CVE-2026-73309: xenforo — Exploit

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.

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CVSS v3.1 Score

7.4

HIGH

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS — Exploit Prediction

0.0048

Probability of exploitation

0.40%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-697 CWE-697

Affected Products

Vendor	Product	Versions
xenforo	xenforo	< 2.3.13

References

Exploits

https://bombobombone.github.io/posts/cve-2026-73309/

Other References

https://github.com/BomboBombone/CVE-2026-73309 https://www.vulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-endpoint https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/ https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/

Frequently Asked Questions

What is 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. It has a CVSS v3.1 base score of 7.4 (HIGH).

How severe is CVE-2026-73309? +

CVE-2026-73309 has a CVSS v3.1 score of 7.4 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0048, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-73309? +

CVE-2026-73309 affects products from xenforo, specifically: xenforo. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-73309? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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