A high-severity security vulnerability has been discovered in Python’s asyncio A module on Windows, potentially enabling attackers to write data beyond the boundaries of an allocated memory buffer, is a class of bug that can lead to memory corruption or even arbitrary code execution.
Tracked as CVE-2026-3298, the flaw was publicly disclosed on April 21, 2026, by Python security developer Seth Larson via the official Python Security Announcement mailing list.
The vulnerability resides in the sock_recvfrom_into() method of asyncio.ProactorEventLoop a Windows-specific event loop implementation that handles asynchronous I/O operations.
Python asyncio Vulnerability
The root cause is a missing boundary check on the data buffer when the optional nbytes The parameter is passed to the method.
When a network response exceeds the pre-allocated buffer size, Python fails to enforce a size limit. This allows excess incoming data to overwrite adjacent memory regions, a condition known as an out-of-bounds (OOB) write.
Depending on which memory is overwritten, this bug can trigger application crashes, memory corruption, or, in worst-case scenarios, arbitrary code execution.
This vulnerability is Windows-only. Linux, macOS, and other Unix-based systems rely on SelectorEventLoop as their backend and are completely unaffected.
However, Windows users running Python applications that leverage asyncio-based networking, particularly those invoking sock_recvfrom_into() with the nbytes argument are at risk.
The attack surface is particularly relevant for:
- Windows-hosted Python web servers and API backends
- Asynchronous network applications using UDP socket operations
- Services receiving variable-length network data into fixed-size buffers
It’s worth noting that asyncio.ProactorEventLoop has been the default event loop on Windows since Python 3.8, meaning this flaw impacts a broad range of modern Python deployments in production environments.
The Python security team rated CVE-2026-3298 as HIGH severity. Out-of-bounds write vulnerabilities are frequently leveraged in memory corruption attack chains, and their presence in a core standard library component like asyncio significantly elevates the risk for Windows-based deployments.
Production servers and network-facing services that process untrusted or variable-length data are the most exposed.
Patch and Remediation
A fix has already been submitted to the CPython repository via GitHub Pull Request #148809. The patch introduces the missing boundary check, ensuring that received data cannot exceed the buffer size defined by the nbytes parameter.
Until an official patched Python release is available, Windows users should take the following steps:
- Monitor the official CVE record at
cve.org/CVERecord?id=CVE-2026-3298for patched version details - Apply the updated Python release as soon as it becomes available
- Temporarily avoid using
sock_recvfrom_into()with thenbytesparameter in untrusted or internet-facing network environments
Developers building network-facing applications on Windows are strongly encouraged to prioritize this patch and review any existing code paths that call sock_recvfrom_into() with externally influenced buffer sizes.
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