Mistune: Denial of service via nested bracket parsing in parse_link_text
Summary
Mistune, a Python Markdown parser, versions 3.0.0 before 3.3.0 are affected by an algorithmic complexity flaw in the parse_link_text function. An attacker can submit Markdown with deeply nested square brackets, which causes quadratic parsing time and exhausts server CPU with a small request. No authentication is required. Applications that render user-supplied Markdown, including web applications, CMS platforms and API services, are at risk.
Affected
- mistune from 3.0.0 before 3.3.0 (GitHub advisory range: >=3.0.0 and <=3.2.1)
- Distribution packages: Debian lists all previous versions as affected. Ubuntu status: needs evaluation.
- Transitive dependencies, for example through nbconvert and Jupyter, may also be affected.
Fixed in
- mistune 3.3.0 (vendor-stated fix)
- Note: a separate DoS issue, CVE-2026-76098, affects 3.3.0 to 3.3.2 and is fixed in 3.3.3. Use the latest upstream release.
- Distribution patch status: unknown.
Workaround
No vendor-stated workaround. Upgrading is the fix.
Unverified interim measures: limit Markdown input size, enforce rendering timeouts or run rendering in isolated worker processes, and restrict who can submit Markdown.
Exploitation
- CISA SSVC assessment: exploitation none, automatable yes, technical impact partial.
- A research note states the vulnerability is exploited, but this is not confirmed by a primary source and conflicts with the CISA assessment. Treat exploitation status as unverified.
Our recommendation
- Identify all systems and applications using mistune, including transitive Python dependencies, for example with pip show or a software composition analysis tool.
- Upgrade to the latest upstream release, at least 3.3.3, not only 3.3.0.
- Until patched, limit Markdown input size, apply timeouts to rendering and monitor CPU usage of services that process user-supplied Markdown.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- GitHub Security Advisory GHSA-3q64-rw38-243v github.com
- VulnCheck advisory: Mistune 3.0.0 before 3.3.0 Denial of Service via Nested Bracket Parsing www.vulncheck.com
- Ubuntu security notice CVE-2026-115349 ubuntu.com
- OSV Debian entry DEBIAN-CVE-2026-115349 osv.dev
- VulDB entry 425395 vuldb.com
- HOL Guard advisory CVE-2026-115349 hol.org
- Miggo database entry CVE-2026-76098 www.miggo.io
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 00:41.
Original description (NVD)
Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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