CVE-2026-104855 in Wasmtime
Summary
by MITRE • 10/02/2026
Wasmtime is a runtime for WebAssembly. From 46.0.0 until 46.0.2 and 47.0.3, fuel and epoch preemption checks inside bulk operations including memory.copy, table.grow, and array.copy can expose invalid intermediate state when an embedder mutates a Store in Store::epoch_deadline_callback or continues using a Store after cancellation or a trap. A cancelled non-nullable table growth can leave null elements, linear-memory growth during memory.copy can invalidate retained raw pointers, and callback-triggered garbage collection during array.copy can invalidate GC pointers, resulting in a crash, invalid memory access, or GC heap corruption. Embeddings whose callbacks only access the host data in Store<T>, and embeddings that discard a Store after timeout or epoch deadline, are not affected. This issue is fixed in versions 46.0.2 and 47.0.3.
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Analysis
by VulDB Data Team • 10/02/2026
The vulnerability identified within Wasmtime, specifically affecting versions from 46.0.0 through 46.0.1 and 47.0.0 through 47.0.2, represents a critical flaw in the handling of fuel consumption and epoch preemption checks during bulk memory operations. WebAssembly runtimes utilize mechanisms such as fuel accounting to prevent infinite loops and epoch-based time slicing to manage long-running computations or enforce timeouts. These safety features are essential for maintaining system stability and preventing denial-of-service conditions. However, when these checks interact with complex bulk operations like linear memory copying via the memory.copy instruction, table growth through table.grow, or array element duplication via array.copy, a race condition emerges if the embedding environment mutates the Store context during specific callback intervals. The core technical flaw lies in the fact that intermediate states become visible and potentially persistent when an embedder modifies the Store within the Store::epoch_deadline_callback function or continues to utilize a Store instance after it has been cancelled or encountered a trap state. This design oversight allows for scenarios where the runtime's internal consistency guarantees are violated, leading to undefined behavior rather than safe termination of execution.
The operational impact of this vulnerability is severe and multifaceted, primarily manifesting as crashes, invalid memory accesses, or corruption of the garbage collection heap. In specific instances involving non-nullable tables, a cancelled growth operation can leave behind null elements where valid references were expected, violating type safety constraints inherent to WebAssembly's strict typing model. Similarly, during linear-memory operations such as memory.copy, if the underlying memory is grown while the copy is in progress due to callback-triggered events, previously retained raw pointers may become invalid or point to unmapped regions of memory. This scenario creates a classic use-after-free condition where subsequent accesses result in segmentation faults or arbitrary code execution potential depending on the attacker's ability to control heap layout. Furthermore, during array.copy operations, if garbage collection is triggered by callback mechanisms while elements are being moved, GC pointers can be invalidated prematurely. This leads to heap corruption as the runtime attempts to manage objects that have already been collected or reallocated, destabilizing the entire execution environment and potentially allowing an attacker to achieve arbitrary read-write primitives through carefully crafted WebAssembly modules designed to exploit these timing windows.
From a classification perspective, this vulnerability aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, as it involves race conditions between the runtime's internal state management and external embedder callbacks. It also relates closely to CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer, particularly regarding invalid memory access resulting from pointer invalidation during bulk operations. In terms of offensive security frameworks such as MITRE ATT&CK, this flaw could be leveraged in techniques associated with Execution through WebAssembly or potentially Defense Evasion if an attacker uses these crashes to bypass sandboxing mechanisms that rely on predictable termination states. The vulnerability underscores the complexity of integrating asynchronous host callbacks with synchronous guest execution contexts without proper synchronization primitives or state isolation barriers.
Mitigation strategies for this issue are primarily centered around upgrading the Wasmtime runtime to patched versions, specifically 46.0.2 and 47.0.3, which address these race conditions by ensuring that intermediate states during bulk operations are not exposed in a manner that allows embedder mutations to corrupt internal consistency. For developers who cannot immediately upgrade, it is crucial to review embedding implementations for patterns that mutate the Store within epoch deadline callbacks or continue using stores after cancellation traps. Embeddings that strictly limit callback access to host data stored in generic type parameters of the Store and discard the store instance immediately upon timeout or epoch deadline expiration are noted as not being affected by this specific flaw. Therefore, isolating state mutations from critical execution paths and ensuring strict lifecycle management of Store instances can serve as a temporary defensive measure until patching is completed.