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CVE-2026-73309 is an authentication-bypass vulnerability affecting XenForo versions before 2.3.13. The OAuth2 token endpoint uses PHP truthy evaluation for client-secret and PKCE code-verifier checks. Supplying empty values for the client_secret and code_verifier parameters causes the affected validation logic to be skipped, allowing an attacker to redeem a valid authorization code without authenticating the OAuth client or demonstrating possession of the PKCE verifier.
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1 valid exploit after Mallory filtered fakes, detection scripts, and README-only repos.
This three-file repository contains a standalone Python proof of concept for CVE-2026-73309 affecting XenForo versions before 2.3.13. The sole executable, poc.py, accepts a target base URL, OAuth client ID, authorization code, and redirect URI. It POSTs an authorization_code grant to the target's /api/oauth2/token endpoint while explicitly setting code_verifier to an empty string. The claimed flaw is that XenForo checks parameter presence but conditionally validates only truthy string values, allowing an empty verifier to bypass PKCE comparison. On receipt of an access token, the script prints a SHA-256 digest rather than the raw credential and sends it as a Bearer token to /api/me to confirm authenticated access. README.md documents the affected and fixed versions, prerequisites, expected HTTP responses, and references. This is an operational but narrow PoC: it does not generate, obtain, intercept, or brute-force authorization codes; exploitation depends on the operator already having a valid code and matching client/redirect details.
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6 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
An authentication-bypass vulnerability in XenForo's OAuth2 token endpoint prior to version 2.3.13. Empty client_secret and code_verifier values can exploit PHP truthiness handling to bypass client-secret and PKCE verifier validation, enabling exchange of a valid authorization code for OAuth2 token pairs without proving client identity or satisfying the PKCE commitment.
An OAuth2 token-endpoint authentication-bypass flaw in XenForo versions before 2.3.13. Empty client_secret and code_verifier values can exploit PHP truthy evaluation to bypass client-secret and PKCE validation, allowing an unauthenticated attacker with a valid authorization code to exchange it for a token pair.
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