# 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.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 6 May 2026
- **Updated:** 28 Aug 2026
- **Also known as:** AZL-86318, CLEANSTART-2026-LT48571, CLEANSTART-2026-RE32707, DEBIAN-CVE-2026-33079, GHSA-8mp2-v27r-99xp, PYSEC-2026-2651, UBUNTU-CVE-2026-33079

## 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 (Inefficient Regular Expression Complexity)

## Which versions are affected by CVE-2026-33079?

3 open source packages ship an affected version, including mistune.


| Package | Ecosystem | 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.

## Is CVE-2026-33079 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33079 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33079 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33079 exists.

Affected packages and fixed versions:
- mistune (PyPI): affected from 3.0.0a1; fixed in 3.2.1
- mistune (PyPI): affected from 3.0.0a1; fixed in 3.2.1
- mistune (PyPI): affected from 3.0.0a1; fixed in 3.2.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-33079/ (Markdown: https://insights.sbomify.com/CVE-2026-33079/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33079
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33079
- OSV: https://osv.dev/vulnerability/CVE-2026-33079

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33079/
Summaries are generated and may contain mistakes; the official description above is the record.