# CVE-2025-6272: wasm3 and pywasm3 out-of-bounds write in MarkSlotAllocated

Source: https://junglewise.ai/threats/cve-2025-6272-wasm3-and-pywasm3-out-of-bounds-write-in-markslotallocated  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-06-19. Updated: 2026-06-09  
License: CC BY 4.0 with a link back

CVE-2025-6272 · Severity: medium · CVSS 4 · Published 2025-06-19

A memory management vulnerability exists in pywasm3, a Python wrapper for the Wasm3 WebAssembly runtime. This runtime is used to execute WebAssembly code in various environments, including embedded systems and cloud applications. An attacker with local access could provide a specially crafted WebAssembly file that causes the application to crash, potentially leading to a denial-of-service condition.

Technologies: [pywasm3 (PyPI)](https://junglewise.ai/threats/technologies/pywasm3). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2025-6272 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2025-6272 being exploited in the wild.

**How severe is CVE-2025-6272?** Medium severity, with a CVSS score of 4, published 19 June 2025.

**Which products and versions are affected?** pywasm3 (PyPI) <= 0.5.0, ECOSYSTEM:introduced=0,last_affected=0.5.0.

**Is there a fix for CVE-2025-6272?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2025-6272 attacked?** It is attacked with local access, in a software library.

**Am I affected by CVE-2025-6272?** Check whether you run pywasm3 (PyPI) 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.

## Technical details

An out-of-bounds (OOB) write vulnerability exists in the MarkSlotAllocated function within source/m3_compile.c of wasm3 and its Python wrapper pywasm3. The issue occurs during the compilation phase of WebAssembly modules when the runtime improperly restricts memory buffer operations while marking slots as allocated. A local attacker can exploit this by providing a malicious WebAssembly binary, triggering a segmentation fault (SEGV) and resulting in a denial-of-service. The vulnerability was identified via fuzzing and affects version 0.5.0; as of the advisory date, no official patch has been confirmed in the provided text.

## Affected products

- wasm3 project pywasm3 <= 0.5.0
- wasm3 project wasm3 0.5.0

## Identifiers

- CVE-2025-6272
- GHSA-rrmw-gv85-w824
- PYSEC-2025-186

## Timeline

- 2025-03-29: disclosed: Issue reported on GitHub repository
- 2025-06-19: advisory: GitHub and NVD advisories published

## References

- [https://github.com/wasm3/wasm3/issues/531](https://github.com/wasm3/wasm3/issues/531)
- [https://github.com/user-attachments/files/19516600/wasm3_crash.txt](https://github.com/user-attachments/files/19516600/wasm3_crash.txt)
- [https://vuldb.com/?ctiid.313276](https://vuldb.com/?ctiid.313276)
- [https://vuldb.com/?id.313276](https://vuldb.com/?id.313276)
- [https://vuldb.com/?submit.593008](https://vuldb.com/?submit.593008)

## Related threats

- [wasm3 pywasm3 use-after-free in ForEachModule](https://junglewise.ai/threats/cve-2024-27530-wasm3-pywasm3-use-after-free-in-foreachmodule)
- [wasm3 pywasm3 out-of-bounds read in m3_exec.h](https://junglewise.ai/threats/cve-2024-27528-wasm3-pywasm3-out-of-bounds-read-in-m3-exec-h)
- [Wasm3 uncontrolled memory allocation in Read_utf8](https://junglewise.ai/threats/cve-2024-27529-wasm3-uncontrolled-memory-allocation-in-read-utf8)
- [PYSEC-2024-303 - wasm3 139076a is vulnerable to Denial of Service (DoS).](https://junglewise.ai/threats/cve-2024-27527-pysec-2024-303-wasm3-139076a-is-vulnerable-to-denial-of-service)
- [PYSEC-2024-307 - wasm3 v0.5.0 was discovered to contain an out-of-bound memory read which leads to segmentation fault…](https://junglewise.ai/threats/cve-2024-34246-pysec-2024-307-wasm3-v0-5-0-was-discovered-to-contain-an-out-of)