XenForo: Incorrect Comparison
CVE-2026-73309 is a critical-severity incorrect comparison vulnerability in XenForo, with a CVSS 3.1 base score of 9.1 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists version 2.3.13 as the first without the flaw, so updating XenForo to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.69% chance of exploitation within 30 days, higher than 53% of scored CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Update XenForo to 2.3.13 or a later version.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 9.1 | Critical | CNA (vulncheck.com) |
| CVSS 3.1 | 7.4 | High | CNA (vulncheck.com) |
| CVSS 3.1 | 9.1 | Critical | NVD |
Official 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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Incorrect Comparison
The product compares two entities in a security-relevant context, but the comparison is incorrect.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: poc, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
Third-party and government advisories
- github.com/BomboBombone/CVE-2026-73309Third Party Advisory
- vulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-endpointThird Party Advisory
Technical write-ups (may include proof of concept)
- bombobombone.github.io/posts/cve-2026-73309/Exploit, Third Party Advisory
Other vulnerabilities in XenForo
- 7.2 High severity
- 7.5 High severity
- 8.6 High severity
- 7.5 High severity
- 8.1 High severity
- 7.4 High severity
Recent Incorrect Comparison vulnerabilities
- 9.8 Critical severity
- 8.8 High severity
- 8.3 High severity
- 7.2 High severity
- 7.5 High severity
- 8.1 High severity
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