---
id: CVE-2026-73309
title: >-
  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
  parameter…
summary: >-
  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
  parameter…
severity: high
cvss: 7.4
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N'
cwe:
  - CWE-697
vendor: xenforo
product: xenforo
affected:
  - xenforo < 2.3.13
patched:
  - xenforo 2.3.13
published: '2026-09-08'
updated: '2026-09-11'
sourceUpdated: '2026-09-11T20:33:42.980'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-73309'
references:
  - url: 'https://bombobombone.github.io/posts/cve-2026-73309/'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/BomboBombone/CVE-2026-73309'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-endpoint
    label: disclosure@vulncheck.com
  - url: >-
      https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/
    label: disclosure@vulncheck.com
  - url: >-
      https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/
    label: disclosure@vulncheck.com
tags:
  - nvd
  - exploit-available
  - cve.org
epss: 0.00476
epssPercentile: 0.40282
exploits:
  github: 1
  githubRepos:
    - 'https://github.com/BomboBombone/CVE-2026-73309'
  checkedAt: '2026-09-24T07:53:15.380Z'
exploitAvailable: true
ssvc:
  exploitation: poc
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-09-09T16:01:33.140807Z'
ingestedAt: '2026-09-08T15:33:26.984Z'
---

## Overview

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.

## Affected

- `xenforo < 2.3.13`

## Remediation

Upgrade past the affected range:

- `xenforo 2.3.13`