CVE-2026-49851 lepture CVE debrief

Who should care

Users of Mistune, especially those using versions prior to 3.3.0, should be aware of this vulnerability. Developers and administrators who utilize Mistune in their applications or infrastructure should prioritize upgrading to version 3.3.0 or applying appropriate mitigations. Red Hat and other vendors may be affected, as indicated by the presence of references to their security advisories.

Technical summary

The vulnerability in Mistune arises from the parse_link_text function's superlinear behavior, approximately O(n²), when parsing Markdown containing many consecutive [ characters. This leads to repeated scanning of the input using a regex search inside a loop, causing quadratic-time behavior and potentially resulting in CPU exhaustion. An attacker can exploit this by providing a specially crafted Markdown input that triggers excessive CPU usage with a very small payload. The issue is addressed in Mistune version 3.3.0.

Defensive priority

Given the high severity and potential for exploitation, defenders should prioritize upgrading Mistune to version 3.3.0. If immediate upgrade is not possible, implementing compensating controls such as input validation and monitoring for suspicious CPU usage patterns may help mitigate the risk.

Recommended defensive actions

• Upgrade Mistune to version 3.3.0 or later.
• Implement input validation to limit the impact of potentially malicious Markdown inputs.
• Monitor systems for unusual CPU usage patterns that may indicate exploitation attempts.
• Review and apply vendor-provided security advisories and patches.
• Consider applying rate limiting or other traffic shaping techniques to limit the potential impact of an attack.

Evidence notes

The CVE record and NVD detail provide official information about the vulnerability. Multiple source references, including GitHub security advisories and Red Hat security notices, corroborate the existence and details of the vulnerability. The CVE has a CVSS score of 8.7 and is classified as HIGH severity.

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.