Reference page — cumulative record through Sunday, October 4, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.

CVE-2026-73309

CVE-2026-73309CRITICAL

XenForo < 2.3.13 Authentication Bypass via OAuth2 Token Endpoint

  AV  AC  AT  PR  UI  VC  VI  VA   CVSS    EPSS   %ile   KEV
   N   H   N   N   N   H   H   N    9.1   .0069   50.9     —

AFFECTED
  Product  Versions     Fixed
  XenForo  unspecified  —

TIMELINE
  Aug 11  Reserved by VulnCheck
  Sep 8   Published (CNA: VulnCheck)
  Sep 9   EXPLOIT PUBLISHED — CVE-2026-73309 (XenForo). Public exploit reference added.
  Sep 11  EXPLOIT PUBLISHED — CVE-2026-73309 (XenForo). Public exploit reference added.

CWE-697 · CNA: VulnCheck · CVSS v4.0 · 5 references · NVD status: Analyzed

Description

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.

Lifecycle

Complete event history — 4 events, chronological
Date	Event	Detail
August 11, 2026	Reserved	Reserved by VulnCheck
September 8, 2026	Published	Published (CNA: VulnCheck)
September 9, 2026	EXPLOIT PUBLISHED	EXPLOIT PUBLISHED — CVE-2026-73309 (XenForo). Public exploit reference added.
September 11, 2026	EXPLOIT PUBLISHED	EXPLOIT PUBLISHED — CVE-2026-73309 (XenForo). Public exploit reference added.

Affected

Affected products and packages — 1 row
Vendor	Product / Package	Ecosystem	Version introduced	Fixed
XenForo	XenForo	—	—	—

Weaknesses

CWE-697

References (5)

https://bombobombone.github.io/posts/cve-2026-73309/ [technical-description]
https://github.com/BomboBombone/CVE-2026-73309 [exploit]
https://www.vulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-endpoint [third-party-advisory]
https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/ [vendor-advisory, patch]
https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/ [release-notes]

Authoritative record: CVE-2026-73309 at cve.org

Vendors: xenforo

Weaknesses: CWE-697

About this page

This is a reference page, not a dated page of record. It assembles the complete lifecycle of CVE-2026-73309 from the CVE Program record, NVD enrichment, the CISA KEV catalog, EPSS, and OSV advisories. The box score's numbers (CVSS, EPSS, KEV status) are current as of Sunday, October 4, 2026 UTC and are re-derived as the archive grows; only dated daily editions are immutable pages of record. The authoritative source for this identifier is cve.org.