8.6

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3.1

-

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CVE-2022-39393

CVE-2022-39393: Wasmtime may have data leakage between instances in the pooling allocator

Impact

There is a bug in Wasmtime's implementation of it's pooling instance allocator where when a linear memory is reused for another instance the initial heap snapshot of the prior instance can be visible, erroneously to the next instance. The pooling instance allocator in Wasmtime works by preallocating virtual memory for a fixed number of instances to reside in and then new instantiations pick a slot to use. Most conventional modules additionally have an initial copy-on-write "heap image" which is mapped in Wasmtime into the linear memory slot. When a heap slot is deallocated Wasmtime resets all of its contents back to the initial state but it does not unmap the image in case the next instance is an instantiation of the same module.

The bug in Wasmtime occurs when a slot in the pooling allocator previously was used for a module with a heap image, meaning that its current state of memory contains the initial heap contents of that module. If the next instantiation within that slot does not itself contain a heap image then Wasmtime would leave the old heap image in place erroneously and continue with instantiation. This means that instantiations of modules without a heap image can see the initial heap image of the prior instantiation within that slot.

Heap images in Wasmtime are created by precomputing WebAssembly data segments into one large mapping to be placed into linear memory at a particular offset. Most modules produced by toolchains today will have a heap image and an initialization snapshot. Creating a module without a heap image would require a hand-crafted *.wat file or a specially crafted source program. This consequence means that this bug is highly unlikely to be accidentally triggered and would otherwise require an intentional trigger with a hand-crafted module.

One important part of this vulnerability is Wasmtime is highly likely to segfault when the slot is reused again with a module that itself has an initialization image. For example if module A has a heap initialization image and module B does not have a heap initialization image, then the following sequence of events could happen if they all are instantiated into the same instance slot:

Module A is instantiated, and then deallocated. This leaves A's heap image in place, reset to its initial contents.

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CVE-2022-39393

CVE-2022-39393:

8.6

CVSS Score

3.1

-

CVSS Score

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
wasmtime	rust	< 2.0.2	2.0.2

Technical Details

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Root Cause Analysis

The vulnerability stems from improper memory image handling in the pooling allocator. The patch shows critical changes in pooling.rs where memory image slot management was restructured. The original code only acquired the slot when a module had an image, leaving previous images intact for modules without one. The addition of 'drop(slot)' in the else clause and reordered slot acquisition indicates the vulnerable path was in the slot reuse logic when transitioning between image/non-image modules.

Vulnerable functions

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WAF Protection Rules

WAF Rule

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Reasoning

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8.6

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CVE-2022-39393: Wasmtime may have data leakage between instances in the pooling allocator

Impact

CVE-2022-39393:

8.6

-

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8.6

8.6

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Vulnerable functions

CVE-2022-39393: Wasmtime may have data leakage between instances in the pooling allocator

Impact

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8.6

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CVE-2022-39393: Wasmtime may have data leakage between instances in the pooling allocator

Impact

CVE-2022-39393:

8.6

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Technical Details

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8.6

8.6

Technical Details

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Vulnerable functions

CVE-2022-39393: Wasmtime may have data leakage between instances in the pooling allocator

Impact

Patches

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References

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