Wasmtime has a use-after-free bug after cloning `wasmtime::Linker`

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Wasmtime is a runtime for WebAssembly. In 43.0.0, cloning a wasmtime::Linker is unsound and can result in use-after-free bugs. This bug is not controllable by guest Wasm programs. It can only be triggered by a specific sequence of embedder API calls made by the host. Specifically, the following steps must occur to trigger the bug clone a wasmtime::Linker, drop the original linker instance, use the new, cloned linker instance, resulting in a use-after-free. This vulnerability is fixed in 43.0.1.

Source published

9 Apr 2026 18:47:26 UTC

Source modified

13 Apr 2026 15:38:33 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 4.0 · 1.0 CVSS:4.0/AV:P/AC:H/AT:P/PR:H/UI:A/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N

Wasmtime is a runtime for WebAssembly. In 43.0.0, cloning a wasmtime::Linker is unsound and can result in use-after-free bugs. This bug is not controllable by guest Wasm programs. It can only be triggered by a specific sequence of embedder API calls made by the host. Specifically, the following steps must occur to trigger the bug clone a wasmtime::Linker, drop the original linker instance, use the new, cloned linker instance, resulting in a use-after-free. This vulnerability is fixed in 43.0.1.

Source published

Apr 9, 2026, 6:47:26 PM (timezone not stated)

Source modified

Apr 13, 2026, 3:38:33 PM (timezone not stated)

Observed here

1 Nov 2026 06:07:57 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 4.0 · 1.0 CVSS:4.0/AV:P/AC:H/AT:P/PR:H/UI:A/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N

Wasmtime is a runtime for WebAssembly. In 43.0.0, cloning a wasmtime::Linker is unsound and can result in use-after-free bugs. This bug is not controllable by guest Wasm programs. It can only be triggered by a specific sequence of embedder API calls made by the host. Specifically, the following steps must occur to trigger the bug clone a wasmtime::Linker, drop the original linker instance, use the new, cloned linker instance, resulting in a use-after-free. This vulnerability is fixed in 43.0.1.

Source published

9 Apr 2026 19:16:24 UTC

Source modified

26 Oct 2026 21:10:00 UTC

Observed here

27 Oct 2026 09:03:08 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 1.0 CVSS:4.0/AV:P/AC:H/AT:P/PR:H/UI:A/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS 3.1 · [email protected] · Primary · 5.0 CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:H

### Impact In version 43.0.0 of the `wasmtime` crate, cloning a `wasmtime::Linker` is unsound and can result in use-after-free bugs. This bug is not controllable by guest Wasm programs. It can only be triggered by a specific sequence of embedder API calls made by the host. The typical symptom of this use-after-free bug is a segfault. It does not enable heap corruption or data leakage. If you are using the `wasmtime` CLI, rather than the embedding API, you are not affected. If you are using the embedding API but are not calling `wasmtime::Linker`'s `Clone` implementation, you are not affected. Specifically, the following steps must occur to trigger the bug: * Clone a `wasmtime::Linker` * Drop the original linker instance * Use the new, cloned linker instance, resulting in a use-after-free ### Patches This bug has been patched in Wasmtime version 43.0.1 ### Workarounds Wasmtime embedders are highly encouraged to upgrade their `wasmtime` crate dependency. If upgrading is not an option, or as a temporary workaround before upgrading, you can avoid this bug by not cloning `wasmtime::Linker` and instead creating a new, empty `wasmtime::Linker` and manually reregistering the host APIs from the original linker: ```rust use wasmtime::{Linker, Result, Store}; fn clone_linker<T>(linker: &Linker<T>, store: &mut Store<T>) -> Result<Linker<T>> { let mut cloned = Linker::new(); for (module, name, item) in linker.iter(store) { cloned.define(module, name, item)?; } Ok(cloned) } ``` ### References This bug was introduced during an internal refactoring that was part of our efforts to [robustly handle allocation failure in Wasmtime](https://github.com/bytecodealliance/wasmtime/issues/12069). This refactoring introduced an string-interning pool which had an unsound `TryClone`[^try-clone] implementation. [^try-clone]: [The `TryClone` trait](https://github.com/bytecodealliance/wasmtime/blob/33e8b3d955697587b23cf39d87fbcbdb4d26b0c9/crates/core/src/alloc/try_clone.rs#L5-L17) is our version of the Rust standard library's `Clone` trait that allows for returning `OutOfMemory` errors. * The `StringPool` was introduced in https://github.com/bytecodealliance/wasmtime/pull/12536, at which time the bug in `TryClone for StringPool` was already present, although this code path was not yet used anywhere. * `wasmtime::Linker` was refactored to internally use `StringPool` in https://github.com/bytecodealliance/wasmtime/pull/12537, at which time the buggy code path became accessible. * This bug was originally reported to the Wasmtime maintainers as https://github.com/bytecodealliance/wasmtime/pull/12906

Source published

9 Apr 2026 20:23:44 UTC

Source modified

10 Sep 2026 03:51:01 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V4 · CVSS:4.0/AV:P/AC:H/AT:P/PR:H/UI:A/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N

Publisher rating · LOW

This is an entry in the RustSec database for the Wasmtime security advisory located at https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-hfr4-7c6c-48w2 For more information see the GitHub-hosted security advisory.

Source published

9 Apr 2026 12:00:00 UTC

Source modified

12 Oct 2026 18:00:09 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

OSV · upstream source license applies

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:P/AC:H/AT:P/PR:H/UI:A/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N