# CVE-2026-71478 — thephpleague commonmark

> As of 2026-08-07, CVE-2026-71478 is a MEDIUM-severity vulnerability in thephpleague commonmark, CVSS v3.1 6.1, EPSS 0.2% (10.0th percentile). No Threadlinqs-tracked threat campaign has been attributed to CVE-2026-71478 as of 2026-08-07; the identifier is re-checked against the Threadlinqs threat corpus on every daily ingest.

**Last updated:** 2026-08-07

## What is CVE-2026-71478?

league/commonmark is a PHP library for parsing and rendering CommonMark Markdown. From 1.5.0 until 2.9.0, the AttributesExtension's href and src unsafe-link filter can be bypassed by embedding control bytes, such as a tab, carriage return, line feed, or leading C0 control character, in a javascript: URL that browsers discard before parsing the scheme, causing the browser to still execute the script even when the unsafe-link filter is enabled. This issue is fixed in 2.9.0.

The record classifies CVE-2026-71478 under weakness classes [CWE-79](https://intel.threadlinqs.com/cwe/CWE-79), [CWE-86](https://cwe.mitre.org/data/definitions/86.html), [CWE-692](https://cwe.mitre.org/data/definitions/692.html). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs a user to take an action first. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 60 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 6.1 — MEDIUM (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`)
- **EPSS (FIRST):** 0.2% probability of exploitation in the next 30 days, higher than 10.0% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 3.5/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-08-06, last modified 2026-08-07

## Is CVE-2026-71478 being exploited?

It currently carries a trending score of 32 in the Threadlinqs vulnerability feed.

## Affected products and versions

- **thephpleague**: commonmark

Affected software packages:

- `composer:league/commonmark` — patched in 2.9.0

## How to fix CVE-2026-71478

Fixed versions are recorded for 1 package: composer:league/commonmark 2.9.0. No vendor patch reference has been recorded for CVE-2026-71478 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.

## Threat activity tracking CVE-2026-71478

No threat campaign in the Threadlinqs corpus currently references CVE-2026-71478, in its CVE list or as an indicator. The linkage is recomputed from the live corpus on every daily ingest, so this section fills in as soon as a tracked campaign cites the identifier.

## Sources

Enriched from CVE.org, FIRST EPSS, GitHub Security Advisories, NVD. Last verified by Threadlinqs on 2026-08-10. This product uses the NVD API but is not endorsed or certified by the NVD.

**Vendor advisory and patch**

- [github.com](https://github.com/thephpleague/commonmark/security/advisories/GHSA-29pj-957v-52mc)

**Other references**

- [github.com](https://github.com/thephpleague/commonmark/commit/493a5aa7d65754b73846006eaff9c2c4431a8e2c)
- [github.com (2.9)](https://github.com/thephpleague/commonmark/releases/tag/2.9.0)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-71478
Full detection coverage and IOCs for threats exploiting CVE-2026-71478 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp