Mistune versions from 3.0.0a1 up to but not including 3.2.1 contain a Regular Expression Denial of Service (ReDoS) vulnerability in the LINK_TITLE_RE regex used for parsing Markdown link titles. An attacker can craft a small Markdown input (under 100 bytes) with repeated escaped punctuation sequences to cause exponential backtracking in the regex engine, resulting in significant parsing delays (e.g., ~6 seconds for 58 bytes). This affects any application or service that parses user-supplied Markdown using mistune, including web apps, documentation systems, APIs, and Jupyter tooling. The issue is fixed in mistune 3.2.1 by modifying the regex to remove ambiguity in backslash handling, eliminating the exponential backtracking.