies, so oversized writes succeed instead of trapping. A malicious guest can pass large buffers to write on outgoing-body resources, holding host allocations for the store's lifetime and exhausting host memory.
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Analysis
by VulDB Data Team • 10/22/2026
The vulnerability identified in Wasmtime versions prior to 36.0.16, 37.0.0 before 49.0.1, specifically within the wasmtime-wasi-http component, represents a critical failure in resource management and access control enforcement for WebAssembly System Interface (WASI) operations. This flaw centers on the improper handling of outgoing HTTP body streams when interacting with the host environment through the WASI p2 interface. The core technical deficiency lies in the absence of strict validation checks regarding write allowances defined by the wasi:io/streams specification. In a correctly implemented system, any attempt to write data to an output stream must be preceded by a verification that sufficient permissions and resource limits are available for such an operation. However, due to this oversight, the runtime fails to trap or reject oversized write requests from guest applications, allowing them to proceed unchecked against host memory constraints.
From a technical perspective, this lapse allows malicious WebAssembly guests to exploit the lack of boundary enforcement by passing excessively large buffers to write functions on outgoing-body resources. Instead of triggering an error condition that would halt execution and prevent further resource consumption, the runtime accepts these writes and allocates corresponding memory segments on the host side. Because these allocations are tied to the lifetime of the WebAssembly store rather than being transient or garbage-collected immediately upon completion, they persist as long as the instance remains active. This behavior effectively transforms a simple input validation failure into a persistent resource leak mechanism that can be triggered repeatedly by untrusted code executing within the sandboxed environment.
The operational impact of this vulnerability is severe, primarily manifesting as a denial-of-service condition through host memory exhaustion. An attacker controlling or influencing the guest application can systematically allocate large amounts of host memory by issuing continuous oversized write operations. Since each allocation remains bound to the store's lifecycle, even moderate-sized writes performed repeatedly can accumulate significant overhead over time. This leads to increased memory pressure on the hosting system, potentially causing performance degradation for other workloads running in parallel or resulting in a complete crash if available memory is depleted entirely. Such scenarios are particularly dangerous in multi-tenant environments where Wasmtime serves multiple users or services simultaneously, as one compromised guest could degrade service availability for all others sharing the same host resources.
This issue aligns with CWE-787: Out-of-bounds Write and CWE-400: Uncontrolled Resource Consumption, reflecting both the improper memory handling and the resulting resource exhaustion. In terms of adversarial tactics, this vulnerability facilitates techniques associated with MITRE ATT&CK ID T1496: Resource Hijacking, where an attacker consumes computational resources to disrupt service availability or gain a competitive advantage in shared environments. The exploitation vector relies on the guest application's ability to initiate network-related operations via WASI HTTP APIs, making it relevant for any deployment of Wasmtime that exposes web server functionalities or handles external HTTP requests through WebAssembly modules.
Mitigation strategies must prioritize immediate software updates and robust input validation practices. Users running affected versions should upgrade to Wasmtime 36.0.16, 48.0.3, or later releases where the wasi:io/streams check-write allowance is properly enforced for outgoing HTTP bodies. For organizations unable to update immediately, implementing strict resource quotas and memory limits at the host level can provide a layer of defense against excessive allocation attempts. Additionally, developers integrating Wasmtime into their applications should review guest code for potential abuse vectors related to stream writing operations and consider deploying runtime monitoring tools that detect anomalous patterns in memory usage or write frequency. Ensuring that all WASI interfaces adhere strictly to the latest specification standards during development will also help prevent similar enforcement gaps in future implementations.