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CVE-2026-49851: Mistune: Potential DoS via quadratic-time parsing in parse_link_text

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-06-24 · updated 2026-10-25
• Weakness: CWE-400

Description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, Mistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parse_link_text. When parsing Markdown containing many consecutive [ characters, parse_link_text repeatedly scans the input using a regex search inside a loop. Each iteration re-scans a large portion of the remaining string, resulting in quadratic-time behavior. An attacker-controlled Markdown input can therefore trigger excessive CPU usage with a very small payload. This vulnerability is fixed in 3.3.0.

Affected products

• lepture mistune: before 3.3.0

References

• https://github.com/lepture/mistune/security/advisories/GHSA-qcq2-496w-v96p
• https://access.redhat.com/errata/RHSA-2026:60520
• https://access.redhat.com/security/cve/CVE-2026-49851
• https://bugzilla.redhat.com/show_bug.cgi?id=2492304
• https://github.com/lepture/mistune/security/advisories/GHSA-qcq2-496w-v96p
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-49851.json

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