Mistune: Algorithmic complexity denial of service (CVE-2026-115352)

Mistune before 3.3.0 contains an algorithmic complexity vulnerability in the inline link parser. An unauthenticated attacker can exploit this by submitting specifically crafted markdown payloads containing long sequences of unmatched '[' characters. This triggers O(N^2) complexity in the 'parse_link_text' and 'parse_link_label' functions, leading to significant CPU exhaustion and denial of service of rendering workers, with observed impacts of 30+ seconds per 16 KB of input.

Impact

Successful exploitation results in high CPU utilization on servers rendering user-supplied markdown, potentially leading to service degradation or denial of service for the application relying on the Mistune parser.

Recommendation

• Upgrade Mistune to version 3.3.0 or later to include the fix for the algorithmic complexity flaw.
• Implement request size limits on inputs processed by Mistune to mitigate the impact of O(N^2) complexity payloads if immediate patching is not feasible.
• Audit applications utilizing Mistune for public-facing markdown rendering to identify high-risk exposure points.