Описание
Mistune has a ReDoS in LINK_TITLE_RE that allows denial of service via crafted Markdown input
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:
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:
- Inline links:
[text](url "PAYLOAD)→parse_link()→parse_link_title() - Block link reference definitions:
[label]: url "PAYLOAD→BlockParser.parse_ref_link()→parse_link_title()at block_parser.py#L259
PoC
Output (Apple M2, Python 3.14.3, mistune 3.2.0):
Each increment of N roughly doubles the execution time (consistent with O(2^N)).
The same attack works via block link reference definitions:
Impact
This is a denial of service vulnerability. Any application or service that parses user-supplied Markdown using mistune can be made unresponsive by an attacker submitting a small crafted input (under 100 bytes).
Affected use cases include:
- Web applications with Markdown-enabled input fields (comments, posts, descriptions)
- Documentation systems that accept user contributions
- API endpoints that process Markdown
- Jupyter tooling such as nbconvert that relies on mistune for rendering
Suggested Fix
Exclude the backslash character from the catch-all character class to eliminate the alternation overlap:
This ensures a backslash can only be consumed by the escaped-punctuation branch, eliminating the ambiguity in both the double-quote and single-quote branches. Verified on mistune 3.2.0 (Apple M2, Python 3.14.3):
- Reduces N=25 from 4.2 seconds to 0.000006 seconds (700,000x improvement)
- Handles N=50 in 0.000008 seconds
- Passes all existing functional tests (quoted titles, escaped quotes, escaped punctuation)
Пакеты
mistune
>= 3.0.0a1, <= 3.2.0
3.2.1
Связанные уязвимости
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.
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.
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.
Mistune ReDoS in LINK_TITLE_RE allows denial of service with crafted Markdown titles
In versions 3.0.0a1 through 3.2.0 of Mistune, there is a ReDoS (Regula ...