CVE-2026-49851: Mistune: 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 parselinktext. A relatively small input consisting of repeated [ characters causes significant parsing slowdown.

Affected component mistune/inlineparser.py → parselinktext

Description When parsing Markdown containing many consecutive [ characters, parselinktext 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() (inlineparser.py) advances only 1 character at a time when parselink() returns None - Each failed attempt calls parselinktext() 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.createmarkdown()

s = "[" 6400

t = time.perfcounter() md(s) print(time.perfcounter() - 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.createmarkdown()

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

for n in sizes: s = "[" n

t0 = time.perfcounter() md(s) dt = time.perfcounter() - 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 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 parselinktext 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)