Python Vulnerability Allows Out-of-Bounds Write on Windows Systems

A security vulnerability has been discovered in Python’s Windows asyncio implementation, allowing attackers to trigger out-of-bounds memory writes through a missing boundary check in network socket operations.

The vulnerability, tracked as CVE-2026-3298, carries a high severity rating. It exclusively affects Windows platforms and was publicly disclosed on April 21, 2026.

The flaw exists in the sock_recvfrom_into() method of Python’s asyncio.proactorEventLoop class, which is Windows’ native event loop implementation.

When the optional nbytes parameter is used, the method fails to validate whether the incoming network data exceeds the destination buffer size.

As a result, data larger than the allocated buffer could be written beyond its intended memory boundary, creating a classic out-of-bounds write condition.

Out-of-bounds write vulnerabilities are particularly dangerous because they can corrupt adjacent memory regions.

Depending on the runtime environment and how memory is managed, this can potentially lead to application crashes, arbitrary code execution, or privilege escalation.

Windows Python Vulnerability

Only Windows users running Python with asyncio-based network applications are at risk. Specifically, applications that use ProactorEventLoop Python’s default event loop on Windows and invoke sock_recvfrom_into() with the nbytes parameter are vulnerable.

Linux and macOS platforms are not affected, as they rely on a different event loop implementation (SelectorEventLoop) that does not contain this flaw.

The root cause is a missing boundary check introduced in the ProactorEventLoop's socket receive logic.

When a caller specifies nbytes to limit the amount of data read into a buffer, the function does not verify that the actual data received fits within that limit.

This allowed network-supplied data to overflow the buffer during an async receive operation. Seth Larson reported the vulnerability and officially disclosed it through the Python Security Announce mailing list.

Mitigations

The Python development team has issued a fix via a pull request to the CPython repository on GitHub (PR #148809). Users are strongly advised to:

• Update Python to the latest patched version immediately.
• Review asyncio-based Windows applications using sock_recvfrom_into() with the nbytes parameter.
• Monitor the official CVE record for details on the affected version and further updates.

This vulnerability highlights the ongoing risk of missing input validation in low-level async I/O operations.

Windows-based Python deployments running networked asyncio applications should treat this as a high-priority patch given its high severity and potential for memory corruption.

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