CVE-2026-115389 in Wasmtime
Summary
by MITRE • 10/22/2026
Wasmtime before 36.0.16, 37.0.0 before 48.0.3, and 49.0.0 before 49.0.1 does not adequately account for host allocations when lifting guest values through the dynamically-typed Val API, bypassing the hostcall fuel limit. A malicious WebAssembly guest can return crafted values that allocate at least ~100x more host memory than tracked, causing denial of service.
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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 through 48.0.3, and 49.0.0 before 49.0.1 represents a critical failure in resource accounting within the WebAssembly runtime environment. This flaw specifically affects the handling of host allocations during the process of lifting guest values via the dynamically-typed Val API. In standard WebAssembly execution, fuel limits are implemented to prevent infinite loops and excessive computation by capping the number of instructions executed or resources consumed. However, this particular implementation detail allows a maliciously crafted WebAssembly module to bypass these safeguards through an imbalance between tracked resource consumption and actual host-side memory allocation.
The technical root cause lies in how Wasmtime manages data transfer from the guest environment back to the host using the Val API. When a guest function returns values that require conversion or lifting into native host types, the runtime must allocate memory on the host side to store these lifted structures. The vulnerability arises because the fuel accounting mechanism does not adequately correlate with the actual size of these dynamic allocations. A malicious actor can construct WebAssembly code that triggers the creation of large data structures in the host environment while reporting minimal resource usage to the fuel counter. This discrepancy allows the guest module to consume significantly more memory than permitted by the configured limits, effectively rendering the fuel protection mechanism obsolete for this specific class of operations.
The operational impact of this vulnerability is severe, primarily manifesting as a denial of service condition. By exploiting the ability to allocate approximately one hundred times more host memory than tracked, an attacker can rapidly exhaust available system resources on the hosting server or application. This resource exhaustion leads to process crashes, unresponsiveness, or forced termination by operating system out-of-memory killers. In cloud-native environments where WebAssembly is used for sandboxed execution of untrusted code, such as in edge computing platforms or secure plugin systems, this flaw could allow a single malicious tenant to destabilize the entire host infrastructure, affecting other users sharing the same runtime instance.
From a classification perspective, this vulnerability aligns with CWE-787: Out-of-bounds Write and CWE-400: Uncontrolled Resource Consumption. The bypass of fuel limits is particularly concerning as it undermines fundamental security assumptions about resource isolation in sandboxed environments. In terms of offensive tactics, this behavior corresponds to ATT&CK technique T1496: Resource Hijacking, where an adversary uses computing resources for their own benefit or causes denial of service by exhausting system capacity. The specific mechanism involves manipulating the interface between guest and host states to create a resource accounting blind spot that is not covered by standard execution limits.
Mitigation strategies must prioritize immediate software updates as the primary defense vector. Organizations running Wasmtime should upgrade to version 36.0.16 or later, 48.0.3 for the 37.x branch, and 49.0.1 for the 49.x series immediately upon availability. These releases contain patches that correct the resource accounting logic within the Val API lifting process, ensuring that host allocations are properly tracked against fuel limits. For environments where immediate patching is not feasible, administrators should implement strict memory quotas at the container or OS level to limit the maximum memory footprint of any single Wasmtime instance. Additionally, monitoring for anomalous spikes in memory usage associated with WebAssembly execution can help detect exploitation attempts before they lead to complete service disruption.