CVE-2026-104855

Wasmtime: Preemption and traps during bulk operations enable breaking internal VM state

Workarounds

Embedders using Store::epoch_deadline_callback are safe if they only access the T in Store<T>. Embedders that do not continue using a store after a timeout or epoch deadline are also unaffected. Embedders which explicitly mutate the store in an epoch callback, or resume wasm after trapping have no workaround however. The embedding needs to be updated to account for this issue.

Wasmtime's implementation of bulk-data-transfer WebAssembly instructions, such as memory.copy, contains a vulnerability when a preemption via epochs or fuel is combined with altering the store's state or cancelling a computation. To prevent these operations from taking too long Wasmtime injects fuel/epoch checks during these operations, but this enables embedders, and possible WebAssembly, to witness intermediate state in the middle of the operation. Examples of this include:

• When a non-nullable WebAssembly table is grown the new elements initially start as null and are filled in as part of a loop with preemption checks. If this computation is then cancelled this left the table in a grown-but-uninitialized state where subsequent usage via WebAssembly could possibly segfault. Loads from this table are assumed to not be null due to its type, but the runtime implementation was exposed through this cancellation at a preemption point.
• Embedders could mutate the store during an epoch callback, such as growing a WebAssembly linear memory. During a bulk memory.copy operation, however, the pointers being copied to/from weren't recomputed between preemption points. This meant that if the linear memory moved its base address it could be possible to have a preemption, the embedder manually grows memory, and then on resumption the copy operation uses invalid pointers.
• Embedders could execute a GC during epoch callbacks. GC operations such as array.copy, like memory.copy above, maintained raw pointers internally in the operation which were not updated after the preemption point. This could lead to corruption of the GC heap.

All of these situations are examples of embedder-driven mutations of the Store or embedder-induced resumption of a Store after a computation was cancelled. These operations expose the internal state of these WebAssembly operations which is semantically incorrect and additionally can cause segfaults for example. Exposing these bugs, however, requires explicit patterns to be present in the embedding itself such as using Store::epoch_deadline_callback and mutating wasm options. Another example is to cancel one invocation (possibly in a table.grow) and then execute more wasm afterwards within the same store. Embeddings not using Store::epoch_deadline_callback or executing code after timeouts/fuel are not affected by this issue.

Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing. Typical impact: TOCTOU exploits, data corruption, or privilege escalation.

Kodem intelligence

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This issue is fixed in Wasmtime 46.0.2 and 47.0.3. In these versions Wasmtime reverts back to Wasmtime 45-and-earlier behavior for these operations to check fuel once before the operation and then not during the operation. This means that a very large memory.copy does not have preemption points in the middle of the operation any more, for example.

1. What is CVE-2026-104855? CVE-2026-104855 is a low-severity race condition vulnerability in wasmtime (rust), affecting versions >= 46.0.0, < 46.0.2. It is fixed in 46.0.2, 47.0.3. Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing.
2. Which versions of wasmtime are affected by CVE-2026-104855? wasmtime (rust) versions >= 46.0.0, < 46.0.2 is affected.
3. Is there a fix for CVE-2026-104855? Yes. CVE-2026-104855 is fixed in 46.0.2, 47.0.3. Upgrade to this version or later.
4. Is CVE-2026-104855 exploitable, and should I be worried? Whether CVE-2026-104855 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-104855 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-104855?
   • Upgrade wasmtime to 46.0.2 or later
   • Upgrade wasmtime to 47.0.3 or later

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