# CVE-2026-34988: Bytecode Alliance Wasmtime data leakage in pooling allocator

Source: https://junglewise.ai/threats/cve-2026-34988-bytecode-alliance-wasmtime-data-leakage-in-pooling-allocator  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-09. Updated: 2026-05-10  
License: CC BY 4.0 with a link back

CVE-2026-34988 · Severity: medium · CVSS 6.3 · Published 2026-04-09

Wasmtime is a runtime for executing WebAssembly code in a secure, isolated environment. A bug in its memory management system can allow one guest program to access the private memory of a previously running program under specific configurations. This could lead to sensitive data leakage between different users or applications sharing the same infrastructure.

Technologies: [Bytecodealliance Wasmtime](https://junglewise.ai/threats/technologies/bytecodealliance-wasmtime), [wasmtime (crates.io)](https://junglewise.ai/threats/technologies/wasmtime). Vendors: [Bytecodealliance](https://junglewise.ai/threats/vendors/bytecodealliance), [crates.io](https://junglewise.ai/threats/vendors/crates-io).

## Quick answers

**Is CVE-2026-34988 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-34988 being exploited in the wild.

**How severe is CVE-2026-34988?** Medium severity, with a CVSS score of 6.3, published 9 April 2026.

**Which products and versions are affected?** wasmtime (crates.io) SEMVER:introduced=0.0.0-0,fixed=36.0.7,introduced=37.0.0,fixed=42.0.2,introduced=43.0.0,fixed=43.0.1, SEMVER:introduced=28.0.0,fixed=36.0.7.

**Is there a fix for CVE-2026-34988?** Yes. wasmtime (crates.io): upgrade to 36.0.7, 43.0.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-34988 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-34988?** Check whether you run Bytecodealliance Wasmtime or wasmtime (crates.io) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- wasmtime (crates.io): upgrade to 36.0.7, 43.0.1 or later

  ```sh
  cargo update -p wasmtime
  ```


## Technical details

A logic error in Wasmtime's pooling allocator causes a divergence between runtime memory resetting and compile-time assumptions. Specifically, the allocator may use an incorrect predicate to determine if virtual memory permissions need to be reset when a linear memory slot is reused. If the pooling allocator is active, memory guard size is 0, and memory reservation is less than 4GiB (matching max_memory_size), the system fails to trigger a segfault on out-of-bounds loads. This allows a subsequent WebAssembly instance to read the memory remains of a previous instance. The issue is fixed in versions 36.0.7, 42.0.2, and 43.0.1.

## Affected products

- Bytecode Alliance Wasmtime >= 28.0.0, < 36.0.7; >= 37.0.0, < 42.0.2; 43.0.0

## Identifiers

- CVE-2026-34988
- GHSA-6wgr-89rj-399p
- RUSTSEC-2026-0088

## Timeline

- 2026-04-09: disclosed
- 2026-04-09: advisory
- 2026-04-09: patched

## References

- [https://api.github.com/users/shumbo](https://api.github.com/users/shumbo)
- [https://github.com/shumbo](https://github.com/shumbo)
- [https://api.github.com/users/shumbo/gists%7B/gist_id%7D](https://api.github.com/users/shumbo/gists%7B/gist_id%7D)
- [https://api.github.com/users/shumbo/repos](https://api.github.com/users/shumbo/repos)
- [https://avatars.githubusercontent.com/u/10496155?v=4](https://avatars.githubusercontent.com/u/10496155?v=4)
- [https://api.github.com/users/shumbo/events%7B/privacy%7D](https://api.github.com/users/shumbo/events%7B/privacy%7D)

## Related threats

- [RUSTSEC-2026-0422 - Typed functions with `i31` or `nullref` arguments may panic due to insufficient type-check](https://junglewise.ai/threats/rustsec-2026-0422-typed-functions-with-i31-or-nullref-arguments-may-0545dc2b)
- [Wasmtime type confusion with host-created struct types](https://junglewise.ai/threats/rustsec-2026-0423-cross-engine-type-confusion-with-host-created-struct-89fe3f0e)
- [Wasmtime unchecked memory usage in borrowed resource transfer](https://junglewise.ai/threats/rustsec-2026-0424-unchecked-memory-usage-in-transferring-borrowed-b297503c)
- [Wasmtime Cranelift aarch64 br_table sandbox escape](https://junglewise.ai/threats/cve-2026-115893-wasmtime-is-a-runtime-for-webassembly-prior-to-36-0-19-48-0-7)
- [Wasmtime component-model type-check bypass in stream/future lowering](https://junglewise.ai/threats/cve-2026-115892-wasmtime-is-a-runtime-for-webassembly-from-46-0-0-until-48-0-7)