CVE-2026-33079 Mistune ReDoS in LINK_TITLE_RE allows denial of service with crafted Markdown titles
Attackers can cause denial of service in applications using Mistune by supplying crafted Markdown that triggers catastrophic regex backtracking.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-33079?
CVE-2026-33079 is a high-severity denial of service vulnerability in lepture mistune, published on 6 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In versions 3.0.0a1 through 3.2.0 of Mistune, there is a ReDoS (Regular Expression Denial of Service) vulnerability in `LINK_TITLE_RE` that allows an attacker who can supply Markdown for parsing to cause denial of service. The regular expression used for parsing link titles contains overlapping alternatives that can trigger catastrophic backtracking. In both the double-quoted and single-quoted branches, a backslash followed by punctuation can be matched either as an escaped punctuation sequence or as two ordinary characters, creating an ambiguous pattern inside a repeated group. If an attacker supplies Markdown containing repeated ! sequences with no closing quote, the regex engine explores an exponential number of backtracking paths. This is reachable through normal Markdown parsing of inline links and block link reference definitions. A small crafted input can therefore cause significant CPU consumption and make applications using Mistune unresponsive.
Weakness: CWE-1333
Which versions are affected by CVE-2026-33079?
3 open source packages ship an affected version, including mistune.
| Package | Affected | Fixed in |
|---|---|---|
mistune PyPI | From 3.0.0a1 | 3.2.1 |
mistune PyPI | From 3.0.0a1 | 3.2.1 |
mistune PyPI | From 3.0.0a1 | 3.2.1 |
How do I fix CVE-2026-33079?
Upgrade mistune to 3.2.1 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-33079 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33079 exists.
Sources
Also known as AZL-86318, CLEANSTART-2026-LT48571, CLEANSTART-2026-RE32707, DEBIAN-CVE-2026-33079, GHSA-8mp2-v27r-99xp, PYSEC-2026-2651 and 1 more.