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CVE-2026-73311 – OAuth2 Authorization Code Reuse / Token Theft – XenForo before 2.3.13

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Summary

CVE-2026-73311 is an OAuth2 authorization code reuse vulnerability (CWE-294, Authentication Bypass by Capture-replay) affecting XenForo forum software before version 2.3.13. XenForo’s OAuth2 implementation fails to invalidate authorization codes after they are first exchanged for tokens, allowing a previously used code to be resubmitted to mint an additional, independent token pair for the same user and scopes. The flaw is rated CRITICAL (CVSS v4.0 base score 9.1); it also carries a CVSS v3.1 HIGH rating.

Technical details

  • Root cause: After a client exchanges an OAuth2 authorization code for an access/refresh token pair, XenForo does not mark that code as consumed or otherwise invalidate it.
  • Trigger condition: An attacker in possession of a previously issued (and already-used) authorization code can resubmit it to the token endpoint.
  • Attack vector: Network-based; no user interaction required; the OAuth2 authorization code flow’s single-use guarantee is bypassed by replaying a captured code.
  • Impact: The attacker obtains a fresh, unauthorized token pair scoped to the victim account, enabling unauthorized API access and potential account compromise.

Affected software

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

Severity

  • CVSS v3.1: 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: 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 XenForo to version 2.3.13 or later, which corrects the authorization code invalidation logic.
  • If immediate patching isn’t possible: Monitor OAuth2 token issuance logs for repeated token grants against the same authorization code, and consider disabling/restricting OAuth2 client integrations until the update is applied. There is no vendor-documented workaround other than upgrading.

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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