Affected: lepture mistune, Red Hat Migration Toolkit for Applications 8.2, Red Hat OpenShift AI 3.4, Red Hat OpenShift AI (RHOAI), Red Hat Satellite 6

In versions 3.0.0a1 through 3.2.0 of Mistune, there is a ReDoS (Regular Expression Denial of Service) vulnerability in `LINK_TITLE_RE` that allows an attacker who can supply Markdown for parsing to cause denial of service. The regular expression used for parsing link titles contains overlapping alternatives that can trigger catastrophic backtracking. In both the double-quoted and single-quoted branches, a backslash followed by punctuation can be matched either as an escaped punctuation sequence or as two ordinary characters, creating an ambiguous pattern inside a repeated group. If an attacker supplies Markdown containing repeated ! sequences with no closing quote, the regex engine explores an exponential number of backtracking paths. This is reachable through normal Markdown parsing of inline links and block link reference definitions. A small crafted input can therefore cause significant CPU consumption and make applications using Mistune unresponsive.

Affected: PyPI: mistune

### 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`](https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/helpers.py#L20-L25):

```python 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()`](https://github.com/lepture/mistune/blob/df23edd60b43b6…

Affected: PyPI: mistune

### 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`](https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/helpers.py#L20-L25):

```python 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()`](https://github.com/lepture/mistune/blob/df23edd60b43b6…