Vulnerability in Xenforo
CVE-2026-73309
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.4 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N.
EPSS exploit prediction
EPSS: 0.007 (52.1th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 52nd percentile — 52.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000
Affected products
Weakness classification (CWE)
Public proof-of-concept exploits
References
- Vendor Advisory (vendor-advisory, patch, Release Notes)
- Release Notes (release-notes, Release Notes)
- www.vulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-end… (Third Party Advisory, third-party-advisory)
- Researcher Advisory (Exploit, technical-description, Third Party Advisory)
- Researcher PoC (exploit, Third Party Advisory)
Frequently asked questions
- What is CVE-2026-73309?
- CVE-2026-73309 is a high-severity vulnerability in Xenforo, classified under Incorrect Comparison. CVSS score: 7.4/10. Published 2026-09-08.
- How severe is CVE-2026-73309?
- High severity. CVSS v3 base score is 7.4 out of 10.
- Is CVE-2026-73309 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.