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CVE-2026-73309 – Authentication Bypass (OAuth2 Token Endpoint) – XenForo before 2.3.13

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Summary

CVE-2026-73309 is an authentication bypass vulnerability in the OAuth2 token endpoint of XenForo forum software, affecting all versions before 2.3.13. It allows an unauthenticated attacker to obtain a valid OAuth2 token pair without ever proving client identity or possessing the PKCE code verifier, by simply submitting empty values for the client_secret and code_verifier parameters. The issue carries a CVSS score of 9.1 (Critical).

Technical details

  • Root cause: The token endpoint’s validation logic relies on PHP’s truthy evaluation, which treats an empty string as false; when client_secret and code_verifier are submitted as empty strings, the corresponding validation checks are skipped entirely (CWE-697: Incorrect Comparison).
  • Trigger conditions: An attacker in possession of a valid authorization code (or able to obtain one) can send a token exchange request to the OAuth2 token endpoint with empty client_secret and code_verifier values.
  • Attack vector: Network-based, requires no authentication and no user interaction, though the attack complexity is rated high.
  • Impact: Successful exploitation yields a valid access/refresh token pair without proving client identity or holding the PKCE commitment, resulting in high confidentiality and integrity impact on the affected OAuth2 client relationship.

Affected software

  • XenForo, all versions from the initial release up to (but not including) 2.3.13

Severity

  • CVSS v3.1 Base Score: 7.4 (High) — CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
  • CVSS v4.0 Base Score: 9.1 (Critical) — CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade to XenForo 2.3.13 or later, which fixes the OAuth2 token endpoint validation logic.
  • If no patch: If upgrading is not immediately possible, disable or restrict OAuth2 client/token-endpoint functionality until the patch can be applied, and monitor OAuth2 token issuance logs for anomalous requests using empty client_secret or code_verifier values.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Raw response body: jQuery.extend(true, XenForo, Forum software by XenForo™, <!--XF:branding, <html id="XenForo", <html id="XF"
  • JavaScript object present once the page runs: XF.GuestUsername

References

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1

Map your entire attack surface (continously)

IONIX uses multi-factor discovery methods, including DNS analysis, certificate mapping, metadata inspection, and more, to automatically map every internet-facing asset across your environment. This includes cloud instances, third-party platforms, shadow IT, and even forgotten infrastructure that traditional tools miss.

2

Monitor for new CVEs

Dozens of threat intel feeds using agentic technology are continuously analyzed to detect the appearance of proof-of-concept code, exploit kits, and indicators of active targeting. IONIX goes further by applying AI to proactively evaluate whether emerging vulnerabilities are likely to be exploited, even before PoCs go public.

3

Identify Potential External Exposures

Not all CVEs matter. IONIX filters vulnerabilities by asking attacker-centric questions: Can it be reached from the internet? Does it require authentication? Is it being exploited in the wild? This dramatically reduces noise and focuses teams on threats that can actually be weaponized.

4

Create Safe, Scalable Exploit Validations

IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

5

Execute Exploit Validations

By combining context about software stack, versioning, exposure status, and reachability, IONIX ensures that only the right payloads are executed against the right assets, maximizing efficiency and minimizing risk.

6

Drive Fast and Actionable Remediation

Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

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