---
cve: "CVE-2026-86431"
severity: "MEDIUM"
cvss: 6.9
epss: "0.2%"
vendor: "thephpleague"
kev: false
exploited: false
published: "2026-09-07 13:20:42"
tags: [cve, security, medium]
source: tsecurity.de CVE-Dossier
exported: "2026-09-21T18:27:46+02:00"
---

# CVE-2026-86431

> 6.9 MEDIUM · 🧪 PoC

## Beschreibung

league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\x0Conclick="alert(1)"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.

## CVSS-Vektor

```
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Netzwerk | bad |
| AC Komplexität | Gering | bad |
| PR Privilegien | Keine | bad |
| UI Interaktion | Keine | bad |

## Patch verfügbar (OSV)

- 73cb188c785abfa7a7bec73487148202968274c6 (Commit)
- 43207253ea5f14867c77c697cd3838c446cadcea (Commit)

## Referenzen

- <https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8>
- <https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea>
- <https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-86431) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_