Mistune: Quadratic-time denial of service in inline link parser
Summary
Mistune, a Python Markdown parser, takes a long time to process Markdown that contains many unbalanced '[' or '![' characters. An unauthenticated attacker can send a small, crafted input that makes the parser scan the remaining text repeatedly, tying up CPU and worker processes for minutes. The effect is a denial of service; no data loss or code execution is described. Exposure depends on whether an application renders untrusted Markdown with a vulnerable version.
Affected
- lepture mistune (Python package): 3.0.0 up to but not including 3.3.0
- Versions before 3.0.0: Unknown.
Fixed in
- Upgrade to mistune 3.3.0 or later.
Workaround
Unknown. No vendor workaround was found; upgrading to 3.3.0 is the fix.
- Interim measures (suggested by the research team, not from a source): limit Markdown input size, enforce request timeouts and worker recycling in the application server, and apply rate limiting at the reverse proxy for endpoints that render user Markdown.
Exploitation
- No public proof of concept found at the time of research.
- Not known to be exploited in the wild; not verified against the CISA KEV catalogue.
- CISA SSVC: exploitation none, automatable yes, technical impact partial.
Our recommendation
- Inventory Python applications that depend on mistune, including requirements files and lockfiles.
- Upgrade mistune to 3.3.0 or later.
- Check vendor or distribution packages that may bundle an older version.
- Prioritise internet-facing applications that render Markdown from untrusted users, such as comments, tickets, wikis or chat features.
- Monitor for worker timeouts and high CPU usage on these services.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- GitHub Security Advisory GHSA-x2gr-6qf2-fc9x github.com
- VulnCheck Advisory: Mistune before 3.3.0 Quadratic-Time DoS via parse_link_text www.vulncheck.com
- OSV: CVE-2026-115348 osv.dev
- mistune v3.3.0 release notes github.com
- VulDB 425397 vuldb.com
- Ubuntu Security: CVE-2026-115348 ubuntu.com
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, 00:42.
Original description (NVD)
Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity vulnerability in the inline link parser's parse_link_text function that allows unauthenticated attackers to exhaust CPU with unbalanced brackets. Attackers can submit Markdown containing many unbalanced '[' or '![' characters, forcing repeated unbounded forward scans and tying up worker processes for minutes.
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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