Potential DoS via quadratic-time parsing in parse_link_text

Summary

Mistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parse_link_text. A relatively small input consisting of repeated [ characters causes significant parsing slowdown.

Affected component

mistune/inline_parser.py → parse_link_text

Description

When parsing Markdown containing many consecutive [ characters, parse_link_text repeatedly scans the input using a regex search inside a loop. Each iteration re-scans a large portion of the remaining string, resulting in quadratic-time behavior.
An attacker-controlled Markdown input can therefore trigger excessive CPU usage with a very small payload.

Root cause

The vulnerability stems from a two-loop interaction:

• The outer loop in InlineParser.parse() (inline_parser.py) advances
  only 1 character at a time when parse_link() returns None
• Each failed attempt calls parse_link_text() which performs an O(n)
  scan to the end of the string looking for a closing ]
• With n consecutive [ characters, this results in O(n) × O(n) = O(n²)
  total work

PoC

Run below python script

import mistune
import time

md = mistune.create_markdown()

s = "[" * 6400

t = time.perf_counter()
md(s)
print(time.perf_counter() - t)

Benmark poc
Run below code for benchmark

import mistune
import time

md = mistune.create_markdown()

sizes = [100,200,400,800,1600,3200,6400]

for n in sizes:
    s = "[" * n

    t0 = time.perf_counter()
    md(s)
    dt = time.perf_counter() - t0

    print(f"{n:6d} {dt:.6f}")

Observed behaviour

python3 benchmark.py
100 0.001609
200 0.003207
400 0.012906
800 0.050220
1600 0.197307
3200 0.801172
6400 3.190393
Execution time grows superlinearly, consistent with O(n²) complex

Impact

This can be used as a denial-of-service attack in any application that parses user-supplied Markdown using Mistune, including:
Web applications (comments, posts, content rendering)
API services processing Markdown
Documentation rendering systems
A small (~6 KB) payload can block CPU for multiple seconds.

Suggested fix

Return the furthest scanned position from parse_link_text even on failure, so the outer loop can skip ahead instead of advancing 1 character at a time

Security Classification

CWE-400: Uncontrolled Resource Consumption
Denial of Service (CPU exhaustion)