Critical Vulnerability in Python 3.9: CPU Denial of Service via HTML Parser
Summary
ALSA-2026:39798: Important: python3.9 security, bug fix, and enhancement update. Affects python-unversioned-command versions < 3.9.25-7.el9_8.2. Fixed in 3.9.25-7.el9_8.2. How to fix ↓
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Background
Python is a widely-used interpreted, interactive, object-oriented programming language known for its simplicity and versatility. It supports various system calls, libraries, and windowing systems, making it a popular choice for developers. The affected package, Python 3.9, includes numerous features such as modules, classes, and dynamic typing. However, it has recently been identified to have a critical vulnerability that can lead to a CPU Denial of Service (DoS) when the HTML parser encounters repeated unterminated markup declarations.
Vulnerability Detail
The vulnerability, identified as CVE-2026-15308, arises from the HTML parser in Python 3.9, which can be exploited through repeated unterminated markup declarations. This can lead to excessive CPU usage, effectively causing a Denial of Service by making the application unresponsive. The issue is particularly concerning for applications that rely heavily on HTML parsing, as attackers could leverage this vulnerability to disrupt service availability.
How To Fix
Minimum fixed version
3.9.25-7.el9_8.2
This is the first release that contains the fix.
sudo dnf update python-unversioned-commandTip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
Currently, there are no specific workarounds mentioned for this vulnerability. The best course of action is to apply the available patch as soon as possible to mitigate the risk.
Conclusion
The vulnerability identified in Python 3.9 poses a significant risk to applications relying on HTML parsing. It is crucial to update to the patched version 3.9.25-7.el9_8.2 to mitigate the risk of a CPU Denial of Service. Regular monitoring and prompt updates are essential to maintaining application security.
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FAQ
CVE-2026-15308 is a security vulnerability in Python 3.9 that allows for a CPU Denial of Service due to issues in the HTML parser.
You can check if your application is affected by using Vulert, which monitors open-source vulnerabilities in real-time. Visit Vulert Scanner to analyze your manifest file.
The vulnerability can lead to excessive CPU usage, causing applications to become unresponsive and resulting in a Denial of Service.
Yes, a patch is available in version 3.9.25-7.el9_8.2. It is recommended to update to this version to mitigate the risk.
If immediate updating is not possible, ensure to monitor your application closely for unusual behavior and consider implementing additional security measures until the update can be applied.
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