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CVE-2026-49851 — Mistune: Potential DoS via quadratic-time parsing in parse_link_text

2026-10-03T12:59:31.492861+00:00

Affected: lepture mistune, Red Hat OpenShift AI 3.4, Red Hat Migration Toolkit for Applications 8, Red Hat OpenShift AI (RHOAI), Red Hat OpenShift Container Platform 4, Red Hat Satellite 6

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, Mistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parse_link_text. 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. This vulnerability is fixed in 3.3.0.

GHSA-qcq2-496w-v96p — Mistune: Potential DoS via quadratic-time parsing in parse_link_text

2026-10-03T12:59:31.492955+00:00

Affected: PyPI: mistune

### 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_markd…

PYSEC-2026-2652 — Mistune: Potential DoS via quadratic-time parsing in parse_link_text

2026-10-03T12:59:31.493020+00:00