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CVE-2026-73316 – Payment Replay / Unauthorized Account Upgrades – XenForo before 2.3.13

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Summary

CVE-2026-73316 is a payment replay vulnerability in XenForo’s PayPal REST payment provider, caused by insufficient verification of data authenticity (CWE-345). Because the affected code fails to check for duplicate transaction IDs, an attacker can resubmit a valid PayPal webhook payload multiple times to trigger repeated subscription activations and unauthorized account upgrades. The flaw carries a CVSS v4.0 base score of 8.7 (High) and affects all XenForo versions prior to 2.3.13.

Technical details

  • Root cause: The PayPal REST payment provider integration in XenForo does not verify that an incoming webhook’s transaction ID has already been processed, allowing duplicate processing of the same payload.
  • Trigger conditions: An attacker who can capture or otherwise obtain a valid PayPal webhook payload can resend (replay) it to the XenForo endpoint that handles PayPal payment notifications.
  • Attack vector: Network — the payload is replayed directly against the site’s payment webhook handler, requiring no authentication or user interaction.
  • Impact: Repeated processing of the same payment event can cause duplicate subscription activations and unauthorized upgrades of user accounts/entitlements, an integrity impact without direct confidentiality or availability loss.

Affected software

  • XenForo, all versions before 2.3.13 (PayPal REST payment provider component)

Severity

  • CVSS v4.0: 8.7 (High) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
  • CVSS v3.1: 7.5 (High) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Mitigation and recommended actions

  • Immediate: Upgrade XenForo to version 2.3.13 or later, which includes the fix for this payment replay issue.
  • If immediate patching is not possible: Monitor PayPal transaction/subscription logs for duplicate transaction IDs or unexpected repeated subscription activations, and consider restricting or closely auditing access to the PayPal webhook endpoint at the network/WAF layer until the update can be applied.

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"
  • Rendered page (JavaScript object present after execution): XF.GuestUsername

References

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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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