# CVE-2026-86429: commonmark before 2.9.1 Denial of Service via SmartPunct and Attributes

> An attacker can cause denial of service via CPU exhaustion in the commonmark library if specific optional extensions are enabled.

- **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:** 7 Sep 2026
- **Updated:** 8 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-86429, GHSA-7x8h-jg2x-pjm5, GHSA-jjv6-8j6v-6j52, UBUNTU-CVE-2026-86429

## What is CVE-2026-86429?

CVE-2026-86429 is a high-severity denial of service vulnerability in thephpleague commonmark, published on 7 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The league/commonmark (thephpleague/commonmark) library in versions >= 1.5.0 and < 2.9.1 contains quadratic parsing complexity in its SmartPunctExtension and AttributesExtension. When either extension is explicitly registered on the Environment (they are not enabled by default and are excluded from the standard CommonMark and GitHub-Flavored Markdown converters), an unauthenticated attacker can submit small, specially crafted Markdown documents — such as text alternating with unpaired quotes, contiguous runs of block-level attribute blocks, or repeated class attributes — to trigger disproportionate CPU consumption and cause a denial of service. Fixed in 2.9.1.

Weakness: CWE-407 (Inefficient Algorithmic Complexity)

## Which versions are affected by CVE-2026-86429?

2 open source packages ship an affected version, including league/commonmark. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `league/commonmark` | Packagist | From 1.5.0 | 2.9.1 |
| `league/commonmark` | Packagist | From 1.5.0 | 2.9.1 |

Products: Thephpleague Commonmark.

## How do I fix CVE-2026-86429?

Upgrade league/commonmark to 2.9.1 or later.

## Is CVE-2026-86429 being exploited?

There is no known exploitation of CVE-2026-86429 in the wild and no public exploit code.

## Other vulnerabilities in thephpleague commonmark

- [CVE-2026-86428](https://insights.sbomify.com/CVE-2026-86428/): commonmark 1.5.0 before 2.10.0 Denial of Service via Attributes (high 8.7)
- [CVE-2026-86430](https://insights.sbomify.com/CVE-2026-86430/): league/commonmark before 2.9.1 Denial of Service via parsing (high 8.7)
- [CVE-2026-86431](https://insights.sbomify.com/CVE-2026-86431/): commonmark before 2.9.1 XSS via AttributesExtension form feed bypass (medium 6.9)
- [CVE-2026-33347](https://insights.sbomify.com/CVE-2026-33347/): league/commonmark has an embed extension allowed_domains bypass (medium 6.3)
- [CVE-2019-10010](https://insights.sbomify.com/CVE-2019-10010/): Moderate severity vulnerability that affects league/commonmark (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-86429 and fix it if it is affected.

What it is: CVE-2026-86429 is a high-severity denial of service vulnerability in thephpleague commonmark, published on 7 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-86429 in the wild and no public exploit code.

Affected packages and fixed versions:
- league/commonmark (Packagist): affected from 1.5.0; fixed in 2.9.1
- league/commonmark (Packagist): affected from 1.5.0; fixed in 2.9.1

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-2026-86429/ (Markdown: https://insights.sbomify.com/CVE-2026-86429/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-86429
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-86429
- OSV: https://osv.dev/vulnerability/CVE-2026-86429

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-86429/
Summaries are generated and may contain mistakes; the official description above is the record.