Mistune EUVD-2026-108417

No special configuration is required: the vulnerable parse_link_text fallback lives in the shared core inline parser and is reachable from every documented default entry point (mistune.html, mistune.create_markdown(...), Markdown.parse), in both the default escape=False renderer and the escape=True safe-mode renderer, with no plugin or non-default option needed. The only genuine prerequisite is that the application accepts and renders attacker-supplied Markdown through mistune 3.0.0-3.2.1. The exploit is a plain ASCII string of unbalanced '[' / '![' characters requiring no authentication, no user interaction, and no special headers. Limiting factors that bound realistic risk: impact is confined to CPU/availability of the rendering worker process (scope Unchanged - no code execution, data access, or cross-process effect), and deployments that require authentication before accepting Markdown effectively raise the bar to PR:L (CVSS 3.1 ~6.5 per the advisory's conservative variant). Upgrading to 3.3.0 is the fix.

The CVSS 4.0 vector in the input (AV:N/AC:L/AT:N/PR:N/UI:N, VA:H, score 8.7) and the GHSA-proposed CVSS 3.1 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H = 7.5) agree closely: a network-reachable, low-complexity, unauthenticated flaw whose only impact is availability - pure CPU-exhaustion DoS (CWE-400 driven by CWE-1333 algorithmic complexity), with no confidentiality or integrity effect. The signals are internally consistent rather than conflicting: there is no CISA KEV listing and no EPSS score in the provided data, so exploitation-probability cannot be quantified here, and absence of KEV means this is NOT confirmed actively exploited. Offsetting that, the GHSA advisory ships a self-contained, confirmed-reproduced proof-of-concept showing ~4x-per-doubling (quadratic) growth and ~16 KB tying up a core for ~16 seconds on 3.2.1 in both default and safe-mode renderers, which materially lowers the attacker effort/skill barrier. The vuln.today decision label is 'this_week' (not act_now/emergency), so this is a prompt-but-not-emergency priority: a genuinely real, trivially triggerable DoS against any service that renders untrusted Markdown, but one bounded to resource exhaustion of the rendering process rather than code execution or data compromise. Net: treat as a real, actionable this-week patch item; the headline 8.7 reflects easy unauthenticated reachability, not catastrophic impact, since scope is Unchanged and impact is availability-only.

An attacker submits a short (tens of kilobytes) Markdown payload consisting of many consecutive unbalanced '[' or '![' characters to any endpoint that renders user-supplied Markdown with mistune 3.0.0-3.2.1 (comment box, wiki page, chat message, issue tracker, SaaS preview). Because parse_link_text rescans the remaining input from each unmatched bracket, parsing runs in O(n²), so ~50 KB consumes minutes of CPU on one core and a handful of concurrent requests can starve a render worker pool. A confirmed, self-contained PoC demonstrating the quadratic blowup on 3.2.1 (default escape=False and safe-mode escape=True) is published in the GHSA advisory.