GHSA-8mp2-v27r-99xp

Summary

A ReDoS (Regular Expression Denial of Service) vulnerability in LINK_TITLE_RE allows an attacker who can supply Markdown for parsing to cause denial of service. A crafted 58-byte Markdown document blocks the parser for approximately 6 seconds (measured on Apple M2, Python 3.14.3), with exponential growth per additional byte pair.

Details

The vulnerable regex is defined in src/mistune/helpers.py#L20-L25:

LINK_TITLE_RE = re.compile(
    r"[ \t\n]+("
    r'"(?:\\' + PUNCTUATION + r'|[^"\x00])*"|'  # "title"
    r"'(?:\\" + PUNCTUATION + r"|[^'\x00])*'"   # 'title'
    r")"
)

The double-quote branch compiles to "(?:\\[PUNCTUATION]|[^"\x00])*". The two alternatives inside (A|B)* overlap: a backslash followed by a punctuation character (e.g. \!) can be matched by either branch — as a 2-character escaped-punctuation sequence \\!, or as two individual [^"\x00] characters (\ then !). The same ambiguity exists in the single-quoted title branch.

When the input contains repeated \! pairs with no closing ", the regex engine exhaustively backtracks through all 2^N combinations, resulting in exponential O(2^N) time complexity.

This is reachable through normal Markdown parsing via two code paths:

1. Inline links: [text](url "PAYLOAD) → parse_link() → parse_link_title()
2. Block link reference definitions: [label]: url "PAYLOAD → BlockParser.parse_ref_link() →...