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 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 3.190393Execution 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)
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | mistune | < 3.3.0 | 3.3.0 |
References
- https://github.com/lepture/mistune/security/advisories/GHSA-qcq2-496w-v96p
- https://nvd.nist.gov/vuln/detail/CVE-2026-49851
- https://access.redhat.com/security/cve/CVE-2026-49851
- https://bugzilla.redhat.com/show_bug.cgi?id=2492304
- https://github.com/lepture/mistune
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-49851.json
- https://pypi.org/project/mistune
- https://github.com/advisories/GHSA-qcq2-496w-v96p
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Weakness (CWE)
CWE-1333, CWE-400- Platform
- Python (PyPI)
- Package
mistune- Disclosed
- 09.07.2026
- Updated
- 11.10.2026 15:18
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-2652 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-qcq2-496w-v96p - CC BY 4.0
Vulnerability history: mistune
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 20
- First → latest
- 10.2017 09.2026
- Without a fix
- 1
- Exploited in attacks
- 0
Severity breakdown
- High 7
- Medium 13
Disclosures per year
- 2017: 2
- 2018: 0
- 2019: 0
- 2020: 0
- 2021: 0
- 2022: 1
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 17
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Mistune: Potential DoS via quadratic-time parsing in parse_link_text this entry
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