Gradio before version 6.15.0 contains a flaw where a shared module‑level HTTP client is used across all users in the reverse proxy endpoint. This allows a remote attacker to inject a parent‑domain cookie, leading to cross‑Space session fixation. The injected cookie is stored by the shared client and replayed in subsequent proxy requests to other legitimate Spaces, effectively hijacking user sessions across the entire Gradio deployment. This constitutes a CWE‑384 vulnerability that can compromise confidentiality and integrity of user data.

The affected product is the Gradio web‑user interface, specifically all releases prior to 6.15.0. Vulnerability applies to any deployment where multiple Interactive HF Spaces share the same reverse proxy client; all users on the same deployment are at risk if any single Space can be controlled by an attacker.

The CVSS score of 7.6 indicates a high severity. EPSS data is not available, and the issue is not listed in the CISA KEV catalog, but the attack requires only remote control of a single HF Space to inject a cookie that propagates across all spaces. Once injected, the attacker can impersonate any user who then accesses the shared proxy. Because the attack vector is remote and the vulnerability is not limited by user privileges, attackers with internet access can exploit this weakness in any publicly exposed Gradio deployment.