CVE-2026-15308 Python Software Foundation CVE debrief

Who should care

Users of Python, especially those processing uncontrolled data with the incremental HTML parser, should be aware of this vulnerability and take steps to mitigate it. This includes applying patches, restricting input to the HTML parser, and monitoring for unusual CPU usage patterns that may indicate exploitation attempts. Additionally, users should review their Python installations and update them to the latest version.

Technical summary

The incremental HTML parser (html.parser.HTMLParser) in Python allows for CPU denial-of-service through repeated unterminated markup declarations when processing uncontrolled data. This vulnerability has a CVSS score of 8.7 and is classified as HIGH severity. The vulnerability is caused by the parser's inability to handle unterminated markup declarations, which can lead to a denial-of-service attack. The vulnerability affects Python applications that use the incremental HTML parser to process uncontrolled data.

Defensive priority

High priority should be given to patching this vulnerability, especially in environments where uncontrolled data is processed with the incremental HTML parser. This vulnerability has a significant impact on the security of Python applications, and users should take steps to mitigate it as soon as possible.

Recommended defensive actions

• Apply patches from Python
• Restrict input to the HTML parser to prevent unterminated markup declarations
• Monitor for unusual CPU usage patterns that may indicate exploitation attempts
• Consider implementing compensating controls such as rate limiting or IP blocking
• Inventory and update affected Python installations
• Review and update security configurations for Python applications
• Implement additional monitoring and logging to detect potential exploitation attempts

Evidence notes

The CVE record was published on 2026-07-09T17:16:58.260Z and was last modified on 2026-07-10T20:07:52.327Z. The NVD entry is currently Analyzed. This vulnerability affects Python's incremental HTML parser, which is used to process uncontrolled data. The vulnerability allows for CPU denial-of-service through repeated unterminated markup declarations. The source confidence is limited, and defenders should verify the affected scope and severity. The CVE record was obtained from the NVD, and its accuracy is dependent on the information provided by the source.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.