A significant security flaw has been identified within Python’s Windows-based asyncio module, posing a threat of out-of-bounds memory writes due to inadequate boundary checks in network socket operations.
Details of the Vulnerability
Designated as CVE-2026-3298, this vulnerability is rated as highly severe and was made public on April 21, 2026. It affects only Windows systems, stemming from an issue in the sock_recvfrom_into() method of Python’s asyncio.proactorEventLoop class, the default event loop for Windows.
The vulnerability arises when the optional nbytes parameter is utilized. The method fails to ensure that incoming data does not surpass the allocated buffer size, leading to potential out-of-bounds writes. Such vulnerabilities are critical as they may corrupt nearby memory regions.
Impact on Windows Systems
Only applications operating on Windows that utilize Python’s asyncio with the ProactorEventLoop and invoke sock_recvfrom_into() using the nbytes parameter are susceptible. This issue is non-existent on Linux and macOS, which employ a different event loop (SelectorEventLoop) unaffected by this flaw.
The root cause is the absence of a boundary check in the ProactorEventLoop’s socket receiving logic, allowing excess network data to overflow the buffer during asynchronous operations.
Mitigation and Solutions
The Python development team has addressed this issue with a patch available in the CPython repository on GitHub (PR #148809). Users are urged to update to the latest patched version of Python immediately. It is also advised to reassess any asyncio-based Windows applications that use sock_recvfrom_into() with the nbytes parameter.
This incident underscores the importance of input validation in low-level asynchronous I/O operations. Windows Python deployments with networked asyncio applications should prioritize this update due to the high risk of memory corruption.
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