---
cve: "CVE-2026-73309"
severity: "CRITICAL"
cvss: 9.1
epss: "57.9%"
vendor: "Microsoft"
kev: false
exploited: false
published: "2026-09-08 14:17:24"
tags: [cve, security, critical]
source: tsecurity.de CVE-Dossier
exported: "2026-09-16T07:10:34+02:00"
---

# CVE-2026-73309

> 9.1 CRITICAL

## Beschreibung

XenForo before 2.3.13 contains an authentication bypass vulnerability in the OAuth2 token endpoint that allows unauthenticated attackers to obtain valid token pairs by submitting empty values for client_secret and code_verifier parameters. Attackers can exploit PHP truthy evaluation logic, which treats empty strings as false and skips client secret validation and PKCE code verifier validation, to exchange a valid authorization code for a token pair without proving client identity or holding the PKCE commitment.

## CVSS-Vektor

```
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Netzwerk | bad |
| AC Komplexität | Gering | bad |
| PR Privilegien | Keine | bad |
| UI Interaktion | Keine | bad |
| S Scope | Unverändert | good |
| C Vertraulichkeit | Hoch | bad |
| I Integrität | Hoch | bad |
| A Verfügbarkeit | Keine | good |

## Referenzen

- <https://bombobombone.github.io/posts/cve-2026-73309/>
- <https://github.com/BomboBombone/CVE-2026-73309>
- <https://www.vulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-endpoint>
- <https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/>
- <https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-73309) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_